The first or most anterior of these arteries supplies the hyoidean

hemibranch, while the succeeding four supply the holobranchs of the four

branchial arches. The blood is collected from the capillaries of the

branchial lamellae by a series of efferent branchial vessels, a pair for

the two hemibranchs of each branchial arch and a single vessel for the

hyoidean hemibranch, which unite with one another in a somewhat singular

fashion. The efferent arteries from the anterior and posterior hemibranchs

of each branchial cleft unite above and below each cleft in such a way as

to form a series of complete vascular loops round the hyoidean cleft and

the three succeeding branchial clefts, which are connected by short

longitudinal trunks in each arch and also by a longitudinal commissural

vessel between their ventral extremities. As the fifth arch is gill-less,

there is no complete loop round the fifth cleft, the blood collected by the

efferent vessel of the posterior hemibranch of the fourth arch being

conveyed to the corresponding vessel of the anterior hemibranch of the same

arch by one of the short longitudinal vessels above mentioned. Dorsally,

each arterial loop is continuous with an epibranchial artery; and by the

dorsal union of the four {332}epibranchial arteries of the two sides the

dorsal aorta is formed. It may be pointed out that the anterior efferent

vessel of each arch, which is usually larger than the posterior one, is to

be regarded as the primary efferent artery of the corresponding holobranch,

and as such is directly continuous with an epibranchial artery, the

posterior efferent artery being a secondary vessel which opens not into the

primary trunk of its own branchial arch, but into that of the succeeding

arch.[379] The principal arteries which supply the various parts of the

head with blood are derived from the first efferent branchial vessel. From

the ventral end of this artery a mandibular artery is given off, which

subdivides into branches for the muscles of the lower jaw as well as into

nutrient vessels for the hyoidean hemibranch. At about the middle of its

length the same artery gives off an afferent pseudobranchial or hyoidean

artery, to the spiracular or mandibular pseudobranch. From the latter organ

the blood is collected by an anterior carotid artery which, after giving

off an ophthalmic branch to the eye, perforates the orbital wall and enters

the cranial cavity, where it is joined by an anastomotic trunk from the

posterior carotid of the opposite side; finally, the anterior carotid

divides into anterior and posterior cerebral arteries for the brain. The

third and last of the cephalic arteries is the posterior carotid; this

artery arises from the dorsal extremity of the first efferent branchial

vessel, and, on entering the orbit, gives off the anastomotic trunk

previously mentioned. The latter vessel enters the cranial cavity, and,

after crossing its fellow, joins the anterior carotid of the opposite side,

as described above. The main trunk is then continued forwards in the orbit,

and its various branches eventually supply the eye-muscles, the mandibular

adductor muscle, and some other parts of the head.

It is worthy of note that the median dorsal aorta is prolonged forwards in

front of the first pair of epibranchial arteries as a slender median vessel

(_a.d.a_), which ultimately divides into two branches, each branch uniting

with the posterior carotid of its side.

A remarkable system of arteries for the supply of nutrient blood to the

gills and heart has its origin in the following {333}manner. On each side,

the longitudinal commissural vessel, which connects the ventral ends of the

arterial loops surrounding the different gill-clefts, gives origin to a

series of pairs of short transverse vessels, and by their union these

combine to form a median longitudinal hypobranchial artery which lies

beneath the ventral aorta. From the hypobranchial artery are derived the

coronary arteries for the heart; and from the same artery, or from its

lateral connexions with the longitudinal commissural artery, and, in the

case of the hyoidean hemibranch, from the mandibular artery, are derived

the various nutrient vessels for the gills.

The arteries for the trunk, and for the pectoral and pelvic limbs, arise in

succession from the dorsal aorta. The first of the series is the subclavian

artery, which has its origin from the aorta close to the dorsal extremities

of the fourth pair of epibranchial arteries. Each subclavian artery gives

off a brachial artery to the pectoral fin, and is then continued forwards

as a lateral hypobranchial artery, which, with its fellow of the opposite

side, eventually becomes continuous with the hinder end of the median

hypobranchial artery. Behind the subclavian artery there is a median

coeliaco-mesenteric artery, the various branches of which are distributed

to the liver, stomach, and intestine. A lieno-gastric artery supplies the

pancreas and spleen, and also sends branches to the stomach. In addition,

there are also arteries for the gonads, numerous segmental arteries for the

myotomes, and renal arteries for the kidneys. Finally, the aorta gives off

a pair of iliac arteries for the pelvic fins, and then enters the haemal

canal as the caudal artery.

The more important differences in the arterial system of the Holocephali

and the Teleostomi relate to (1) the absence of the posterior efferent

branchial artery in each branchial arch; (2) modifications dependent on the

condition of the spiracular and hyoidean hemibranchs, and the mode of

origin and the course of their afferent and efferent vessels; and (3) the

source from whence the air-bladder derives its blood when that organ is

present.

(1) The branchial arterial system is somewhat more primitive than in the

generality of Elasmobranchs.[380] There are no complete vascular loops

round the gill-clefts, and the blood from the two {334}hemibranchs of each

branchial arch is conveyed to the dorsal aorta by a single efferent vessel

which corresponds to the more anterior of the two in _Mustelus

antarcticus_.[381]

(2) In _Callorhynchus_[382] among the Holocephali, where the spiracle is

absent but the hyoidean hemibranchi is still a true gill, the latter organ

is supplied with venous blood by a branch from the ventral aorta, the

corresponding efferent vessel joining the dorsal aorta (Fig. 196). In the

absence of a spiracular pseudobranch the anterior carotid may be regarded

as continuous with the hyoidean artery,[383] and as having its origin

directly from the efferent artery of the hyoidean hemibranch (Fig. 196). At

its origin the anterior carotid anastomoses with the mandibular artery.

[Illustration: FIG. 196.—Portion of the efferent branchial system of

_Callorhynchus_. _a.c_, Anterior carotid; _a.cb.a_, anterior cerebral

arteries; _d.a_, dorsal aorta; _ef.b.a_, 1-4, efferent branchial arteries;

_ef.hy_, efferent artery from the hyoidean hemibranch; _hy.c_, hyobranchial

cleft; _md.a_, mandibular artery; _my.a_, myelonal artery; _p.c_, posterior

carotid; _p.cb.a_, posterior cerebral artery. (From T. Jeffery Parker.)]

The Sturgeon more closely resembles the Elasmobranchs. The hyoidean gill is

supplied by an afferent branchial artery from the ventral aorta, and its

efferent vessel joins the corresponding trunk from the holobranch of the

first branchial arch. A hyoidean artery supplies the spiracular

pseudobranch, the efferent vessel of which contributes to the blood-supply

of the brain and the eye, and probably represents an anterior carotid.

_Lepidosteus_[384] offers a singularly interesting transition from the

{335}Elasmobranch to the Teleost. As indicated in the preceding chapter,

this Fish possesses both a hyoidean gill and a spiracular pseudobranch

(Figs. 197 and 198). The hyoidean gill is supplied by an afferent artery

direct from the ventral aorta, but the proper efferent vessel of the gill,

which primitively joined the dorsal aorta, is suppressed, and the blood is

collected into a vessel, which, like the hyoidean artery in Elasmobranchs,

becomes the afferent artery of the spiracular pseudobranch. The latter

artery unites, however, with a second hyoidean artery derived from the

efferent branchial vessel of the first branchial arch, and represents the

artery termed "hyoidean" in Teleosts. The efferent vessel from the

spiracular pseudobranch joins an internal branch from the carotid artery,

and then distributes its blood both to the eye and the brain.

[Illustration: FIG. 197.—Blood-vessels of the spiracular pseudobranch and

the hyoidean gill in _Lepidosteus_. _af.a_, _ef.a_, Afferent and efferent

vessels of the hyoidean gill; _af.ps.a_, _ef.ps.a_, afferent and efferent

vessels of the spiracular pseudobranch; _ca_, carotid (posterior); _d.a_,

dorsal aorta; _ef.b.a^{1-4}_, efferent branchial vessels; _hy.a_, hyoidean

artery; _hy.h_, hyoidean gill; _hy.ps_, spiracular pseudobranch; _v.a_,

ventral aorta. (From F. W. Müller, after Johannes Müller.)]

In Teleosts, as already mentioned in a preceding chapter, it is probable

that the hyoidean hemibranch is suppressed, the so-called hyoidean

pseudobranch being a spiracular pseudobranch. The latter is now supplied by

a "hyoidean" artery, which has its origin from the ventral end of the

efferent {336}branchial artery of the first branchial arch, the

corresponding efferent trunk forming an ophthalmic artery, and passing to

the choroid gland of the eye (Fig. 199). Both the proper afferent and

efferent arteries of the hyoidean hemibranch either disappear or, as in the

Cod (_Gadus morrhua_), the efferent artery may be represented on each side

by an anastomosis between the hyoidean artery and the cephalic circle.

Hence, the "hyoidean" artery of Teleosts corresponds to the one which has a

similar origin in _Lepidosteus_.

A brief description of the remaining efferent branchial arteries and their

derivatives in the Cod (_Gadus morrhua_) will illustrate the condition of

these structures in a well-known Teleost.

[Illustration: FIG. 198.—The branchial circulation in _Lepidosteus_

(diagrammatic). _a_, _a_, Afferent branchial arteries; _c_, carotid; _d.a_,

dorsal aorta; _e_, _e_, efferent branchial arteries; _ef.a_, efferent

vessel from the hyoidean gill which, after its union with the hyoidean

artery, becomes the afferent vessel of the spiracular pseudobranch;

_ef.a′_, efferent vessel of the spiracular pseudobranch; _hy.a_, hyoidean

artery; _hy.g_, hyoidean gill; _sp.ps_, spiracular pseudobranch; _v.a_,

ventral aorta; 1-5, the hyo-branchial and succeeding gill-clefts. (After F.

W. Müller and Ramsay Wright.)]

In this Fish the efferent branchial vessels open dorsally into right and

left suprabranchial arteries,[385] which unite behind to form a median

dorsal aorta (Fig. 199). Anteriorly, the paired suprabranchial arteries

extend towards the base of the skull as the so-called "carotid" arteries.

The two carotids enter the cranial cavity, and there unite in the median

line, as in the Cyclostomes. By the union of these arteries in front, and

of the {337}right and left suprabranchial arteries behind, the

characteristic "circulus cephalicus" of Teleosts is completed.[386] From

the anterior part of the cephalic circle are derived two internal carotid

arteries[387] for the brain, and also a pair of orbito-nasal arteries for

the eye-muscles and the nasal sacs, while more posteriorly an external

carotid has its origin from each suprabranchial artery.

[Illustration: FIG. 199.—Branchial arterial system of the Cod (_Gadus

morrhua_). Lateral view. _af.b.a_, First afferent branchial artery; _cl.a_,

coeliac artery; _d.a_, median dorsal aorta; _ef.b.a_, first efferent

branchial artery; _ex.c_, external carotid; _H_, heart; _hy.a_, hyoidean

artery; _Hy.b.a_, hypobranchial artery for the heart and pelvic fins;

_hy.ps_, spiracular pseudobranch; _in.c_, internal carotid;[387] _l.d.a_,

left suprabranchial artery; _m.a_, mesenteric artery; _on_, orbito-nasal

artery; _oph.a_, ophthalmic artery; _r.d.a_, right suprabranchial artery;

_sb.a_, subclavian artery; _v.a_, ventral aorta; 1-5, hyobranchial and

succeeding gill-clefts. (Altered from T. Jeffery Parker.)]

(3) In most Teleostomi the air-bladder is supplied with blood by branches

of the coeliac artery, with the addition of small branches arising directly

from the dorsal aorta. _Polypterus_ and _Amia_[388] are, however,

exceptional, inasmuch as the arteries for the air-bladder are derived from

the last or fourth pair of efferent branchial vessels, and in this respect,

but not in the destination {338}of the corresponding veins, the two genera

exhibit a significant resemblance to the Dipnoi.

In the Dipnoi the ventral aorta is so short that the afferent branchial

arteries arise almost directly from the conus arteriosus with their roots

in close contiguity to one another (Fig. 200).

[Illustration: FIG. 200.—Branchial arterial system of _Neoceratodus_.

Lateral view. The conus arteriosus and the afferent branchial vessels are

represented in solid black, the efferent vessels and their derivatives with

double contours. _a_, Auricle; _a.c.a_, anterior carotid; _a.cb.a_,

anterior cerebral artery; _af.b.a′_, first afferent branchial artery;

_br.a_, brachial artery; _c.a_, coronary artery; _c.ar_, conus arteriosus;

_c.m.a_, coeliaco-mesenteric; _ep.a_, epibranchial artery; _hb.a_,

hypobranchial artery; _hy.a_, hyoid artery; _hy.ar_, hyoidean arch; _l.a_,

lingual; _l.d.a_, _r.d.a_, left and right dorsal aortae; _oc.a_, occipital

artery; _oes.a_, oesophageal artery; _p_, pericardium; _p.a_, pulmonary

artery; _p.c.a_, posterior carotid; _p.cb.a_, posterior cerebral artery;

_s.v_, sinus venosus; _v_, ventricle; 1, hyobranchial cleft; 2-5, branchial

clefts. (After Baldwin Spencer, diagrammatic.)]

In _Neoceratodus_ (Fig. 200),[389] there are two efferent vessels to each

gill-bearing branchial arch, which unite above to form an epibranchial

artery, and by the successive union of the four epibranchial arteries a

short common trunk is formed on each side. Posteriorly, the two trunks

unite to form a median dorsal aorta. Immediately above the gill-clefts each

efferent vessel gives off a branch which, passing either forwards or

backwards, unites with the corresponding branch of the efferent vessel in

front or behind as the case may be. A hyoidean artery arises from the

ventral extremity of the anterior efferent artery of the first branchial

arch, and, after giving off a lingual artery, ascends the hyoid arch and

supplies the hyoidean pseudobranch. The efferent vessel of the pseudobranch

(_a.c.a_) or anterior {339}carotid artery, eventually enters the cranial

cavity and subdivides into anterior and posterior cerebral arteries for the

brain, also giving off a branch which unites with its fellow of the

opposite side directly behind the infundibulum. A posterior carotid springs

from the epibranchial of the first branchial arch and divides into

palatine, orbital, and ocular branches; and from the ventral end of the

anterior efferent vessel of the second branchial arch is derived a

hypobranchial artery for the heart and pericardium. The pulmonary arteries

for the lung-like air-bladder have their origin from the fourth pair of

epibranchial arteries.

As in so many other details of its anatomy, _Neoceratodus_ exhibits in its

arterial system abundant evidence of the wide-spreading affinities of the

group to which it belongs. In its branchial arterial system _Neoceratodus_

presents a singular combination of features which, individually, are

characteristic of Amphibia and Elasmobranchs. Special Amphibian features

may be noted in the origin of the afferent branchial arteries almost

simultaneously from the anterior end of the conus arteriosus; in the mode

of union of the epibranchial arteries to form the dorsal aortae; in the

origin of a lingual artery from the efferent vessel of the first branchial

arch; and in the derivation on either side of a pulmonary artery from the

fourth epibranchial artery. Agreement with Elasmobranchs is to be found in

the presence of two efferent branchial vessels in each branchial arch,

although the relations of these arteries are more primitive than in most

adult Elasmobranchs, inasmuch as the two efferent vessels of the _same_

arch unite to form an epibranchial artery; and also in the origin and

distribution of the anterior and posterior carotids. Lastly may be

mentioned the fact that _Neoceratodus_ agrees not only with the Amphibia

but also with those generalised Teleostomi, _Polypterus_ and _Amia_, in the

mode of origin of the great arteries for the air-bladder.

Of the two remaining Dipnoi, the arterial system of _Protopterus_[390] is

better known than that of _Lepidosiren_, but in both cases further research

is needed before a satisfactory comparison can be made with _Neoceratodus_

and other Vertebrates. It is evident, nevertheless, that both genera differ

from _Neoceratodus_ in approximating more closely to the Amphibia than to

the {340}lower Fishes, in so far as the branchial part of the arterial

system is concerned.

