The SENSE-ORGANS connected with the nervous system are few and simple.

There are sensory cells in the ectoderm, on the margin of the velum, on the

velar tentacles, and especially in clumps on papillae of the cirri around

the mouth, which are probably tactile. In the roof of the oral hood there

is a sensory structure, the "groove of Hatschek," which is supposed to be

an organ of taste. The olfactory pit alluded to above opens externally on

the left-hand side of the snout. It is ciliated internally and leads to the

so-called olfactory lobe, an {129}antero-dorsal hollow outgrowth from the

brain. In the young animal the olfactory pit opens by the neuropore into

the central canal (Fig. 80, A), but that passage is closed in the adult.

Possibly the olfactory pit is homologous with the hypophysis or pituitary

body of Vertebrates, the homologue of which in Tunicata has a ciliated

funnel. Finally, the median cerebral eye (Figs. 80 and 81) is a mere

pigment spot in the anterior wall of the cerebral vesicle, and a series of

somewhat similar pigment spots occurs along the floor of the central canal

in the spinal cord.[114] There is no known auditory organ. On the under

surface of the oral hood patches of ciliated epithelium drawn out into

rounded lobes were called by Johannes Müller the "Räder-organ." This