[Illustration: FIG. 201.—Branchial arterial system of _Protopterus_

(diagrammatic). _a_, Auricle; _a.c.a_, carotid artery; _af.b.a^{1-4}_,

afferent branchial arteries; _af_, _ef_, afferent and efferent vessels of

the hyoidean pseudobranch; _b.a^2_, second branchial arch, the vestigial

first arch being omitted; _c.a_, conus arteriosus; _e.g_, external or

cutaneous gill; _ep.a_, epibranchial artery; _hy.ar_, hyoid arch; _hy.ps_,

hyoidean pseudobranch; _l.a_, lingual artery; _l.d.a_ and _r.d.a_, right

and left dorsal aortae; _l.p.a_, left pulmonary artery; _s.v_, sinosus

venosus; _v_, ventricle; 2-6, the second branchial and succeeding clefts,

the hyobranchial cleft being closed. The vestigial first branchial arch is

not shown. The epibranchial arteries unite to form the right or left dorsal

aorta at the same point and not in succession as in the figure. (Altered

from Newton Parker.)]

In their origin from the conus the four afferent branchial arteries of

_Protopterus_ resemble those of _Neoceratodus_, but their relations to the

branchial clefts are somewhat different (Fig. 201). The first or hyoidean

cleft is closed, and the first afferent vessel lies between the second

cleft and the third, and is therefore in relation with the second branchial

arch. The remaining afferent arteries are disposed between the succeeding

clefts and are related to the corresponding arches. As the second and third

arches, like the vestigial first arch, bear no gill-lamellae, their

afferent arteries are directly continuous with the corresponding efferent

vessels, as in those Teleosts in which certain arches are gill-less, as

well as in the Tadpole-stage of the tailless Amphibia when the internal

gills begin to degenerate; and they apparently transmit arterial blood

directly to the dorsal aorta.[391] The third and fourth afferent arteries,

on the contrary, supply venous blood to the two hemibranchs which are borne

by each of the two corresponding arches, viz.: the fourth and fifth, and

from each pair of hemibranchs the blood is collected into two efferent

vessels which unite dorsally {341}to form an epibranchial artery. From the

dorsal end of the fourth afferent artery there arises a recurrent branch

which curves round the upper margin of the sixth cleft and supplies the

gill-lamellae on the posterior margin of that cleft, a fact which lends

support to the view that these lamellae are "emigrants" from the anterior

margin of the cleft; the efferent vessel from the "emigrant" lamellae joins

the fourth epibranchial artery. The blood-supply of the external or

cutaneous gills is derived from the dorsal extremities of the second,

third, and fourth afferent arteries, while the efferent vessels from these

organs join the corresponding epibranchial arteries; in this respect there

is a close resemblance between _Protopterus_ and those larval Amphibians

which possess similar cutaneous gills. All four epibranchial arteries unite

together at about the same point to form a short common trunk, the right or

left dorsal aorta, which subsequently unites with its fellow to form the

median dorsal aorta.

There is a so-called "hyoidean" artery, which, however, has its origin, not

from an anterior efferent branchial vessel as in _Neoceratodus_, but from

the first _afferent_ branchial artery. After giving off a submaxillary or

lingual artery, the "hyoidean" artery (_af_) becomes the afferent vessel

for the "opercular gill" or "hyoidean pseudobranch,"[392] and supplies the

latter with arterial blood. The efferent vessel (_ef_) from the

pseudobranch unites with the four epibranchial arteries in forming the

right or left dorsal aorta. A "carotid" artery arises from the efferent

vessel of the "hyoidean pseudobranch," and a pulmonary artery has its

origin from the root of the dorsal aorta of its side, and not from the

fourth epibranchial artery as in _Neoceratodus_.

THE BLOOD.—The blood consists of a nutritive fluid plasma in which float

red corpuscles and leucocytes. In the Cyclostomata (e.g. _Petromyzon_) the

red corpuscles are circular, but in _Myxine_ they have the usual oval

shape. In Fishes the red corpuscles are almost invariably flat, oval,

biconvex, and nucleated, and owe their colour to the presence of the

characteristic oxygen-absorbing, iron-containing pigment, haemoglobin. They

are unusually large in the Dipnoi and are only exceeded in size by those of

certain Urodele Amphibians. The leucocytes are much less numerous than the

red corpuscles, although their relative proportions are very variable, even

in the same species under different {342}conditions. They appear to be more

numerous in the Dipnoi (e.g. _Protopterus_) than in any other Vertebrates,

except under pathological conditions.[393]

THE LYMPHATIC SYSTEM.—In addition to blood-vessels, Fishes possess a

lymphatic system, consisting of smaller vessels, lymph-capillaries or

lymph-spaces, distributed in the connective tissue of different parts of

the body, and by their union ultimately forming larger lymph-vessels or

sinuses which communicate with certain of the principal veins, the whole

forming a series of channels for the collection of the blood-plasma which

has exuded from the blood-capillaries for the nutrition of the tissues, and

for its conveyance to the general venous system. The fluid in the

lymphatics, or lymph, consists of dilute blood-plasma containing leucocytes

but devoid of red corpuscles. At the points where the larger lymphatics

open into the veins, lymph-hearts may be developed. In the Eel (_Anguilla

vulgaris_) there is a lymph-heart in the tail, which communicates by a

valvular orifice with the smaller of the two caudal veins, and by its

rhythmical pulsations propels the lymph into the vein. In _Silurus_ there

are two caudal lymph-hearts. Apart from the lymphoid tissue, which is so

abundantly present in certain parts of the body, Fishes appear to be devoid

of the special "lymphatic glands" of the higher Vertebrates.

THE DUCTLESS OR BLOOD-GLANDS.—All the important blood-glands of other

Vertebrates have their representatives in Fishes. Nothing is certainly

known of the function of these organs in Fishes, but from the general

structural resemblance which they present to their equivalents in the

higher Vertebrates, it is perhaps not unreasonable to infer that they are

similar in function. If this be so, the blood-glands of Fishes are organs

for leucocyte-formation and phagocytosis, involving the destruction and

removal of effete red blood-corpuscles; in addition, they may also be

concerned with certain obscure chemical changes in the composition of the

blood, which have an important relation to general or local nutrition.

THE SPLEEN.—This lymphoid organ is the largest of all the blood-glands,

and, in the form of a compact or more or less lobulated body, is present in

all Fishes, and possibly in Cyclostomes. In position the spleen is usually

in close proximity to the stomach, to which it is attached by an extension

round it {343}of the peritoneal investment of that organ. Thus, in the

Dog-Fish (_Scyllium_), the spleen is a large reddish body attached to the

convexity of the U-shaped stomach, and, in addition, sends a long narrow

lobe between the distal limb and the valvate portion of the intestine (Fig.

153, _spl_). In the Sturgeon (_Acipenser_), the organ is also large, but is

attached to the left side of the commencement of the intestine. In the Cod

(_Gadus_) among Teleosts the spleen is much elongated and is situated on

the dorsal side of the stomach. In the Dipnoi (e.g. _Protopterus_)[394] the

organ is probably represented by a large compact lymphoid mass, closely

connected with the dorsal and lateral walls of the stomach (Fig. 154, A,

_s_).

THE THYROID GLAND.—This organ[395] usually arises in the form of a small

median evagination of the hypoblastic epithelium of the ventral wall of the

pharynx, in the region of the second visceral arch. Later it becomes

detached from the place of origin and converted into a solid spherical

body. Eventually the component cells form the limiting epithelium of a

series of follicles or vesicles embedded in a matrix of connective tissue

and blood-vessels, and the characteristic adult structure is attained.

Among the Cyclostomata the evagination is relatively large in the young

Lamprey (_Petromyzon fluviatilis_), as also is the orifice of communication

with the pharynx (Fig. 202, _th_).[396] The aperture soon becomes reduced

to a mere pore, and finally disappears. During the larval or

Ammocoetes-stage the organ consists of a median ciliated portion,

communicating with a pair of laterally placed glandular sacs, but in the

adult it is much smaller, and acquires the usual follicular structure. In

adult Elasmobranchs the thyroid is represented by a moderately large

compact organ, situated near the anterior end of the ventral aorta. In

Teleostomi the organ may be paired, or, as in the Perch (_Perca_), more

diffuse, consisting of masses of reddish lobules lying beneath the aorta,

and also scattered for a variable distance along the course of the afferent

branchial arteries.

In the Dipnoi (e.g. _Protopterus_)[397] the thyroid is small,

{344}consisting of two lateral lobes connected by a constricted median

portion, and situated beneath the epithelium of the tongue, immediately

above the hyoidean symphysis. A similar structure has been described by

Bischoff[398] in _Lepidosiren_, and was regarded by him as a salivary

gland.

As in Reptiles, Birds, and Mammals, paired or accessory thyroid bodies

("supra-pericardial organs")[399] are present in many Fishes, and appear to

be similar in structure to the median thyroid. In Elasmobranchs these

bodies originate as a pair of outgrowths from the epithelium of the pharynx

behind the last pair of branchial arches (Fig. 202, B, _a.th_).

Subsequently they become detached from the pharynx, and in the adult are

situated on the dorsal side of the pericardium, remote from the median

thyroid.

[Illustration: FIG. 202.—A, a vertical section through a just-hatched larva

of _Petromyzon_. _a.v_, Auditory vesicle; _br.c_, branchial cleft; _h_,

heart; _m_, mouth; _n_, notochord; _ol_, olfactory pit; _ph_, pharynx;

_sp_, septum or velum between the stomodaeum and the mesenteron; _sp.c_,

spinal cord; _th_, thyroid outgrowth from the floor of the pharynx. (From

Gegenbaur, after Calberla.) B, diagram illustrating the development of the

thyroid, the thymus, and the accessory thyroids, and their relations to the

branchial clefts. _a.th_, Accessory thyroids; _g.p_, gill-pouches; _Ph_,

pharynx; _t_, thymus; _th_, median thyroid. (From Hertwig, after de

Meuron.)]

According to Dohrn the median thyroid is to be regarded as the vestige of a

gill-cleft which primitively existed between the hyomandibular cartilage

and the hyoidean arch. This conclusion seems, however, to be less in

harmony with the facts of development than the view[400] that the organ is

derived from the characteristic hypobranchial groove or "endostyle" of

Ascidians {345}and Amphioxus, which has undergone a change of function from

a mucus-conveying groove to a blood-gland. On the other hand, the mode of

origin of the paired thyroids certainly favours the suggestion that they

represent a posterior pair of vestigial gill-clefts, a view which derives

some support from the fact that in _Notidanus_, where additional branchial

arches and clefts are present, the paired thyroids are absent.

THE THYMUS.—In the embryo Elasmobranch and Teleost[401] the thymus has a

multiple origin, being derived from a series of distinct epithelial

thickenings, one of which is developed at the dorsal extremity of each of

the gill-clefts except of the spiracle. These rudiments subsequently detach

themselves from the epithelial surface and sink inwards, eventually fusing

together on each side to form a single independent structure. Later, the

epithelial mass thus formed becomes invaded by connective tissue, and by

leucocytes which form lymph follicles, and the thymus gradually assumes the

structure of a lymphoid organ. From its mode of development it has been

suggested that the thymus owes its evolution to the metamorphosis and

ingrowth of branchial filaments,[402] but it is also noteworthy that each

embryonic rudiment of the organ closely resembles, both in position and

origin, one of the developing branchial tongue-bars of Amphioxus.[403] The

abundance of leucocytes which it contains has also prompted the further

suggestion that the origin of the thymus may be due to the necessity of

providing for the phagocytic protection of the gills themselves from the

ravages of harmful micro-organisms, fungoid spores, etc., as well as to aid

in the removal of such portions of the gills as may have been injured.[404]

A thymus is probably present in all Fishes, if not in the adult at all

events in the embryo, but is always relatively small in size. In

Elasmobranchs the organ lies on each side above the branchial arches and

beneath the dorsal musculature; and in Teleostomi at the dorsal extremity

of the last branchial arch, in close proximity to the mucous membrane of

the branchial cavity. In a similar position in the Dipnoi (e.g.

_Protopterus_)[405] there are, on each side, {346}two contiguous lobes of

lymphoid tissue which apparently represent a thymus.

[Illustration: FIG. 203.—Supra-renal and inter-renal bodies of Fishes. A,

of _Scyllium catulus_; B, of _Acipenser sturio_; C, of _Pagellus

centrodontus_. _d.a_, Dorsal aorta; _tr_, inter-renal body; _l.m_, lymphoid

portion of the mesonephros; _m_, mesonephros; _mtn_, metanephros; _oes_,

oesophagus; _sg.a_, segmental arteries; _sr_, supra-renal bodies; _sy.n_,

sympathetic nerves. (From Swale Vincent.)]

THE SUPRA-RENAL BODIES.—The supra-renal bodies are organs of problematic

function, which are present in the Cyclostomata, and probably in all

Fishes, and situated in close proximity to the kidneys.

In the Cyclostomata (_Petromyzon_) these bodies are represented by lobules

of cells along the posterior cardinal veins, and also by masses of peculiar

cells ("chromaffin cells") along the sides of the aorta and segmental

arteries.[406] In Elasmobranchs there are two distinct structures, the

paired supra-renals and the inter-renals (Fig. 203, A). The former are a

series of pairs of segmentally arranged bodies, situated on the successive

pairs of segmental arteries given off from the dorsal aorta. The two bodies

which form the first pair are much larger than any of the others, and were

formerly spoken of as "axillary hearts." The inter-renal is usually a thin

elongated "ochre-yellow" body, from which one or two lobes may be detached

in front, and extends for a variable distance in {347}the median line

between the two kidneys, or is unsymmetrically placed on the ventral

surface of either kidney.[407] Sometimes (e.g. in _Raia_) the inter-renals

are paired, in which case they are applied to the inner and hinder margins

of the kidneys. In the Sturgeon (_Acipenser sturio_) the "supra-renals"

appear as numerous "ochre-yellow" bodies, variable in size and distribution

(Fig. 203, B). Some of them are visible on the surface of the kidneys,

while others are scattered about in their substance, but on the whole are

more anteriorly placed than in Teleosts. In the latter group the

"supra-renals" are usually two in number (Fig. 203, C), but may be as many

as five or reduced to one. They are disposed either on the ventral or the

dorsal surface of the kidneys, generally near their hinder extremities, or

more or less deeply embedded in their substance. Besides these bodies there

are also chromaffin cells in the walls of the anterior cardinal veins.[408]

Histologically, the paired segmentally arranged bodies of Elasmobranchs

differ considerably in structure from the inter-renal bodies, the former

resembling the "medulla," while the inter-renals, as well as the so-called

supra-renals of _Acipenser_, exhibit a striking resemblance to the alveolar

"cortical" substance of the Mammalian supra-renals.[409] In Cyclostomes the

cortex is apparently represented by the lobules of cells along the

posterior cardinal veins and the medulla by the "chromaffin" cells, while

in Teleosts the cortex and the medulla have their respective counterparts

in the supra-renals and the "chromaffin" cells in the walls of the anterior

cardinal veins. It may be concluded, therefore, that Elasmobranchs,

Cyclostomes, and Teleosts possess anatomically distinct representatives of

both the "medulla" and "cortex" of Mammalia, although the Sturgeon is at

present only known to possess the equivalent of the "cortex." In Amphibia,

Reptilia, and Aves both "cortex" and "medulla" are present, and in the

varying intimacy of their relations offer a transition to the Mammalian

arrangement of a central medulla closely invested by a sheath of cortical

substance. A more or less intimate connexion exists between the paired

supra-renals of Elasmobranchs and the sympathetic nervous {348}system. The

former are usually well supplied with sympathetic nerve fibres, and contain

ganglion-cells in their substance.

The primitive origin of these organs is very obscure, and as regards their

development there is much diversity of opinion. It seems certain, however,

that the cortex and medulla of the higher Vertebrates, including their

equivalents in the Elasmobranchs, have independent origins, and the balance

of opinion seems to point to the derivation of the cortex from some portion

of the germinal coelomic epithelium, while the medulla is derived from the

embryonic nerve cells of the sympathetic ganglia.

LYMPHOID TISSUE.—In addition to certain of the ductless glands, and the

local or diffused masses of their characteristic tissue already mentioned

in connexion with the alimentary canal, lymphoid tissue is often abundantly

present in other parts of the body. There is, for example, a mass of this

tissue on the heart of the Sturgeon (_Acipenser_). The anterior enlarged

portion of the mesonephros, commonly termed the "head-kidney" of the

Teleostomi (Fig. 203, B, C), is almost entirely composed of lymphoid

tissue,[410] which has replaced, wholly or partially, the proper renal

structure; and from the presence of free red blood-corpuscles and of

crystals of oxy-haemoglobin and other derivatives of haemoglobin, it may be

inferred that the "head-kidney," in common with the more orthodox

blood-glands, performs a blood-destroying function.[411] On the other hand,

the example of the spleen, which is alike the seat of leucocyte-formation

and of blood-destruction, renders it unnecessary to reject the view that

the "head-kidney" is an organ in which leucocytes or blood-corpuscles are

formed. In but few Teleostomi is a purely lymphoid "head-kidney" entirely

wanting, as, for example, in the Sun-Fish (_Orthagoriscus mola_).[412] As

previously mentioned the Dipnoi are remarkable for the extraordinary

development of lymphoid tissue, inasmuch as it forms a thick investing mass

round the kidneys and gonads in addition to its exceptional abundance in

the walls of the alimentary canal.

The absence of ordinary lymphatic glands in Fishes is well known, and it is

at least probable that, functionally, the want of these lymphoid organs may

be compensated for by the superabundance of lymphoid tissue in other parts

of the body.[413]

{349}CHAPTER XIII

MUSCULAR SYSTEM—LOCOMOTION—SOUND-PRODUCING ORGANS—ELECTRIC ORGANS

MUSCULAR SYSTEM.—The various muscles of the body may be arranged in two

systems: (i.) the _somatic_ or _parietal_, composed of striated or

voluntary muscle-fibres; and (ii.) the _splanchnic_ or _visceral_,

consisting for the most part of unstriated or involuntary fibres. Somatic

muscles form the great lateral longitudinal muscles of the trunk and tail,

which retain the primitive embryonic metamerism to a greater extent in

Fishes than in any other Vertebrates, and are the principal muscles

associated with locomotion. The lateral muscles are composed of a series of

transverse muscle-segments or myotomes, which are >-shaped, or S-shaped,

or they even take a zigzag course from above downward. The myotomes are

disposed in pairs, and they are separated from one another by fibrous septa

or myocommata. Each myotome is divided into a dorsal or epiaxial portion,

and a ventral or hypaxial portion, by a longitudinal, horizontal, fibrous

septum extending outwards from the vertebral centra to the skin. The

muscles of the pectoral and pelvic fins are derivatives from more or fewer

of the adjacent myotomes. The splanchnic muscles include the musculature of

the walls of the alimentary canal, as well as those specialised portions of

the visceral system which are represented by the muscles of the branchial

arches and the jaws, and are composed of striated fibres.

LOCOMOTION.—A Fish and a Bird are equally remarkable for the many and

various ways in which they are adapted for locomotion in the particular

medium in which they live. In its shape the Fish is admirably adapted for

cleaving the water. Spindle-like in shape, but thicker in front than

behind, a Fish resembles {350}a double wedge, the thick part of which is

represented by the head and one of the thin edges by the free hinder margin

of the caudal fin. The body is bounded by smooth flowing contour lines,

unbroken by any sharp separation of the body regions from one another, and

with no points of resistance to its forward motion through the water. The

body being thicker in front than behind, and, as seen in transverse

section, broader above than below, it follows that its centre of gravity

will be nearer the head than the tail, and nearer the dorsal than the

ventral surface. The dorsal position of the centre of gravity necessarily

renders the equilibrium of the body unstable, and were it not for the

balancing action of the paired fins the Fish would float belly upwards, as

is always the case after death. Most Fishes are provided with a membranous

gas-containing sac, the air-bladder, the principal function of which is to

render the Fish, bulk for bulk, of the same weight as the water, so that in

this position of equilibrium, or plane of least effort, the animal can

execute its various locomotor movements with a minimum expenditure of

muscular effort—an advantage which no other animal possesses.[414] To give

stability to the body, and to steady its course when swimming, the Fish has

a dorsal and a ventral keel, formed by the anal and dorsal fins, which,

like the sliding keel of a yacht, can be raised or lowered as occasion

requires. When these fins are removed the course of the Fish becomes

zigzag, and the animal wobbles.

The organs more directly concerned with swimming are the tail and the

caudal fin, and the pectoral and pelvic fins, but the relative share which

these structures take in the actual propulsion of the Fish differs greatly.

The principal organ of locomotion in the typical Fish is the powerful

muscular tail, which, in swimming, is lashed from side to side by the

alternating contraction of the great longitudinal muscles on opposite sides

of the vertebral column.[415] In such movements the tail is first flexed or

bent, say to the right side: this stroke has been termed the non-effective

or back stroke. By a stroke in the reverse direction the tail is then

extended and straightened, that is to say, the Fish makes the forward or

effective stroke. By a rapid succession of such strokes to the right and

left sides alternately the Fish is {351}forced through the water. It is

obvious, however, that the extension or effective stroke must have a

considerable surplus of power over the flexion or non-effective stroke, and

how this result is achieved will now be briefly considered. Experiment, and

the observation of Fishes like the Sturgeon, which habitually move with

sufficient slowness to allow the phases of their swimming movements to be

followed without much difficulty, show that in swimming a Fish throws its

body into two opposite and complementary curves, a cephalic curve formed by

the anterior half of the body and a caudal curve by the tail. The double

curve enables the Fish always to present a convex, less resisting or

non-biting surface to the water during the flexion of the tail to the right

or left as the case may be, and a concave or biting surface during

extension, that is when the tail is straightening itself during the

effective stroke.

[Illustration: FIG. 204.—To illustrate the mode in which the tail of an

ordinary Fish is used in swimming. See the text for the lettering.

(Slightly altered from Pettigrew.)]

Fig. 204, which represents a Fish in two successive positions while

swimming, will serve to illustrate these conclusions. A Fish in the

position A has its body thrown into a cephalic concavity directed to the

right and a caudal concave surface facing the left. The tail is bent to the

right of the line _a b_, which corresponds to the axis of the Fish when at

rest and to the course pursued by the animal when swimming, and is in the

position which it assumes during a flexion stroke, with its convex

non-biting surface directed outwards and its concave biting surface

inwards. The tail is now ready for an extension stroke, and while this is

in progress it is clear that the concave biting surface of the tail will

meet the water, while at the conclusion of the stroke the tail will be in a

line with _a b_. At the same time the cephalic curve has so far diminished

that the long axis of the body for a momentary period will also coincide

with _a b_, and the Fish is free to advance without impediment. The tail,

{352}however, continues its movement to the left, but now as a flexion

stroke, and assumes the curvature and position indicated in B, with a

reversal in the direction of both the cephalic and caudal curves, but in

the meantime the force of the preceding extension stroke has forced the

Fish along the line _a b_ to the new position indicated by B. By a rapid

succession of alternating flexions and extensions, during which the tail

describes figure-of-8 curves, the Fish travels in an undulating forward

course with a maximum of propelling power and a minimum of "slip." In

short, the action of the tail precisely resembles the action of the

stern-oar in the operation of sculling a boat.

There are also other considerations which add to the surplus power of the

extension stroke by lessening the resistance of the water to the flexion or

non-effective stroke. During the flexion stroke the tail fin is less

expanded and its area diminished, and by the rotation of the Fish on its

long axis the surface of the tail strikes the water obliquely, and further,

the tail moves with less rapidity. On the contrary, when the extension

stroke is made these conditions are reversed. The caudal fin is expanded,

the stroke is more rapid, and by the reverse rotation of the Fish the tail

now strikes the water with its flat surface. In other words, the action of

the tail during the two strokes may be compared to the "feathering" of an

oar in rowing. Nor is this all. A Fish in motion through the water produces

a suction current behind it. The current offers but little resistance to

the flexion stroke, inasmuch as the direction of the two coincide, but

during the extension stroke the tail meets the full force of the current,

and consequently its grip and propelling power are greatly enhanced. There

is a striking analogy between the movements of a Fish's tail in swimming

and the action of the screw of a steamer, but as a propelling organ the

former is far superior to the latter. As we have seen, the tail of a living

Fish can so adjust its shape and surface that it alternately eludes and

grips the water in accordance with the needs of particular strokes.

The curves into which the body of a Fish is thrown when swimming are never

less than two, but in long-bodied Fishes, such as the Eels, the number may

be increased, and in every case the curves occur in pairs and are

complementary to one another.

{353}Many Fishes can jump out of the water, either in pursuit of insect

food, like the Trout, or to enable them to escape the pursuit of their

foes, like the Flying-Fish (_Exocoetus_), by means of a single forcible

stroke of the tail, when the Fish is in a nearly vertical position close to

the surface of the water. It is thus that the Salmon executes its

remarkable leaps over weirs or up salmon-ladders when ascending rivers for

spawning.

The tail is also used for steering. If kept bent to one side when the Fish

is moving the tail acts like a rudder, and the course of the Fish is

deflected to that side; or the direction may be altered by single strokes

of the tail to the right or left, according to the course which the Fish

desires to pursue.

In the majority of Fishes the paired fins are probably of little use for

propulsion, and their action in this as in other functions is not always

clear. In the Sharks and Dog-Fishes as well as in some Teleosts their

planes are nearly horizontal when the fins are extended from the body; in

others they are more oblique, so that the surfaces of the fins look upwards

and backwards, and downwards and forwards; and in others again their

surfaces are so nearly vertical that their strokes will be backwards and

forwards. The pectoral fins also vary in their position on the sides of the

body, being much more dorsal in some Fishes than in others. The paired fins

may act as lateral keels in steadying the course of the Fish especially

when the fins are extended and their planes are horizontal. They certainly

seem to act as balancers in keeping the Fish on an even keel, and in

counteracting the tendency of the Fish to turn belly upwards—a result which

is attained by a slight upward and downward movement of the fins, and

particularly of the pectoral fins. A Fish deprived of its pectoral members

sinks downwards at the head and assumes an oblique position in the water.

Removal of both the pectoral and pelvic fins of one side causes the Fish to

roll over to that side; and if the fins are removed from both sides the

animal turns belly upwards like a dead Fish. The pectoral fins may also be

used for steering: a backward stroke of one fin while the other is kept

folded back against the body will wheel the Fish round to the opposite

side. From the ventral position of its mouth a Shark is forced to turn over

to one side in order to seize its prey, and this movement of rotation is

probably produced by the down strokes of the pectoral fin of one side. In

{354}some Fishes it would seem that the pectoral fins may assist locomotion

by acting as paddles. The 15-spined Stickleback (_Gastrosteus spinosus_)

frequently progresses by their aid alone; and, as their action can be

reversed at pleasure, it is not unusual to see this Fish move backwards.

The fins appear to be rotated or twisted in spiral movements like the tail

when used for swimming, or like the wings of Insects in flying.

It has been mentioned that the function of the median fins (dorsal and

anal) is to give stability to the Fish by acting as dorsal and ventral

keels. This is certainly the case in the generality of Fishes.

Nevertheless, there are exceptional instances in which one, or even both,

of these fins are important swimming organs, acting either as a substitute

for a tail which has become adapted for other uses, or as supplementary to

that organ. Thus, in some of the Syngnathidae (Pipe-Fishes and Sea-Horses)

the small size or absence of the caudal fin, and its use as a prehensile

organ, renders the tail of little or no value as a propelling organ: hence

it is that these Fishes swim by a lateral undulating movement of the dorsal

fin. To enable them to do this the supporting skeleton presents certain

interesting modifications. In the majority of Teleosts the arrangement of

the fin-muscles, and the nature of the articulation between the dermal

fin-rays and their basal radial supports, which is generally some form of a

hinge-joint, are such as to limit the motion of the rays to simple

elevation or depression in the vertical plane, and no lateral motion of the

fin is possible. But in the Syngnathidae, as in the Pipe-Fish

(_Siphonostoma typhle_), there is an exceptionally mobile articulation

between the dermal fin-rays and the distal radial nodules which their cleft

bases embrace and the bony proximal or basal radials, so that the fin can

be flexed or bent to the right or to the left. In addition to this, by a

change in the insertion of their tendons, the muscles corresponding to the

ordinary elevator and depressor muscles of the fin-rays in other Fishes are

capable of producing extensive lateral movements of the fin, or, by

contracting in orderly sequence, of bringing about the characteristic

undulating motion of the fin. A similar mechanism exists in many

Plectognathi (_e.g._ species of _Balistes_, _Monacanthus_, _Diodon_,

_Tetrodon_ and _Orthagoriscus_)[416] in connexion with both the dorsal and

anal fins, but in these Fishes the {355}action of the median fins in

swimming must be regarded as supplementary to that of the tail.

Swimming is by no means the only form of locomotion in vogue amongst

Fishes. A few, like the Angler-Fishes (_Lophius_), habitually use the

pectoral fins for crawling about the sea-bottom. The East Indian Goby,

_Periophthalmus_, uses its pectoral fins, which are bent at an angle like

an elbow-joint, for hopping over sandy flats left bare by the retreating

tide. The Flying-Fish (_Exocoetus_), when projected from the water by a

stroke of its powerful tail, expands its large pectoral fins, and, using

them after the fashion of a parachute, floats through the air for

considerable distances before returning to its natural medium. The "Flying

Gurnards" (_Dactylopterus_) are also capable of short aerial excursions in

a similar fashion. Nor is tree-climbing beyond the province of a Fish, if

credit be given to the assertion that the Indian "Climbing-Perch" (_Anabas

scandens_) uses its opercular spines for ascending trees. Many freshwater

Fishes are known to migrate across land from one pool or river to another,

usually during the night. Eels do so by a serpentine or wriggling motion of

their long bodies, but in others the pectoral fins seem to be the principal

organs used for the purpose, aided, it may be, by a perverted use of the

tail.

SOUND-PRODUCING ORGANS.—Contrary to popular belief sound-producing or vocal

organs are by no means uncommon in Fishes, especially in certain families

of Teleosts. It is not always easy, however, to discriminate between

involuntary, abnormal, or accidental sounds, and those due to the action of

special vocal organs. There are, moreover, some Fishes which observations

have shown to utter highly characteristic sounds, although the precise

nature of the sound-producing mechanism is at present unknown; while other

Fishes appear to possess organs which, on anatomical grounds, are perhaps

vocal in function, although nothing is known of the nature of the sounds

they emit. Here those organs only will be considered which, either with

certainty or with some degree of probability, may be regarded as vocal

structures. For most of our knowledge of these interesting structures we

are indebted to the researches of Sörensen and Dufossé.[417]

{356}[Illustration: FIG. 205.—Stridulating apparatus of _Callomystax

gagata_. _is^1_, The first interspinous bone, the lower part of which forms

the double file and fits into the interval between the cleft neural spines

_ns^4_ and _ns^5_; _is^2_, _is^3_, second and third interspinous bones;

_ns^3_, _ns^4_, _ns^5_, neural spines of the third, fourth, and fifth

vertebrae; _s^1_, _s^2_, spine-like rays of the dorsal fin; _so_,

supra-occipital. (After Haddon.)]

(_a_) _Stridulation_.—Stridulation as a method of sound-production has been

recorded in many Teleosts, and one of the most interesting examples occurs

in the singular Indian Siluroid, (_Callomystax gagata_).[418] In this Fish

(Fig. 205) the first five vertebrae are rigidly connected with one another

and with the skull, mainly through the union of the neural spines of the

third, fourth, and fifth vertebrae, and their articulation with the

supra-occipital bone. The united spines together form a high,

laterally-compressed lamina of bone, the hinder portion of which is

vertically cleft into two thin plates separated by an interval sufficiently

wide to receive the first interspinous bone of the dorsal fin. The inner

surface of each of the two plates is traversed by a series of about thirty

parallel, close-set, vertical ridges, while the first interspinous bone is

similarly ridged on both its faces like a double file. Lastly, it may be

mentioned that owing to the width of the intervertebral ligament between

them the fifth and sixth vertebral centra are articulated by a joint of

unusual mobility. The action of the mechanism is simple. By the vertical

movements of the sixth and succeeding trunk vertebrae, with the

interspinous bones which they support, on the rigid structure formed by the

head and first five vertebrae, the file-like first interspinous bone moves

backwards and forwards, and, by scraping against the ridges on the inner

surfaces of the cleft neural spines, gives rise to a harsh grating noise,

which is particularly unpleasant when artificially produced. The lateral

movements of the trunk in ordinary locomotion do not affect the mechanism:

it is only when the trunk is alternately flexed and extended in the

vertical plane that the mechanism comes into play and a noise is

{357}produced. In the Bull-head (_Cottus scorpius_) the preoperculum is

modified for stridulation, and in _Dactylopterus_ the hyomandibular bone;

in other Fishes, as in some Siluroids (_e.g._ species of _Doras_),

stridulation takes place between a basal process from the great spine of

the pectoral fin and the wall of a socket in the cleithrum into which the

process is received, or between the small first spine of the dorsal fin and

a roof-like process at the upper extremity of the first interspinous bone;

also, in a somewhat similar fashion in the anterior dorsal fin of such

widely different Fishes as certain Trigger-Fishes (Sclerodermi) pertaining

to the genera _Balistes_, _Monacanthus_, and _Triacanthus_, _Acanthurus

chirurgus_ (Acanthuridae), the Boar-Fish (_Capros aper_), _Centriscus

scolopax_ (Centriscidae), and the Three-spined Stickleback (_Gastrosteus

aculeatus_); and even between the spinose ray of the pelvic fin and the

basipterygium in _Triacanthus_, _Capros_, and _Gastrosteus_.

In the "Drumming" Trigger-Fish (_Balistes aculeatus_),[419] which frequents

the coral-reefs off the Island of Mauritius, stridulation takes place

between the postclavicles and a longitudinally grooved area on the inner

surface of each cleithrum. Both the cleithra and postclavicles are in

intimate relation with the air-bladder, and the sound produced by friction

is apparently strengthened by the transference of the vibrations to the

walls and gaseous contents of that organ. The passage of the

sound-vibrations to the surrounding medium is facilitated by the fact that

for a portion of their extent the lateral walls of the air-bladder are in

contact with the superficial skin, which visibly shares in the vibratory

movement of the bladder when the characteristic drumming sounds of

_Balistes_ are being emitted.

Stridulating sounds may also be produced by the friction of the upper and

lower pharyngeal teeth, as in a species of Mackerel (_Scomber brachyurus_).

By the grating of its teeth the Sun-Fish (_Orthagoriscus mola_) is said to

emit sounds similar to those produced by the grinding of the teeth in Pigs

and Ruminants; and Moseley[420] has remarked of a species of _Balistes_

that the "living Fish when held in the hand makes a curious metallic

clicking noise by grating its teeth."

(_b_) _Breathing sounds_.—Characteristic breathing or murmuring sounds, or

"bruits de souffle" as Dufossé terms them, are {358}produced by a few

Teleosts, among which may be mentioned the Eels, certain Cyprinidae, as,

for example, the Carp (_Cyprinus carpio_), several species of Loaches (e.g.

_Misgurnus fossilis_ and _Cobitis taenia_), and the European Siluroid,

_Silurus glanis_. According to Dufossé these sounds originate in some cases

from the expulsion of gas from the air-bladder through the ductus

pneumaticus and mouth, and in others, as in _Misgurnus fossilis_, they are

produced by the rapid ejection through the anus of bubbles of air

previously taken in at the mouth.

(_c_) _Sounds produced through the agency of muscles connected with the

air-bladder_.—In addition to its usual function as a hydrostatic organ or

"float" the air-bladder is often modified in various ways in different

Teleosts, and adapted for use as a sound-producing organ.

[Illustration: FIG. 206.—The air-bladder and elastic-spring-mechanism in

_Auchenipterus nodosus_. A, Cavity of the bladder exposed by the removal of

its ventral wall: _a.c_, anterior chamber; _cl_, clavicle; _c.tr_,

crescentic process of the tripus; _l.c_, left lateral chamber; _l.s_,

longitudinal septum separating the two lateral chambers; _oes_, oesophagus;

_p.s_, pectoral spine; _t.s_, the narrow transverse septum which partially

separates the anterior from the two lateral chambers. B, Ventral view of

the anterior vertebrae, to show the elastic springs: _es_, the oval bony

plates in which the elastic springs terminate; _r^1_, first rib; _t.p^5_,

transverse process of the fifth vertebra; _v^1_, first vertebral centrum;

_cl_, _oes_, and _ps_, as in A. (From Bridge and Haddon.)]

In the South American Siluroid, _Auchenipterus nodosus_, the transverse

processes of the fourth vertebra are bent downwards and backwards, and at

the same time become converted into flexible and highly elastic springs

(Fig. 206, B). Their distal extremities expand into oval bony plates which

are imbedded in the anterior wall of the air-bladder, and often cause the

latter to bulge inwards (Fig. 206, A). From the occipital region of the

skull arise two powerful muscles which pass backwards to {359}their

insertion into the anterior faces of the two springs. By the contraction of

these muscles the springs, and consequently also the front wall of the

bladder, are drawn forwards; but directly the muscles relax, the elasticity

of the springs causes them to move backwards to their former position,

carrying with them the wall of the air-bladder. Hence it follows that the

rapid alternating contraction and relaxation of the muscles will impart a

vibratory movement to the anterior wall of the bladder and to the gaseous

contents of that organ, with the result that a sound is produced. As a

rule, those Fishes in which an elastic-spring-mechanism is present have the

air-bladder subdivided by internal septa into a series of chambers freely

communicating with one another; and no doubt the intensity of the sound is

greatly increased by the vibratory movements of the gases across the free

edges of the septa, and from one chamber to another. The elastic-spring

type of vocal organ is apparently restricted to the Siluridae,[421] and

besides occurring in _Auchenipterus_ is found also in the South American

genera _Doras_, _Oxydoras_, _Rhinodoras_, and _Euanemus_; in the African

genera _Synodontis_ and _Malopterurus_; and in at least four species of the

Indian genus _Pangasius_.[422] There are also a few Teleosts in which the

air-bladder is provided with special muscles, but, instead of being

connected with elastic springs, the muscles extend from the skull, and are

inserted directly into the wall of the bladder (Fig. 207); or, without

being in any way attached to the skeleton, the muscles simply invest some

portion of the surface of the air-bladder. In other Fishes the air-bladder,

without possessing special muscles of its own, may, nevertheless, be

partially invested by tendinous, or partly muscular and partly tendinous,

extensions from the muscles of the body-wall (Fig. 208), or may be

intimately related to certain muscles connected with the pectoral girdle.

{360}[Illustration: FIG. 207.—Ventral view of the air-bladder and its

extrinsic muscle in _Platystoma_. _a.b_, Air-bladder; _a.l.c_, left

antero-lateral caecum of the bladder; _b.o_, basioccipital; _b.w_,

body-wall in contact with the lateral wall of the bladder; _c^1_, centrum

of the first vertebra; _cl_, clavicle; _d.p_, ductus pneumaticus; _m^1_ and

_m^2_, extrinsic muscles of the bladder; _pt.i_, post-temporal. (From

Bridge and Haddon.)]

[Illustration: FIG. 208.—Air-bladder and its muscles in _Micropogon

undulatus_. _a.b_, Air-bladder; _l.b_, right lateral caecum; _m_, _m_,

musculo-tendinous extensions from the muscles of the body-wall, which

partially invest the surface of the air-bladder. (From Sörensen.)]

Whatever the precise relation of the air-bladder to its muscles it is

probable that the physiological effect is in most cases the same. By the

rapid alternating contraction and relaxation of the muscles, some part of

the wall of the bladder becomes alternately compressed and relaxed in such

a way as to initiate a series of vibratory movements in the gases of that

organ, and so produce definite sounds. In not a few of the Fishes the

cavity of the bladder is subdivided by external constrictions or by

internal septa, or is complicated by the development of lateral, tubular,

caecal branches; and hence the vibratory movements of the gases will be

greatly strengthened by their passage across the edges of the septa, or the

apertures of the caeca, and the intensity of the resultant sounds also

increased. It will be readily understood that the nature and quality of the

sounds emitted by different Fishes will necessarily vary with the shape of

the air-bladder, the number and arrangement of the internal septa and the

caeca, and the strength and disposition of the contracting muscles. In a

few Teleosts (Triglidae and Zeidae) sounds are said to be produced by the

rapid vibration of an annular, or centrally-perforated, muscular diaphragm,

which stretches across the cavity of the air-bladder.[423] Nevertheless, it

must be strongly emphasised that, while in some Fishes the air-bladder and

its muscles {361}undoubtedly constitute a vocal organ, there are many

others in which the bladder can only be inferred to be sound-producing from

its general agreement in anatomical structure with the same organ in Fishes

where its vocal function has been clearly proved.

By one or other of these various methods the air-bladder is either known to

be sound-producing, or is believed with good reason to be such, in the

following Teleosts,[424] and many others:—Certain species of the

South-American genera of Siluridae, _Pimelodus_, _Sorubim_, _Platystoma_,

_Piratinga_, _Centromochlus_, and _Trachelyopterus_; species of the

South-American family Characinidae; _Amblyopsis spelaea_, the blind Fish

from the Mammoth Cave of Kentucky (Amblyopsidae); among the Syngnathidae,

the short-snouted Sea-Horse (_Hippocampus brevirostris_) of the British

Coasts; certain Sclerodermi, such as the Trigger-Fishes, _Batistes vetula_,

_Triacanthus brevirostris_, _T. biaculeatus_, and _Monacanthus pardalis_,

and also some "Coffer Fishes" (_e.g._ species of _Ostracion_); some

Gymnodontes (species of _Diodon_ and _Tetrodon_); a few Serranidae (_e.g._

species of _Therapon_ and _Pristipoma_); species of _Holacanthus_

(Chaetodontidae) and in _Holocentrum sogho_ (Berycidae); such Sciaenidae as

the "Drum" (_Pogonias chromis_), the "Maigre" (_Sciaena aquila_), which has

sometimes been taken in British waters, _Umbrina cirrhosa_, _Otolithus

regalis_, and _Micropogon undulatus_, and, with more or less probability,

many other species of the same family; one species of Zeidae, the John Dory

(_Zeus faber_); _Batrachus tau_ among the Batrachidae; several species of

Gurnards (Triglidae) belonging to the genera _Prionotus_ and _Trigla_; the

so-called Flying Gurnard, _Dactylopterus volitans_ (Dactylopteridae); the

Indian species _Ophiocephalus marulius_ and _O. gachua_ (Ophiocephalidae);

amongst the Gadidae, the Cod (_Gadus morrhua_) and the Haddock (_G.

aeglefinus_); in such Zoarcidae as the blind Fish (_Lucifuga subterranea_)

from the subterranean waters of the caves of Cuba, and also in some

Ophidiidae (_e.g._ species of _Ophidium_).

In Fishes other than Teleosts, instances of normal sound-production by

special vocal structures are rare. No recorded instances are known in the

Cyclostomes or the Elasmobranchs,[425] {362}but there is evidence that

sounds are emitted by _Polypterus_ among the Crossopterygii, and by the

Dipnoids _Neoceratodus_,[426] _Protopterus_, and _Lepidosiren_, although it

is not certainly known how they are produced, or that they may not be the

accidental concomitants of the inspiratory or expiratory action of the

lungs in breathing.

As to the nature of the sounds produced by the air-bladder and its muscles

in different Teleosts, a few examples may be given.

The sound produced by the elastic-spring-apparatus of a recently caught

_Doras maculatus_, has been described as a "deep growling tone," which may

be distinctly heard at a distance of 100 feet when the Fish is out of the

water. Under like conditions the air-bladder and its muscles, in a species

of _Platystoma_, emit a similar sound. On the other hand, the sound

produced by the elastic springs of the Electric Siluroid (_Malopterurus

electricus_) has been compared to the hissing of a cat. The Sea-Horse

(_Hippocampus brevirostris_) utters a monotonous sound analogous to that of

a tambour, which is characteristic of both sexes, but is more intense and

frequent in the breeding season. The "Coffer Fish" (_Ostracion trigonus_)

emits a growling sound, as also does the "Globe Fish" (_Tetrodon

honckenii_) when taken out of the water.[427] The air-bladder and its

muscles in the "Drum" (_Pogonias chromis_), constitute the most powerful

sound-producing organ yet found in any Fish. The sounds emitted by the

"Drum" are better expressed by the word drumming than by any other, and

have frequently been heard by persons in vessels lying at anchor on the

coasts of the United States, where these Fishes abound.[428] The "Drum"

begins its drumming noise in the spawning season in April, but is rarely

heard afterwards. The "Maigre" (_Sciaena aquila_), whose musical

performances are perhaps responsible for the Homeric fable of the song of

the Sirens, is remarkable among Fishes for the variety of its sounds, which

have been compared to bellowing, purring, buzzing, and whistling.[429] The

sound is often so intense that it may be heard when the Fish is at a depth

of 18 metres, and the {363}ear of the observer two metres above the water;

and it has been recorded that by listening for these sounds, shoals of

Maigres have been successfully netted. They rarely emit sounds when

isolated; but in shoals, during the breeding season, they do not cease to

make sounds with a vigour and a persistency which apparently must soon wear

out their strength. One of the Indian Horse-Mackerels (_Caranx hippos_)

grunts like a young Pig when captured, and the sound is repeated whenever

it is moved, as long as vitality remains. A West Indian species of the same

family (_Argyriosus vomer_) has been observed to produce a like sound,

while an Egyptian _Caranx_ (_C. rhonchus_) is known to the Arabs as the

"Chakoura" or "Snorter."[430] The sounds produced by the different British

Gurnards, such as the Grey Gurnard (_Trigla gurnardus_), the Piper (_T.

lyra_), the Elleck or Cuckoo Gurnard (_T. cuculus_), and the Tub-Fish (_T.

hirundo_), have been compared to snoring, a sonorous and prolonged

grunting, crooning (whence, perhaps, the term "crooner," by which the Grey

Gurnard is known in Ireland), and croaking. The John Dory (_Zeus

faber_)[431] also utters sounds analogous to those of the Gurnards. Among

the Dipnoi _Lepidosiren_ is said to make a growling sound, and

_Neoceratodus_ a grunting noise which may be heard at night for some

distance.

Whatever the nature of the vocal mechanism, it is highly probable that the

sounds produced by Fishes travel to considerable distances in the water,

inasmuch as the latter medium is a far better conductor of sound than air,

and, moreover, the transmission of sound-vibrations from the air-bladder to

the water is facilitated in many Fishes by the fact that, for a portion of

its extent on each side the bladder is in direct contact with the

superficial skin behind the pectoral girdle.

From the by no means exhaustive list of examples given above, it is obvious

that in some form or other vocal organs are present in a considerable

number of Fishes, both freshwater and marine, belonging to widely different

groups; and further, that even in the same species (e.g. _Doras maculatus_

and other Siluridae), both stridulation and the action of extrinsic muscles

on the air-bladder may be utilised as a means of sound-production. Certain

Teleostean families like the Siluridae, the Sciaenidae, and the Triglidae,

seem to be distinguished above all others by the {364}prevalence of some

form of vocal organ. According to Sörensen, the first mentioned of the

three families includes no less than 68 species, which utilise the

air-bladder alone as a sound-producing organ. Nevertheless, there still

remain many Teleostean families, rich in genera and species, and with an

almost world-wide geographical distribution, in which such organs have not

yet been found.

The advantages which Fishes derive from the possession of sound-producing

organs are sufficiently obvious.

A characteristic feature in the reproduction of most Fishes is the general

absence of any process of conjugation between the sexes, the eggs being

fertilised in the water after their extrusion from the body of the female,

and, consequently, any device which will facilitate the formation of shoals

during the breeding season must be of great advantage to the species by

largely increasing the chances that the ova will be fertilised, and thus

secure the more successful propagation of the race. Hence it may be

concluded that the vocal organs of Fishes are a means to this end, and that

the sounds they produce are in fact recognition-sounds which enable Fishes

of the same species to congregate together at periods when reproductive

activity is greatest. This view is in harmony with much that is known of

the habits of these Fishes, especially with the fact that particular sounds

are often characteristic of particular species, and that the sounds are

produced most frequently and with greater intensity during the breeding

season than at any other time. While useful to all Fishes that possess

them, vocal organs are, no doubt, specially serviceable to those Fishes

which, from the nature of their habitat, can make but little use of their

eyes; and this fact may perhaps explain the prevalence of such organs in

the Siluridae, which are frequently bottom- or ground-feeding Fishes, and

often live in muddy waters.

The sounds emitted by Fishes may also, in some instances at least, be

warning sounds. Many of the sound-producing Fishes are provided with

exceptionally strong spines either in connexion with the median and paired

fins, as in many Siluridae, or on the general surface of the body, as in

_Diodon hystrix_. Such spines are very effective weapons for offensive or

defensive purposes, and are capable of inflicting very severe wounds. The

natural enemies of these Fishes learn by experience or instinct to

{365}associate particular sounds with the possession of dangerous spines,

and warned by the sounds, they refrain from attacking the owner of the

spines, to the mutual advantage of both.

[Illustration: FIG. 209.—An Electric Ray (_Torpedo_) dissected to show its

electric organs. On the left the nerves supplying the organ are dissected

out. The prismatic areas on the surface of the organ indicate the vertical

columns of electric plates, of which there may be 500,000 in each organ.

The dorsal surface of the brain is exposed. _br_, Gills; _f_, spiracle;

_o_, eye; _o.e_, electric organs; _t_, mucus canals; _tr_, tri-geminal

nerve; _tr′_, its electric branch; _v_, vagus; _I_, fore-brain; _II_,

mid-brain; _III_, cerebellum; _IV_, electric lobe of the medulla oblongata.

(From Parker and Haswell, after Gegenbaur.)]

ELECTRIC ORGANS.—Electric organs capable of generating more or less

powerful electric discharges are present in certain Fishes, both marine and

freshwater. They occur in a few Elasmobranchs (species of _Raia_,

_Torpedo_, and _Hypnos_), in such Teleosts as the African Silurid

_Malopterurus_ the "Electric Eel" (_Gymnotus_), and in species of

Mormyridae (e.g. _Mormyrus_). With one exception electric organs are

composed of metamorphosed muscular fibres, and their nerve-endings or motor

end-plates. The species of _Raia_ have two small electric organs, one on

each side of the terminal portion of the tail.[432] In _Gymnotus_[433] the

{366}organs are much larger, and extend the whole length of the tail, which

is fully four-fifths of the total length of the Fish. The Mormyridae also

have their feeble electric organs in the caudal region. In all these Fishes

the electric organs are modified portions of the caudal muscles. In the

Torpedo, however, these organs are two large oval masses, one on each side

of the head, between the gills and the cephalic prolongation of the

pectoral fin (Fig. 209). _Malopterurus_[434] is exceptional in possessing

an electric organ derived from the epidermis and not from the muscular

system. In this Fish the organ envelops nearly the whole body like a

mantle, between the skin and the subjacent muscles of the trunk and tail.

An electric organ is composed of an immense number of "electric plates"

(modified motor end-plates), abundantly supplied with nerves on one of

their surfaces, and disposed in a series of vertical (_Torpedo_) or

longitudinal (_Gymnotus_) columns, separated by septa of connective tissue.

In the active state of the organ in the Torpedo[435] the ventral surfaces

of the plates, on which the nerves are distributed, become negative to the

dorsal, and "the effect in all the plates of a column when summed up is,

therefore, such that the dorsal end of a column becomes positive to the

ventral end."[436] Hence the current in the form of a succession of shocks

passes from the ventral to the dorsal surface of the head. In _Gymnotus_,

where the columns are longitudinally arranged, it is the anterior and

posterior surfaces which become oppositely electrified, and the current

passes from the tail to the head. The shock imparted by an electric

discharge is most powerful in _Gymnotus_,[437] _Malopterurus_, and

_Torpedo_, in the order named, and relatively weak in the remaining genera.

The strength of the shock increases with the number of electric plates

included in the circuit. Thus in _Gymnotus_ the maximum shock is given when

the body of the Fish is so curved that the head and the tail are in contact

with different points on the surface of some other Fish. The discharge may

be reflex or voluntary. Repeated discharges induce fatigue and weaken the

shocks. Electric organs are powerful offensive or defensive structures,

enabling the Fish to repel the attacks of enemies, or to stun or kill their

prey.

{367}CHAPTER XIV

NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE

The nervous system consists of the brain and the spinal cord, and of the

cranial and spinal nerves. The rudiment of the future brain and spinal cord

first appears in the embryos of some Cyclostomes (e.g. _Bdellostoma_), of

Elasmobranchs, and of Chondrostei (e.g. _Acipenser_), and of _Neoceratodus_

among the Dipnoi, in the form of a tubular medullary canal pinched off from

the epiblast of the dorsal surface of the body. By a somewhat different

method, but with the same final result, a medullary canal is formed in

other Cyclostomes (e.g. _Petromyzon_), in the Holostei and Teleostei, and

in _Lepidosiren_,[438] from a solid ingrowing keel of epiblast which

subsequently becomes tubular. Later, the medullary canal in the head

enlarges, and becomes divided by two transverse constrictions into three

vesicles, the primary fore-, mid-, and hind-brain, leaving the rest of the

canal to form the spinal cord.

THE SPINAL CORD.—This portion of the medullary canal retains a simpler and

more uniform cylindrical structure. Its walls thicken and their component

cells become converted into nerve cells and nerve fibres, but a remnant of

the original cavity remains in the adult as a minute axial canal, with a

ciliated epithelial lining, the central canal of the spinal cord or

myelocoele. In most Fishes the spinal cord extends the whole length of the

body, but in some Teleosts, especially in certain Plectognathi, it is

remarkably short. In a Sun-Fish (_Orthagoriscus_), 2½ metres long, and

weighing about a ton and a half, the cord was only 15 mm. in length, or

shorter than the brain.

THE BRAIN.—At an early stage in its embryonic history the {368}brain

consists of three simple vesicles, the _fore_-, the _mid_-, and the

_hind-brain_, the first of which lies in front of the anterior end of the

notochord and is therefore pre-chordal in position. As development proceeds

the walls of the vesicles undergo local thickenings, or they give rise to

hollow paired or median outgrowths, and by one or other of these methods

the different parts of the complex adult brain are evolved, while the

original cavities of the vesicles or of their outgrowths persist as a

continuous system of epithelium-lined spaces or "ventricles."[439] The

fore-brain is remarkable for the number and importance of the parts to

which it gives rise. First, it bulges out in front into a hollow vesicle,

the _prosencephalon_, leaving the rest of the fore-brain as the

_thalamencephalon_ or _diencephalon_ (Fig. 210). The cavity of the

prosencephalon is the _prosocoele_, and a pair of thickenings in its floor

form two basal ganglia or _corpora striata_. In many Fishes the

prosencephalon retains this simple vesicular condition, in which case the

roof or _pallium_ is usually epithelial and non-nervous; but in others two

hollow lobes grow out from it in front and give rise to two _cerebral

hemispheres_ or _parencephala_.[440] Both contain extensions of the

prosocoele, the _paracoeles_ or _lateral ventricles_, from the floor of

which the corpora striata now project. The prolongation of the pallium

forming the roof of the lateral ventricles either remains partially

epithelial, or it may acquire a wholly nervous structure and thicken to an

extent which differs greatly in different Fishes. With the formation of the

hemispheres the prosencephalon and its prosocoele become of secondary

importance, and may cease to be recognisable as distinct from the

thalamencephalon and its ventricle. The lateral ventricles then appear to

communicate directly with the third ventricle by two apertures, the

_foramina of Munro_. The forward growth of the brain is completed by the

development of two hollow lobes, the _olfactory lobes_ or _rhinencephala_,

each of which contains a ventricle or _rhinocoele_ communicating behind

with the prosocoele, or, if hemispheres are present, with the corresponding

lateral ventricle.

{369}[Illustration: FIG. 210.—Diagram of the general structure of the brain

in Craniates. A, vertical longitudinal section; B, dorsal view showing the

brain cavities on the right side. _c_, Cerebellum; _c.c_, central canal of

the spinal cord; _c.h_, cerebral hemispheres; _c.s_, corpus striatum;

_F.B_, fore-brain; _f.m_, foramen of Munro; _H.B_, hind-brain; _in_,

infundibulum; _l.v_, lateral ventricle; _m_, mesocoele; _M.B_, mid-brain;

_m.o_, medulla oblongata; _o.l_, olfactory lobe; _op.l_, optic lobe;

_op.t_, optic thalamus; _p_, paraphysis; _pc_, prosocoele; _pn.o_, pineal

organ; _p.o_, parietal organ; _pr_, prosencephalon; _pt_, pituitary body;

_rh_, rhinocoele; _sp.c_, spinal cord; _s.v_, saccus vasculosus; _th_,

thalamencephalon; iii, iv, third and fourth ventricles. (After Parker and

Haswell.)]

Scarcely less complicated, and perhaps even more interesting from a

morphological standpoint, are the structures arising out of the

thalamencephalon. By thickenings of its lateral walls two large ganglia,

the _optic thalami_, are formed, and on the inner or dorsal aspect of each

of these a _ganglion habenulae_ is developed. From the sides of the

thalamencephalon the primary _optic vesicles_ are derived, which later

become transformed into the retinal parts of the paired eyes and the optic

nerves. Besides the optic vesicles there is a second pair of embryonic

outgrowths which arise from the roof of the thalamencephalon. These

outgrowths form stalked vesicles and represent a pair of degenerate visual

{370}organs. Usually they become so displaced that the left one lies in

front of the right, and they appear as if median. The subsequent fate of

the vesicles differs greatly in different Craniates. Both persist in the

Lamprey, the right vesicle to some extent retaining its primitive visual

function as a _parietal eye_ and directly overlying the left or pineal

vesicle. In Elasmobranchs the two unite to form a glandular organ, the

so-called _pineal body_ of the adult, and in Teleosts the left vesicle

disappears, leaving the right as a pineal body.[441] There is also an

embryonic median outgrowth from the roof of the prosencephalon, the

_paraphysis_, which soon disappears and whose significance is not known. A

median hollow downgrowth from the floor of the thalamencephalon forms the

_infundibulum_, which becomes attached to a caecal diverticulum from the

roof of the mouth. With rare exceptions the diverticulum loses all

connexion with the mouth, and, as the _pituitary body_ or _hypophysis_, it

appears as an appendage to the extremity of the infundibulum. In the

Crossopterygii the connexion is retained even in the adult by means of a

slender canal extending from the pituitary body and opening into the oral

cavity. Laterally, the base of the infundibulum grows out into a pair of

rounded lobes, the _lobi inferiores_, and distally into a thin-walled

glandular sac, the _saccus vasculosus_, which lies just behind the

pituitary body. The cavity of the thalamencephalon persists as the _third

ventricle_ or _diacoele_. The parts of the brain developed from the

mid-brain and the hind-brain are much less complicated, and, except for

variations in size, they present a fairly uniform character in most Fishes.

In the mid-brain the roof bulges out into a pair of _optic lobes_, and by

the growth of lateral thickenings in its floor two thick strands of

longitudinally disposed nerve fibres, the _crura cerebri_, are formed. The

cavity of the mid-brain remains as the _mesocoele_, and from it an

extension may be prolonged into each optic lobe.

From the hind-brain are formed the _cerebellum_ or _epencephalon_ and the

_medulla oblongata_ or _metencephalon_, the former as a dorsal bulging, the

latter as a ventral thickening. Except where the cerebellum is developed

the dorsal wall remains epithelial, and forms the roof of the persistent

cavity of the {371}hind-brain, the _fourth ventricle_ or _metacoele_, which

retains its primitive continuity with the central canal of the spinal cord.

Lateral lobe-like outgrowths from the dorsal columns of the medulla are

conspicuous structures in some Fishes, and are known as _corpora

restiformia_. The paired portions of the brain are connected across the

middle line by a series of _transverse commissures_. The more important

modifications of the brain in Cyclostomes and Fishes will now be briefly

dealt with.

[Illustration: FIG. 211.—Dorsal (A) and ventral (B) views of the brain of

_Petromyzon marinus_. _ch.pl.1_, Anterior choroid plexus forming the roof

of the prosencephalon and thalamencephalon; _ch.pl.2_, aperture in the roof

of the mid-brain exposed by the removal of the middle choroid plexus;

_ch.pl.3_, the fourth ventricle exposed by the removal of the posterior

plexus; _cr.crb_, crura cerebri; _crb_, cerebellum; _crb.h_, cerebral

hemispheres; _dien_, thalamencephalon; _inf_, infundibulum; _l.gn.hb_, left

ganglion habenulae; _med.obl_, medulla oblongata; _nv.1_, olfactory;

_nv.2_, optic; _nv.3_, oculomotor; _nv.5_, trigeminal; and _nv.8_, auditory

nerves; _olf.l_, olfactory lobes; _opt.l_, optic lobes; _pn_, pineal organ;

_r.gn.hb_, right ganglion habenulae. (From Parker and Haswell, after

Ahlborn.)]

{372}[Illustration: FIG. 212.—Dorsal view of the brain of _Myxine_. _c.r_,

Corpora restiformia; _m.o_, medulla oblongata; _n.p_, naso-pituitary canal;

_ol.o_, olfactory organ enclosed in its fenestrated cartilaginous capsule;

_op.l_, optic lobes; _pr_, prosencephalon; _s_, _s_, dorsal roots of spinal

nerves; _sp.c_, spinal cord; _th_, thalamencephalon. (From Wiedersheim,

after Retzius.)]

In the Cyclostome _Petromyzon_ there is a small prosencephalon with an

undivided prosocoele, and on each side of it a small cerebral hemisphere

which appears as a mere appendage to the much larger olfactory lobe (Fig.

211). The prosocoele divides in front into two outwardly directed branches,

and of the two diverticula into which each branch divides one extends as a

lateral ventricle into the hemisphere of its side, and the other as a

rhinocoele into the corresponding olfactory lobe. The ganglia habenulae are

unusually large, the right one being larger than the left. The optic lobes

are large, but not obviously double. So small is the cerebellum that it

seems to be little more than a narrow transverse band crossing the

fore-part of the fourth ventricle. The roof of the brain is largely

epithelial, especially in the prosencephalon, the thalamencephalon, and the

hind-brain. Over these epithelial areas the pia mater is unusually vascular

and forms a series of "choroid plexuses." The ventricular system is

complete and continuous. By contrast with the Lamprey the brain of

_Myxine_[442] is very primitive, more so perhaps than in any other Craniate

(Fig. 212). In a dorsal view the brain is divided into four pairs of

laterally expanded and longitudinally compressed lobes by a median

longitudinal fissure and three transverse fissures. The two anterior lobes

are little more than the thickened anterior wall of the thalamencephalon,

although, judging from their histological structure, they represent a very

imperfectly differentiated prosencephalon and olfactory lobes. The second

and largest pair constitute the thalamencephalon. The last two pairs of

lobes represent a transversely divided pair of optic lobes, or "corpora

quadrigemina." There is a large medulla oblongata with a pair of corpora

restiformia, but the {373}cerebellum is entirely absent. The ventricles are

subject to some individual variation. Third and fourth ventricles are

generally recognisable, either as isolated cavities or connected by a

remnant of the mesocoele. In the feeble development of the prosencephalon,

in the striking preponderance of the mid-brain over the rest of the brain,

and in the absence of a cerebellum, _Myxine_ is unique amongst Craniates.

[Illustration: FIG. 213.—The brain of a Dog-Fish (_Scyllium canicula_). A,

dorsal view; B, ventral view. The choroid plexuses covering the roof of the

third and fourth ventricles have been removed. _b.o_, Olfactory lobe; _ep_,

origin of the stalk of the pineal body; _f.b_ (in A), prosencephalon; _f.b_

(in B), cerebral hemispheres; _fr_, fourth ventricle; _h.b_, cerebellum;

_h.p_, pituitary body; _i.f_, lobi inferiores; _m.b_, optic lobes; _m.d_,

medulla oblongata; _sc_, saccus vasculosus; _th_, thalamencephalon; _t.o_

(i) olfactory peduncle; i.-x. cranial nerves. (From Wiedersheim.)]

In Elasmobranchs among Fishes the brain attains a much higher grade of

structure. In _Scyllium_ (Fig. 213) there is a large prosencephalon, and

directly in front of it a pair of imperfectly differentiated cerebral

hemispheres, while from its antero-lateral regions the large olfactory

lobes arise. The prosocoele divides in front into four diverticula, of

which the two {374}inner ones extend into the hemispheres as lateral

ventricles, and the two outer as rhinocoeles into the olfactory lobes (Fig.

214). In connexion with the infundibulum there is a pair of sacci

vasculosi, consisting mainly of gland-tubules, opening into the

infundibular cavity.[443] The cerebellum is exceptionally large, but it

does not form a "valvula cerebelli." Large ear-like corpora restiformia are

present. The third and fourth ventricles alone retain an epithelial roof in

relation with choroid plexuses.

In all essentials the brain of the Holocephali is a repetition of the

Elasmobranch type, more especially of the elongated form seen in

_Notidanus_. Indications of a higher grade of structure are, however, to be

seen in the reduction of the prosencephalon which, with its prosocoele, is

now scarcely distinguishable from the thalamencephalon and its ventricle;

and in the more complete differentiation of the cerebral hemispheres from

one another and from the rest of the brain. Large frilled corpora

restiformia are conspicuous structures on each side of the medulla

oblongata. Besides the usual intra-cranial pituitary body, there is also a

separate extra-cranial portion lodged in a pit on the ventral surface of

the basis cranii: in the embryo the two are continuous.

[Illustration: FIG. 214.—Horizontal longitudinal section of brain of

_Chiloscyllium_, to show the ventricles; semi-diagrammatic. _cer_, Origin

of cerebellar ventricle or epicoele; _dia_, third ventricle; _iter_,

mesocoele; _meta_, fourth ventricle; _opt_, optocoele, or cavity of an

optic lobe; _para_, lateral ventricles; _pros_, prosocoele; _rh_,

rhinocoele. (From Parker and Haswell.)]

In the Teleostomi the brain is distinctly of a more primitive type than in

any other Fishes (Fig. 215).[444] The most striking feature is the absence

of cerebral hemispheres, the evolution of the primary fore-brain proceeding

no farther than the formation of an undivided prosencephalon with a

non-nervous roof, and a prosocoele which forms a continuous cavity with the

third ventricle, or at the most is only separated from it by an infolding

of the epithelial roof or velum transversum.

{375}[Illustration: FIG. 215.—A, dorsal view of the brain of a Trout

(_Salmo fario_); B, a vertical longitudinal section. _c.il_, Commissura

interlobularis; _g.h_, ganglion habenulae; _h.c_, habenular commissure;

_i.c_, inferior commissure; _l.i_, lobus inferior; _myc_, myelocoele;

_p.c_, posterior commissure; _v.o_, valvula cerebelli; _v.t_, velum

transversum; _ii_., optic nerve; _v.iii_., _v.iv_., third and fourth

ventricles; _v_, _vii_, _viii_, _ix_, _x_, fifth, seventh, eighth, ninth,

and tenth cranial nerves; remaining reference letters as in Fig. 210. (A,

From Wiedersheim; B, after Haller.)]

Amongst other diagnostic characters may be mentioned the predominance of

the mid-brain over the other divisions, the anterior extension of the large

cerebellum into the mesocoele as a "valvula cerebelli," {376}and the

absence of corpora restiformia. This type of brain is most strongly marked

in the Teleostei, but in other Teleostomes some, like _Acipenser_,[445] are

typically Teleostean in this respect (Fig. 216), while others, such as

_Lepidosteus_, have small cerebral hemispheres with lateral ventricles as

well as a prosencephalon.

[Illustration: FIG. 216.—Vertical longitudinal section of the brain of a

Sturgeon (_Acipenser ruthenus_.) _c.p_, Posterior commissure; _c.r_,

cranial roof; _mc_, mesocoele; _op.ch_, optic chiasma; _p.ch.p_, posterior

choroid plexus; _v.c_, valvula cerebelli; _v.t_, velum transversum;

_v.iii_, _v.iv_., the third and fourth ventricles; other lettering as in

Fig. 210. (From Goronowitsch.)]

The most obvious feature in the brain of the Dipnoi is the great

development of the cerebral hemispheres. In this respect these Fishes

approach the Amphibia, but in other features of brain-structure they

present points of agreement with most other groups of Fishes without being

closely related to any one of them. In _Protopterus_[446] (Fig. 217) the

hemispheres are quite distinct except behind, and the walls of their

spacious lateral ventricles are entirely nervous. Olfactory lobes are

sessile on their anterior extremities, and behind and below they enlarge

into ventral lobes which probably represent the hippocampal lobes of the

higher Vertebrates. A vesicular pineal body at the end of a slender stalk

overlies a singular conical projection from the roof of the

thalamencephalon or "pineal pillow."

{377}[Illustration: FIG. 217.—Dorsal (A), ventral (B), and lateral (C)

views of the brain of _Protopterus annectens_. _C_, Cerebellum; _C.H_,

cerebral hemisphere; _D.S.E_, branches of the sinus endolymphaticus; _In_,

infundibulum; _L.I_,, lobi inferiores; _M.O_, medulla oblongata; _O.L_,

olfactory lobe; _Op.L_, optic lobe; _P_, pituitary body; _P.B_, "pineal

pillow"; _S.E_, sinus endolymphaticus; _Sp.c_, spinal cord; _Sp.n_, spinal

nerve; _Vel_, velum transversum; _Z_, pineal body; IV.V., fourth ventricle;

ii., iii., iv., v., vi., vii., viii.1, viii.2, viii.3.4, ix., and x., roots

of the cranial nerves. (From Burckhardt.)]

The optic lobes form a single oval body, and, as in _Petromyzon_ and the

Amphibia, the cerebellum is very small. A posterior choroid plexus covers

the roof of the fourth ventricle, and an anterior plexus in connexion with

the roof of the thalamencephalon projects downwards into the third

ventricle, and is also prolonged forwards into each lateral ventricle. In

_Neoceratodus_[447] the brain is certainly more primitive and distinctly

less Amphibian. As compared with _Protopterus_ the olfactory lobes and the

cerebellum are larger, and the optic lobes are paired. The smaller

hemispheres are {378}non-nervous dorsally and medianly, the roof and inner

wall of each being formed by an extension of the thick, glandular choroid

plexus which forms the roof of the thalamencephalon.

THE SPINAL NERVES.—The spinal nerves of Cyclostomes (e.g. _Petromyzon_)

consist of a series of dorsal nerves arising on each side from the dorsal

surface of the spinal cord, and of a similar double series arising from the

ventral surface, the dorsal nerves regularly alternating with the ventral

nerves. Each myotome is supplied by a dorsal and a ventral nerve which pass

separately to their peripheral distribution in the skin and muscles. In

Fishes, as in the higher Vertebrates, each dorsal nerve, now termed a

dorsal root, enlarges into a ganglion and then unites, either before or

directly after issuing from the neural canal, with the next ventral nerve

or ventral root in front to form a main spinal nerve. At the same time the

spinal nerves of opposite sides tend to form pairs in the same transverse

plane. After the union of the two roots the spinal nerve divides into three

typical branches: a dorsal nerve (ramus dorsalis), and a ventral nerve

(ramus ventralis), both of which include somatic sensory or afferent

fibres, and somatic motor or efferent fibres, for the innervation of the

skin and muscles of the dorsal and lateral portions of a myotome; and a

visceral branch (ramus visceralis), composed of afferent and efferent

visceral fibres, which supplies the adjacent viscera (alimentary canal and

its glands and blood-vessels), and helps to form the sympathetic nervous

system.[448] The somatic afferent and the visceral afferent fibres enter

the spinal cord by the dorsal roots, the somatic efferent leaving the cord

through the ventral roots, although the visceral efferent fibres traverse

both roots. In the region of the paired fins more or fewer of the rami

ventrales unite to form a plexus, the brachial or the pelvic plexus, from

which the nerves to the fins take their origin.

THE CRANIAL NERVES.—It is usual to describe the cranial nerves of

Cyclostomes and Fishes as consisting of ten serially disposed pairs, viz.:

the _olfactory_ (i.), _optic_ (ii.), _oculomotor_ (iii.), _trochlear_

(iv.), _trigeminal_ (v.), _abducens_ (vi.), _facial_ (vii.), _auditory_

(viii.), _glossopharyngeal_ (ix.), and the _vagus_ (x.) Like the spinal

nerves, the cranial nerves collectively include somatic sensory (general

cutaneous) and motor fibres, and also visceral sensory {379}and motor

fibres, all of which have their own special centres in the brain, but the

proportions of these nerve components differ greatly in different nerves.

Certain preoral nerves (iii., iv., and vi.) are exclusively somatic motor;

others (i. and ii.) are special sensory nerves for the olfactory and visual

organs; but most of the other cranial nerves include several components,

and are therefore "mixed" nerves. Besides these components some cranial

nerves include also a quasi-independent system of nerve-fibres, which

converge from certain cutaneous sense-organs to an independent centre in

the medulla oblongata, the _tuber acusticum_,[449] and is probably derived

from the general cutaneous system of nerve components. Such nerve fibres,

including also the auditory nerve, which has its origin from the same

centre, constitute the _lateralis system_. Perhaps the most striking

feature in the postoral cranial nerves is the predominance of the

visceralis or sympathetic system over the somatic. Omitting the lateralis

fibres and a relatively few somatic sensory fibres, visceral fibres,

sensory and motor, are the principal components of all these nerves,

including v. but excluding viii. The reason for this is to be found in the

fact that splanchnic or visceral muscles in relation with the jaws and

branchial arches have usurped the place of somatic muscles in the muscular

system of the head. For developmental and other reasons the olfactory and

optic nerves stand in a category of their own, and the same may be said of

the third, fourth, and sixth nerves, which innervate the muscles of the

eyeball. The remaining nerves, all of which have their origin in the

medulla oblongata, possess certain features in common, and as they are

related to the gill-clefts in such a way that each forks over a cleft, they

may be conveniently distinguished as "_branchial_" or "_branchiomeric

nerves_." A typical branchial nerve consists of (1) a _principal ganglion_

near the origin of the nerve from the brain; (2) a _main trunk_ which gives

off (3) a somatic sensory branch or _dorsal nerve_ to the skin; (4) a

_palatine nerve_ (visceral sensory) to the oral or pharyngeal mucous

membrane; (5) an _epibranchial ganglion_ which is associated with a

transitory embryonic _epibranchial sensory organ_ at the dorsal border of a

branchial cleft; (6) a _pre-branchial_ nerve (visceral sensory), skirting

the anterior margin of a cleft in its ventral course; and (7) a

_post-branchial_ branch (visceral motor) similarly related to the hinder

margin.

{380}[Illustration: FIG. 218.—Diagram showing the principal branches of the

cranial nerves in a Fish, _mk.c_, Meckel's cartilage; _ol.o_, olfactory

organ; _p.q_, palato-quadrate; _s_, spiracle; i-v, branchial clefts; i, ii,

iii, iv, vi, the first, second, third, fourth, and sixth cranial nerves.

The remaining nerves are differently shaded. _Black_.—The trigeminal nerve:

_g.g_, Gasserian ganglion; _md_, mandibularis; _mx_, maxillaris; _op.p_,

ophthalmicus profundus. _Oblique shading_.—The lateralis system and its

centre (_t.a_), the tuber acusticum: _bucc_.vii, buccalis branch of vii;

_md.ex_, external mandibular branches of vii; _l.n._x, lateralis nerve,

with its supra-temporal branch (_s.t_), and its commissural connexion (_c_)

with _op.s._vii, the ophthalmicus superficialis of vii; viii, auditory

nerve. _Dotted_.—The facialis proper, including _c.t_, chorda tympani;

_gn.g_, geniculate ganglion; _hy_, hyomandibularis, with its motor branches

_m_, _m_, _m_; _p.n._vii., palatine. _Dark grey_.—The glossopharyngeal

(ix), with its pre- and post-branchial branches and its palatine nerve,

_p.n._ix; anastomosing with the palatine branch of vii (Jacobson's

anastomosis). _White_.—The vagus: x^{1-4}, the branchial nerves,

ganglionated and forking over clefts ii-v; _v.n._x, visceral nerve; _oc_,

occipito-spinal nerves; _d.r_ and _v.r_, the dorsal and ventral roots of

the first two spinal nerves. (Slightly modified after Wiedersheim.)]

{381}The first six cranial nerves resemble those of the higher Craniates in

their mode of origin from the brain, in the physiological nature of their

component fibres, and in their peripheral distribution, and therefore they

need not be specially referred to here. The principal branches of the fifth

or _trigeminal_ nerve are shown in Fig. 218. Comparing this nerve with a

typical branchial nerve it would seem that the _profundus_ and

_superficialis ophthalmic nerves_ are dorsal nerves; the _maxillaris_ and

_mandibularis_, pre- and post-branchial branches, respectively, in relation

with the modified gill-cleft which forms the mouth, while the branch to the

oral surface represents a _palatine_ nerve. The most important of the

distinctive features in the cranial nerves of Fishes are to be found in the

relations of nerves vii., ix., and x. to branchial clefts, and in the

lateralis system of nerve components and its association with the lateral

line sensory organs. The seventh or _facial_ nerve is an exceptionally

interesting nerve. Besides the usual components of a typical branchial

nerve certain of its so-called branches are wholly or largely derived from

the lateralis system. For this reason the nerve may be said to consist of

two portions, the _facial proper_, or those fibres which constitute the

facial nerve in air-breathing Craniates, and the _lateralis branches_ which

solely innervate lateral line sense-organs, and are therefore peculiar to

aquatic forms. The facial proper has a ganglion (_the facial or geniculate

ganglion_) on its root, and on entering the orbit after traversing the

cranial wall it gives off a _palatine_ nerve. Just over the spiracle a

_pre-branchial_ nerve, the representative of the _chorda tympani_ of

Mammals, leaves the main trunk, and passes ventrally in relation with the

anterior wall of the spiracle to its ultimate distribution in the walls of

the mouth cavity. The main trunk, now called the _ramus hyomandibularis_,

then pursues a ventral course behind the spiracle as a post-branchial

nerve, and certain of its mainly motor branches which pass downwards in

connexion with the hyoid arch supply the muscles of that arch, and, if an

operculum is present, the opercular muscles as well. The lateralis portion

of the facial includes the following principal branches, {382}each of which

may have a ganglion on its root: (1) an _ophthalmicus superficialis_; (2) a

_buccalis nerve_ with its _ramus oticus_; and (3) _external mandibular_

nerves which course in the ramus hyomandibularis. The addition of the great

_lateralis_ nerve, which is usually described as the lateral branch of the

tenth nerve, and of the eighth or _auditory_ nerve which supplies the

auditory organ, completes the enumeration of the main factors of the

lateralis system. The ninth or _glosso-pharyngeal_ nerve, perhaps the most

typical of all the branchial nerves, has pre- and post-branchial branches

which enclose the hyo-branchial cleft. Its palatine nerve usually extends

forwards and anastomoses with the corresponding branch of the seventh, thus

forming a connexion (_Jacobson's anastomosis_) between the two cranial

nerves. In some Elasmobranchs and Teleosts fibres derived from the dorsal

branch of the ninth nerve innervate a few sense-organs of the lateral

sensory canal of the head, and hence that nerve sometimes contains

lateralis fibres. The tenth or _vagus_ is a compound nerve. Besides the

great lateralis nerve generally associated with it, the _vagus_ includes as

many typical branchial nerves as there are branchial clefts behind the

hyo-branchial cleft, and in Elasmobranchs and in _Chimaera_ these nerves

have independent origins from the medulla oblongata. Each nerve has the

typical structure, a ganglionated trunk which forks over a gill-cleft into

the usual pre- and post-branchial branches, and palatine branches to the

pharyngeal walls. In the Dipnoi the lateralis nerve is connected with the

superficial ophthalmic branch of the seventh nerve by a commisural nerve

which curves across the outer face of the auditory capsule. A somewhat

similar anastomosis is also present in _Petromyzon_. The _vagus_ also

includes a large _ramus intestinalis_, which in Elasmobranchs, at all

events, has a distinct ganglionated root. The nerve forms characteristic

plexuses on the oesophagus and stomach, and in Cyclostomes its branches may

extend nearly the whole length of the intestine. In Ganoids and Teleosts

there is an interesting nerve known as the "_lateralis accessorius_." It is

a compound nerve, and owes its formation to the union of somatic sensory

fibres derived in succession from dorsal branches of the v., vii., ix., and

x. nerves, and also from the corresponding branches of a variable number of

spinal nerves. The finer branches of the nerve are distributed to the skin

of one or more of the fins, or {383}even, as in _Gadus_, to all the fins,

especially to the numerous "end-buds" which are present on those organs. In

many Fishes a variable number of the anterior spinal nerves

(_spino-occipital_) perforate the occipital region of the skull. They

probably represent the ventral roots only of the ordinary spinal nerves of

this region.

SENSE-ORGANS

THE CUTANEOUS SENSE-ORGANS.—These organs, the most remarkable and certainly

the most characteristic of the sense-organs of Cyclostomes and Fishes, are

bud-like groups of epidermic cells in relation with the ends of sensory

nerve fibres. Each consists of a central core of sensory cells, provided

with terminal cuticular sensory hairs, and surrounded by a zone of

supporting and mucus-secreting cells which leave the hairs exposed at the

apex of the bud. Two kinds of these organs can be distinguished, which

differ in their innervation and in their position in the skin. Of the two,

the so-called _end-buds_ are the more primitive. They occupy a superficial

position in the epidermis, and their sense-cells are as long as the

supporting cells. They are present in Cyclostomes and Elasmobranchs, and

especially in Teleosts, where they are irregularly distributed over the

surface of the body, on the fins, lips, and barbels, and also in the

epithelium of the mouth and pharynx. In the Dipnoi they are limited to the

oral cavity, and in the higher Craniates they become taste-buds.[450] Their

somatic sensory nerves[451] are derived from the vii., ix., and x. cranial

nerves, and the lateralis accessorius. In the second type, usually called

"_nerve-eminences_," the sensory cells are shorter than the supporting

cells, and they are always innervated by the lateralis system. When first

developed in the embryo they are quite superficial, like end-buds, but

later the epidermis in which they lie sinks inwards so as to line a series

of pits, closed sacs, tubes, open grooves, or closed canals. _Pit-organs_,

so abundant on the head and trunk of Teleosts (Fig. 220), are simple

epidermic pits with insunken nerve-eminences, disposed in groups or in

{384}lines (accessory lateral lines) or irregularly distributed. The

"_Spalt-papillen_" of Elasmobranchs are pit-organs in which the orifice of

the pit is reduced to a slit. The more deeply-seated _Savi's vesicles_ on

the ventral surface of the Torpedo, and the _nerve-sacs_ of Ganoids, are

similar organs converted into closed sacs and pinched off from the rest of

the epidermis. _Lorenzini's ampullae_ or mucus canals, which are found in

definitely located groups on the lateral and upper surfaces of the head in

Elasmobranchs, may perhaps be compared to pit-organs prolonged inwards to

form subcutaneous tubes, each of which terminates in a radially-septate,

chambered dilatation or ampulla, containing groups of sensory cells.

Besides the more diffusely scattered sense-organs there are others which

become disposed in definite lines along the sides of the body and on the

head, and, enclosed in grooves or closed canals, constitute the highly

characteristic _lateral line system_ of Cyclostomes and Fishes.[452] The

auditory organ must also be included as a specialised portion of this

system. Both organs are innervated by the lateralis system, and both arise

from a common rudiment in the embryonic epidermis in the position of the

future auditory organ. This rudiment grows backwards along the side of the

body in the form of a cord of cells differentiated from the epidermis, and

also forwards, where it soon divides into the rudiments of future

supra-orbital and infra-orbital canals. Sense-organs are differentiated at

intervals along the line of the cord; and in the body, but not on the head,

they frequently exhibit a segmental disposition. Each sensory organ then

sinks down into a short epidermic groove, which by the subsequent meeting

of its lips becomes a canal detached from the epidermis. The short canals

then become continuous, leaving, however, an externally opening primary

pore between every two consecutive canals, and the result is a continuous

canal having sense-organs imbedded in its epidermic lining and connected

with the exterior by pores at intervals (Fig. 219).[453] The enclosure of

the canals in the scales of the lateral line of the trunk or in special

drain-pipe ossicles on the head, and the dichotomous subdivision of the

primary pores into groups of surface-pores, complete the evolution of the

system in its more advanced condition.

{385}[Illustration: FIG. 219.—Vertical longitudinal section through the

lateral canal of _Amia calva_. _l.n_, Lateralis nerve with its branches,

_n_, _n_, to the sensory organs, _s.o_, _s.o_; _p_, _p_, _p_, external

pores; _s.c_, sensory canal; _s_, _s_, scales of the lateral line. (From

Wiedersheim, after Allis.)]

Typically, the lateral line system consists of certain canals or grooves,

usually but not invariably continuous, and defined by their innervation,

(i.) a _lateral canal_ extending along the side of the body and the hinder

part of the head, and having its sensory organs supplied by the great

lateralis nerve (Fig. 220); (ii.) a _supra-orbital canal_ passing forwards

over the eye and innervated by the superficial ophthalmic branch of the

facial nerve; (iii.) an _infra-orbital canal_ supplied by the buccalis and

otic branches of the same nerve; and (iv.) a _hyo-mandibular_ or

_operculo-mandibular canal_, situated on the outer side of the hyoid

region, and thence prolonged downward and forward in relation with the

lower jaw, and innervated by the external mandibular branches of the facial

nerve. The hyo-mandibular canal is sometimes distinct from the other

canals, as in Elasmobranchs and some Teleosts (Fig. 220); and in certain

North American Siluroids the same may be said of the supra-orbital. But, as

a rule, the infra-orbital is continuous behind both with the lateral and

the supra-orbital canals, while the hyo-mandibular canal joins the

infra-orbital, or, exceptionally, {386}the supra-orbital canal. Transverse

commissural canals often connect the lateral and supra-orbital canals of

opposite sides across the dorsal surface of the head, and the corresponding

infra-orbital and hyo-mandibular canals may also be continuous at the

extremity of the snout or at the mandibular symphysis.

[Illustration: FIG. 220.—Sensory canals of the left side of the head of

_Gadus virens_. _e_, Eye; _i.o_, infra-orbital canal (dotted); _l.c_,

lateral canal (oblique shading); _n_, nasal apertures; _op_, operculum;

_op.m_, operculo-mandibular canal (longitudinal shading); _p.o_,

pit-organs; _s.o_, supra-orbital canal (cross-hatched); _s.o.c_,

supra-orbital commissure; _s.t_, supra-temporal branch; _t.t_, tubuli by

which the canals communicate with the exterior. (From Cole.)]

Throughout their extent the canals communicate with the exterior by pores,

or short canals terminating in pores, or by branched canals ending in

groups of pores. In Cyclostomes[454] the lateral line system is represented

by pit-organs disposed as in Fishes, and innervated by a true lateralis

nerve. Some Elasmobranchs have the lateral canal of the trunk represented

by an open groove protected by marginal denticles. _Chimaera_ is more

primitive still in this respect, for on the head as well as on the body the

sensory organs are in open grooves. Amongst Fishes these organs are most

primitive in the Dipnoi, where they retain their superficial position in

the epidermis. In Teleostomes the lateral canals perforate the scales of

the lateral line, and at intervals they open externally by simple or

multiple pores which perforate the scales. On the head they are more or

less completely enclosed in special ossicles which either remain distinct

or fuse with certain of the adjacent dermal or cartilage bones of the

skull. The use of the lateral line organs is not certainly known. They

occur only in Fishes and Amphibia, and as blind Fishes are able to avoid

obstacles with the greatest ease when swimming, it is possible that these

organs enable their possessors {387}to appreciate undulatory movements in

water in the shape of reflex waves from contiguous surfaces or

objects.[455] Their great antiquity is shown by their existence in most of

the Heterostraci, and in the Antiarchi and Arthrodira, although they have

not yet been discovered in the Osteostraci.

THE AUDITORY ORGANS.—In its more typical condition each auditory organ

consists of a membranous sac or vestibule, partially constricted into an

upper portion or utriculus and a lower or sacculus (Fig. 221, A). Three

semicircular canals are connected with the utriculus, of which two are

vertical and at right angles to one another, and the third is horizontal.

One end of each canal is dilated into an ampulla. A slender tube, the

ductus endolymphaticus, leaves the sacculus, and ends in a sac-like

swelling, the sinus endolymphaticus, which apparently represents a portion

of the embryonic epidermic involution from which the auditory organ is

formed. A smaller sac-like outgrowth from the sacculus, the _lagena_,

corresponds to the cochlea of the higher Vertebrates. The epidermic lining

of this system of cavities is differentiated into patches or ridges of

sense-cells (maculae or cristae), separated by supporting cells and

innervated by the terminal branches of the auditory nerve. There is a

crista acustica in each ampulla; and maculae acusticae are present in the

utriculus, sacculus, and lagena. A fluid, the _endolymph_, fills all the

cavities, and a similar fluid or _perilymph_ occupies the spaces in the

periotic capsule in which the various chambers are lodged. Among the more

notable deviations from this type of auditory organ the Cyclostome

_Myxine_, apparently, has but a single semicircular canal with an ampulla

at each end, and the vestibule is a simple sac (Fig. 221, B). _Petromyzon_

has two canals, but lacks the horizontal canal. In Elasmobranchs, including

_Chimaera_ (C), the ductus endolymphaticus retains its primitive connexion

with the exterior by means of a pore on the dorsal surface of the head. In

the Dipnoi (e.g. _Protopterus_) the paired endolymphatic sinuses divide

into a number of caecal branches containing otoliths, which meet and

interlace over the fourth ventricle (Fig. 217).[456] Otoliths, either in

the form of fine, {388}mucus-connected, calcareous particles, as in

Elasmobranchs, or as massive solid concretions in Teleosts, are present in

relation with the sensory areas of the utriculus, sacculus, and lagena.

[Illustration: FIG. 221.—Auditory organs of Fishes. A, of a typical Fish;

B, of _Myxine_; C, of _Chimaera_; and D, of _Perca_. _a.c_, Anterior canal;

_am_', _am_", _am_'", ampullae; _am.n_, nerves to ampullae; _c_,

semicircular canal (in _Myxine_); _d.e_, ductus endo-lymphaticus; _h.c._

horizontal canal; _l_, lagena; _mc_, macula acustica; _m.s_, macula

acustica of the sacculus; _n_, nerves to ampullae; _o_, external aperture

of the ductus endo-lymphaticus; _p.c_, posterior canal; _s_, sacculus;

_s.e_, sinus endo-lymphaticus; _sk_, superficial skin; _s.s_, sinus

superior; _u_, utriculus; viii, auditory nerve. (From Wiedersheim, after

Retzius.)]

In a few marine and in a large number of freshwater Teleosts the auditory

organ enters into a more or less intimate connexion with the air-bladder by

one of three different methods.

{389}[Illustration: FIG. 222.—Cavity of the air-bladder of a Siluroid

(_Macrones nemurus_) exposed by the removal of its ventral wall. _a.c_,

Anterior chamber; _b.o_, basioccipital; _b.w_, body wall, here reduced to

the external skin; _cl_, clavicle; _l.c_, lateral chamber; _l.s_,

longitudinal septum; _pt_, post-temporal; _tr.a_, anterior portion of the

tripus; _tr.c_, crescentic portion of the tripus; _t.s_, transverse septum;

_t.s_', shorter transverse septum. (From Bridge and Haddon.)]

The first and simplest is by the apposition of the extremities of a pair of

caecal tubular prolongations from the air-bladder to the outer surfaces of

the fibrous membranes which close a pair of vacuities in the outer bony

walls of the periotic capsules, the inner surfaces being bathed by the

perilymph surrounding the auditory organs. This method is characteristic of

certain Serranidae, Berycidae, Sparidae, Gadidae, and Notopteridae,[457]

and probably in the Hyodontidae. In the second method, of which several

Clupeidae (_e.g._ Herring, Pilchard, etc.) furnish examples, the periotic

vacuities are open instead of closed, and the sac-like ends of the tubular

extensions from the air-bladder are in actual contact with protruding

outgrowths from the utriculus.[458] The third method, by far the most

elaborate, is by the intervention of a series of movably connected

"Weberian" ossicles, of which the most posterior on each side (the tripus)

is inserted into the dorsal wall of the air-bladder (Fig. 223), while the

anterior one (scaphium) forms the outer wall of a median backward

prolongation (sinus impar) of the perilymph-containing spaces surrounding

the two auditory organs. This in turn encloses a similar median

prolongation (sinus endolymphaticus) from the two sub-cerebrally united

endolymphatic ducts (Fig. 223).[459] This complex mechanism is present in

the Cyprinidae, Siluridae, Characinidae, and Gymnotidae; and hence the term

"Ostariophysi"[460] as a collective name for these families.[461] {390}The

physiological _raison d'être_ of the connexion between the air-bladder and

the auditory organ cannot yet be regarded as satisfactorily determined. It

is possible, as Weber thought, that it may be an auxiliary to the function

of hearing by transmitting to the ear sound-waves impinging on the surface

of the body and affecting the gases in the air-bladder.[462] On the other

hand, it may be urged with perhaps greater probability that the connexion

exists for the purpose of conveying to the ear stimuli due to the varying

degrees of distension of the air-bladder, such as, it may be presumed, are

naturally brought about by the variations of hydrostatic pressure which a

Fish encounters in the course of its ascent or descent in the water.[463]

Whether regarded as an accessory to hearing, or as a means of regulating

the distension of the air-bladder, the physiological value of the connexion

must be considerable, and on this point it is at least significant that the

Weberian mechanism is characteristic of the dominant families of freshwater

Teleosts at the present day.[464]

[Illustration: FIG. 223.—Diagram to show the Weberian ossicles and their

relations to the ear and the air-bladder. _at_, Atrium, an extension of the

sinus impar; _a.v.c_, anterior vertical canal; _b.w_, bony wall of the

periotic capsule; _d.e_, the medianly-united endolymphatic ducts; _h.c_,

horizontal canal; _in_, intercalarium, a third ossicle imbedded in the

ligament (_i.lg_) connecting the scaphium with the tripus; _n_, bony

nodules on the sides of the complex vertebral centrum; _p.v.c_, posterior

vertical canal; _s_, sacculus; _sc_, scaphium; _s.e_, sinus

endolymphaticus; _s.i_, sinus impar; _tr.a_, _tr.c_, the anterior and

crescentic parts of the tripus; _ut_, utriculus. The radial lines represent

the fibres of the dorsal wall of the air-bladder. (From Bridge and

Haddon.)]

THE OLFACTORY ORGANS.—These organs are essentially a pair of pit-like

inpushings of the skin of the ventral side of the head in front of the

mouth, with their lining epidermis differentiated into sensory cells

separated by supporting cells, and connected with the olfactory lobes of

the brain by olfactory nerves.

{391}[Illustration: FIG. 224.—Two stages in the development of the

olfactory organ and the pituitary involution in _Petromyzon_. A is the

earlier, B a much later stage. _br_, Brain; _in_, infundibulum; _l.lp_,

lower lip; _ms_, mesenteron; _n_, notochord; _ol.o_, olfactory organ; _pn_,

pineal body; _pt.s_, pituitary sac; _st_, stomodaeum; _u.lp_, upper lip.

(From Parker and Haswell, after Dohrn.)]

The Cyclostomata are unique amongst Craniates in the apparently unpaired

condition of the olfactory organ, and in its remarkable relation to the

pituitary involution. In the embryo Lamprey the median and ventral

olfactory pit is carried inwards with the pituitary invagination, so that

the former appears as a dorsal outgrowth from the latter, and the two have

a common external opening, the naso-pituitary aperture (Fig. 224). Later

the extraordinary forward growth of the upper lip to form the roof of the

buccal funnel has the effect of shifting the naso-pituitary involution and

its aperture to a final position on the dorsal side of the head. It is due

to this dorsal displacement that, as we shall see, the pituitary caecum

reaches the ventral surface of the brain by perforating the basis cranii

from above, instead of from below as in all other Craniates. The pituitary

body is pinched off from the dorsal side of the naso-pituitary involution.

In the adult Lamprey the olfactory organ appears as a round sac divided by

a median septum into two lateral chambers (Fig. 225), the lining epithelium

of which is raised into prominent ridges. Behind the sac the pituitary

involution is prolonged backwards beneath the brain, and, after traversing

the basi-cranial fontanelle, it widens out into a spacious cul-de-sac and

terminates on the dorsal side of the pharynx, beneath the anterior end of

the notochord. In _Myxine_ the pituitary involution ends by opening into

the pharynx.

{392}[Illustration: FIG. 225.—Side view of the brain of _Petromyzon_, with

the olfactory organ and the pituitary caecum in section. _cblm_,

Cerebellum; _crb.h_, cerebral hemisphere; _dien_, thalamencephalon; _f_,

fold in the nasal tube; _gl_, nasal glands; _inf_, infundibulum; _l.gn.hb_,

left ganglion habenulae; _med.obl_, medulla oblongata; _na.ap_,

naso-pituitary aperture; _n.ch_, notochord; _Nv^1-nv^{10}_, cranial nerves;

_Nv^{12}_, first ventral spinal nerve; _olf.cp_, olfactory capsule;

_olf.l_, olfactory lobe; _olf.m.m_, olfactory mucous membrane; _opt.l_,

optic lobe; _pn_, pineal body; _pn′_, inferior pineal body; _pn.e_,

parietal eye; _pty.b_, pituitary body; _pty.p_, pituitary cul-de-sac; _sp_,

median septum of the olfactory sac; _sp^1_, first dorsal spinal nerve.

(From Parker and Haswell, after Ahlborn and Kaenische.)]

The apparently monorhinal condition of the Cyclostomes is probably a

secondary acquisition. At the earliest embryonic stage at which any trace

of an olfactory organ is apparent, there is a median thickening of the

epidermis, possibly a vestige of some older sensory organ comparable, it

may be, to the so-called olfactory organ of Amphioxus; on each side of it

there is a lateral thickening, the rudiments of the paired organs.[465] The

three thickenings, or "plakodes," then sink inwards to form an olfactory

pit. The partial subdivision of the adult organ by a vertical septum, and

the presence of two olfactory nerves, point to the same conclusion.[466]

All Fishes possess olfactory organs which are obviously paired. In

Elasmobranchs and Dipnoi they retain their primitive ventral position. Many

Sharks and Dog-Fishes possess an oro-nasal groove leading from each

olfactory organ to the corresponding angle of the mouth. The Dipnoi proceed

a stage farther, and, by the conversion of the grooves into short canals,

the olfactory pits communicate with the mouth by true internal nostrils, as

in the higher {393}Vertebrates. In the adults of existing Teleostomi the

orifice of each organ is usually divided into two by the growth of a fold

of skin across it, and the two apertures rotate outwards and upwards on to

the lateral or the upper surface of the snout. Of the two nostrils the

posterior one probably corresponds to an external nostril, and the anterior

one to the internal nostril. Occasionally each olfactory organ has only a

single orifice. In the Crossopterygii and in some Teleostei the nostrils

become tubular. The lining epithelium of the olfactory pits is usually

produced into ridges, disposed longitudinally or transversely, or in the

form of radii from a central point in the roof. Many Teleosts have each

olfactory organ prolonged backwards into one or two sacs, the nasal sacs,

which are either simple reservoirs, or glandular and mucus-secreting. In a

species of Chinese Sole (_Cynoglossus semilaevis_) the two sacs, one from

each olfactory organ, unite over the roof of the mouth in a common median

sac, and in one unique specimen the latter communicated with the mouth by a

large naso-pharyngeal aperture.[467]

THE EYES.—In essential structure the eyes of Cyclostomes and Fishes

resemble those of the higher Craniates. As a rule, in Fishes they are

relatively larger, however, and the lens is globular and the cornea

somewhat flatter. Ciliary processes and ciliary muscles are absent. As the

eyes are nearly always lateral in position it is probable that monocular

vision is the rule. In Teleosts and in _Amia_ a "choroid gland," consisting

of a mass of capillary blood-vessels, surrounds the optic nerve externally

to the retina, and derives its blood from the efferent artery of the

pseudobranch (Fig. 226). In most Teleostomi, but not in Cyclostomes,

Elasmobranchs, and Dipnoi, there is a singular prolongation of the choroid

coat, known as the "processus falciformis," which extends across the

vitreous humour to the inner face of the lens, where it ends in an

expansion, the "campanula Halleri" (Fig. 226). Accommodation to vision at

different distances is not effected by alterations in the convexity of the

lens, but by a change in its position with regard to the retina, apparently

brought about by the contraction of a special retractor muscle.[468] Some

oceanic pelagic Teleosts are remarkable for their curious telescopic eyes

in the shape of short protruding {394}cylinders, each terminating in a

strongly convex cornea (Fig. 227).[469] The eyes are directed either

upwards or forwards, and, as their long axes are parallel in either

position, it is probable that these Fishes are capable of binocular vision.

In the young of certain Teleosts occurring in the Antarctic and Indian

Oceans the large eyes are situated at the extremities of extraordinary long

stalks extending from the sides of the head.

[Illustration: FIG. 226.—Vertical section of the eye of _Salmo fario_

(semi-diagrammatic). _arg_, Argentea; _ch_, choroid; _ch.gld_, choroid

gland; _cn_, cornea; _cp.hal_, campanula Halleri; _ir_, iris; _l_, lens;

_opt.nv_, optic nerve; _pg_, pigmentary layer; _pr.fl_, processus

falciformis; _rt_, retina; _scl_, sclerotic. (From Parker and Haswell.)]

In the quasi-parasitic Cyclostome, _Myxine_, and in many Teleosts belonging

to widely different families, which live at great depths in the sea or

inhabit subterranean waters, the eyes suffer from disuse and degenerate in

structure. The influence of a deep-sea habitat on the eyes of Fishes is

somewhat varied. The eyes are often small. A few abyssal Fishes are totally

blind, and no external trace of eyes can be seen (Fig. 430). In such Fishes

compensation is often afforded by an extraordinary development of tactile

organs in the form of long barbels, or of trailing filaments derived from

the median or the paired fins (Fig. 371, B). Many deep-sea forms possess

eyes of the normal size, or even exceptionally large eyes, probably because

either they occasionally migrate towards the surface, or else they possess

phosphorescent organs and are able to see by the aid of the light they

themselves emit. A blind Siluroid (_Amiurus nigrilabris_) frequents the

cave streams of Pennsylvania, and many members of the same family which

live in muddy waters have very small or even minute eyes. One of the Gobies

(_Typhlogobius_),[470] which buries itself in the sand, or is found under

stones in the holes of a burrowing Crab on the coast of California, is also

{395}blind. Amongst other blind Fishes _Amblyopsis_ and _Typhlichthys_

(Amblyopsidae)[471] and _Lucifuga_ (Zoarcidae) may be mentioned, the first

two inhabiting the cave streams of North America, while the third has a

similar habitat in Cuba. When the eyes degenerate they dwindle in size and

recede from the surface. The lens and the iris wholly or partially

disappear, and although it is generally recognisable the retina loses

certain of its characteristic layers, or the latter are but imperfectly

formed. In _Myxine_ even the eye-muscles are absent.

The eyelids of Fishes are little more than marginal folds of skin, capable

of little if any movement, and leave the eyes largely uncovered. Some

Sharks have a third eyelid or "membrana nictitans" at the anterior corner

of the eye. Lachrymal glands are unknown.

[Illustration: FIG. 227.—The telescopic eyes of _Opisthoproctus soleatus_,

Vaill. (A), and of a species of a new family of Teleosts from the Indian

Ocean (B). Nat. size. (From Chun.)]

THE PARIETAL EYE.—It is only in the Cyclostomes that this structure can

have any claim to be regarded as a visual organ. In the Lamprey (Fig. 228)

the parietal eye is a slightly flattened vesicle lying directly over the

pineal vesicle, and connected by a slender stalk or nerve with the right

ganglion habenulae. The dorsal or more external half of the vesicle is

bi-convex, and forms the "pellucida," while the inner half or retina is

said to consist of supporting cells with interspersed deeply pigmented

sense-cells and ganglion cells.[472] The external skin over the parietal

eye is partially transparent in the living animal.

In many of the oldest known Fishes, such as the Ostracodermi, the

Antiarchi, and the Crossopterygian Osteolepida, there are indications of

the existence either of one or of two median sense-organs on the upper

surface of the skull, in the shape of one or two foramina, or hollow

protuberances, or pit-like grooves or {396}depressions, but, as a rule,

when one of them is present the other is absent. It is probable that both

these structures were associated with sensory organs, of which one may have

been a parietal eye and the other a pineal eye. Some Teleosts (_e.g._ many

deep-sea Scopelidae) have a transparent, convex, cornea-like prominence on

the upper surface of the head which may be related to one of these singular

organs.[473]

[Illustration: FIG. 228.—Vertical section through the parietal eye and the

pineal vesicle of _Petromyzon marinus_. _c.t_, Connective tissue; _p_,

pellucida; _p.o_, pineal organ; _pt.o_, parietal eye; _r_, retina; iii _v_,

third ventricle. (From Wiedersheim, after Studnička.)]

{397}CHAPTER XV

THE KIDNEYS AND THE REPRODUCTIVE ORGANS—BREEDING

The kidneys and the reproductive organs are so intimately connected that it

is necessary to deal with them together. Both organs are specialised

portions of the coelom and its epithelial lining. The KIDNEYS are

essentially a series of tubular and at first segmentally-disposed

outgrowths from the coelom (urocoeles) which acquire a connexion with the

exterior, while the gonads have their origin from local modifications of

the coelomic epithelium. At a very early embryonic stage each lateral half

of the coelom presents three well-marked divisions: (1) a series of dorsal

portions ("myocoeles"), the cavities of the myotomes or muscle-segments;

(2) a longitudinally continuous unsegmented portion extending round the

alimentary canal, the "ventral coelom"; and (3) a series of intermediate

tubular portions or "nephrotomes," each of which leads from a myocoele to

the ventral coelom (Fig. 229, A). The essential components of the kidneys,

the urocoeles or renal tubules, are derived from the nephrotomes. In its

typical condition each kidney consists of three portions, which, in

accordance with their embryological and evolutionary sequence, are termed

the "pronephros," the "mesonephros," and the "metanephros." The pronephros,

the larval or provisional kidney, is formed from a limited number of the

nephrotomes immediately behind the head. From each nephrotome a hollow

tubular outgrowth is formed, which grows towards the lateral surface of the

body, and then unites with its fellows of the same side to form a main

longitudinal duct—the "archinephric" or "pronephric duct" (Fig. 229, A,

Fig. 230, A). This duct grows backwards until it opens into the

cloaca.[474]

{398}[Illustration: FIG. 229.—Diagrammatic transverse sections through an

embryo Craniate to show the mode of development of the pronephros (A) and

of the mesonephros (B). The right side of each figure shows an earlier

stage than the left. In B (left side) the connexion of a vas efferens with

a mesonephric tubule, and the division of the archinephric duct into

Müllerian and mesonephric ducts are shown, _a_, Aorta; _a.c_, alimentary

canal; _a.d_, archinephric duct; _g_, glomus; _gl_, glomerulus; _i.n_,

inner nephrostome; _mb_, Malpighian body; _md_, Müllerian duct; _mnd_,

mesonephric duct; _mnt_, mesonephric tubule; _myc_, myocoele; _myt_,

myotome; _n_, notochord; _np_, nephrotome; _nt_, nephrostome; _o.n_, outer

nephrostome; _pn.t_, pronephric tubule; _s.c_, spinal cord; _t_, testes;

_v.c_, ventral coelom; _v.ef_, vas efferens. (After Kingsley and Semon.)]

At the same time the nephrotomes lose their connexion with the myocoeles,

although they still retain their "nephrostomes" or apertures through which

they communicate with the ventral coelom. When fully developed the

pronephros consists of a few tubules, more or less convoluted, opening at

their inner extremities into the coelom by means of their ciliated

nephrostomes, and at their outer ends communicating with the exterior

through the archinephric duct. In relation with the pronephros a branch

from the dorsal aorta forms a tuft of capillary blood-vessels or "glomus,"

opposite the nephrostomes, which projects into the ventral coelom on each

side. Later, a second series of much more numerous tubules is formed behind

the pronephros, which constitute the mesonephros. In forming mesonephric

tubules the nephrotomes become disconnected from the myotomes and their

myocoeles, and curving outwards they {399}come to open into the

archinephric duct, although they do not in any way contribute to its

formation (Fig. 229, B). Segmentally-arranged twigs from the dorsal aorta

end in tufts of capillaries or glomeruli, each of which projects into a

small sac-like enlargement of a mesonephric tubule, pushing before it the

wall of the sac. In this way a double-walled "Malpighian body," containing

a "glomerulus," is formed in connexion with each tubule. Subsequently, the

mesonephric tubules increase in number by budding. New nephrostomes and

Malpighian bodies are developed on the secondary branches, and the original

segmental arrangement of the tubules becomes obscured. With the growth of

new tubules, and the formation of blood-vessels and of connective and

lymphoid tissues between them, each mesonephros finally assumes the

condition of a compact gland imbedded in the dorsal wall of the coelom,

with its ventral surface invested by the peritoneum. A "metanephros," which

in the higher Vertebrates replaces the mesonephros as the functional

kidney, is perhaps not represented in Fishes.

A more or less well-developed pronephros is present in the embryos or

larvae of the Cyclostomes and of all Fishes, but as a rule it completely

disappears at an early period and is replaced by the mesonephros. It is

retained throughout life, however, in the Myxinoid Cyclostomes (Fig. 230,

B), and has its persistent nephrostomes opening into the pericardial

cavity.[475] In a few Teleosts the pronephros is also persistent, as in

_Fierasfer_ and _Dactylopterus_, and in others the organ may not completely

disappear until the approach of sexual maturity. But with these exceptions

the mesonephros is the sole functional kidney in the adults of the

Cyclostomes and of all Fishes. As regards the nature of the duct by which

the excretion of the mesonephros is conveyed outwards, there are notable

differences in different Craniates. The Cyclostomes and the Teleostomi

retain that part of the archinephric duct into which the mesonephric

tubules open, and which remains after the atrophy of the pronephros (Fig.

230, B, E, F). In Elasmobranchs, and probably also in the Dipnoi, a special

mesonephric duct is developed in a way which will be described later (Fig.

230, C, D).

{400}[Illustration: FIG. 230.—Showing the principal modifications of the

kidneys and reproductive organs in Cyclostomes and Fishes. A, The

pronephros and its duct in the embryo; B, the kidneys and genital pores in

_Petromyzon_, the vestigial pronephros represented as in _Myxine_; C and D,

the urinogenital organs of a male and female Elasmobranch; E, of a male or

female Teleost, or a male _Lepidosteus_; F, of a female _Polypterus_,

_Acipenser_, _Amia_, or _Osmerus_. _a_, Anus; _a.d_, archinephric duct;

_c_, cloaca; _c.a_, the coelomic aperture of the Müllerian duct; _c.p_,

cutaneous pit; _g_, gonad; _gd_, gonoduct; _g.p_, genital pore; _i_,

intestine; _m_, Malpighian body; _m.d_, Müllerian duct; _mn_, mesonephros;

_mn^1_, vestigial mesonephros; _mn^2_, excretory portion of the mesonephros

("metanephros"); _mn^3_, genital portion of mesonephros; _mn.d_,

mesonephric duct; _mtn.d_, metanephric duct; _n_, nephrostome; _ov_, ovary;

_p.a_, abdominal pore; _p.f_, peritoneal funnel; _pn_, pronephros; _pn′_,

vestigial pronephros; _s.g_, shell gland; _s.s_, sperm sac; _t_, testis;

_ug.s_, _u.s_, urinogenital sinus; _v.ef_, vasa efferentia; _v.s_, vesicula

seminalis.]

{401}In the males of Elasmobranchs some of the hinder mesonephric tubules

unite to form a single main duct opening into the terminal part of the

mesonephric duct, and these tubules and their separate duct are sometimes

regarded as a metanephros and a metanephric duct. The mesonephric

nephrostomes are persistent throughout life in a few Elasmobranchs (_e.g._

Notidanidae, Heterodontidae, Rhinidae, and some Scylliidae), and also in

_Amia_:[476] in all other Fishes as well as in the Cyclostomes they become

closed in early life.

[Illustration: FIG. 231.—Diagrammatic horizontal section through the

abdominal pores and cloaca of an Elasmobranch. _a.p_, Abdominal pore; _c_,

coelom; _cl_, cloaca; _cl.p_, cloacal papilla; _c.p_, cloacal pit; _od_,

oviducal apertures in the female; _r_, rectum; _u.s_, cloacal aperture of

the urinary sinus (female), or the urogenital sinus (male). In some

Elasmobranchs the abdominal pore opens at the base of the cloacal papilla,

as shown at _a.p^1_. (Modified from Bles.)]

In many Fishes the hinder extremity of the coelom communicates directly

with the exterior through "abdominal pores," of which there is usually a

pair, rarely a single pore, situated close to the cloacal or the anal

aperture.[477] Elasmobranchs usually have a pair, often at the extremities

of a pair of cloacal papillae (Fig. 231), but they are absent in some

families (_e.g._ Heterodontidae and Rhinidae); and in some Scylliidae (e.g.

_Scyllium canicula_) they are very variable, being either present or absent

on both sides, or an open pore is present on one side only. Pores are

present and paired in the Crossopterygii, the Chondrostei, and the

Holostei. Amongst the Dipnoi _Neoceratodus_ has a pair of pores.

_Protopterus_ sometimes has two pores opening into the cloaca, but as a

rule the two become confluent and have a single external aperture. In

_Lepidosiren_ pores are wanting. Abdominal pores are rarely present in

Teleostei. They exist, however, in the Mormyridae (_Gymnarchus_ and several

species of _Mormyrus_), and also in the {402}Salmonidae,[478] where they

are as singularly variable in different species and individuals as in the

Elasmobranch Scylliidae. The use of abdominal pores is not certainly known,

unless the coelom of those Fishes which possess them continues to retain

some measure of its primitive excretory function, and the pores act as

excretory ducts. That the nephrostomes are excretory organs has been shown

by experiment, and it is worthy of note that there exists a reciprocal

relation between these structures and abdominal pores, to the extent that

while there are a few Fishes (_e.g._ certain Elasmobranchs and _Amia_) in

which both coexist, there are many others in which the presence of

nephrostomes is correlated with the absence of pores and _vice versâ_.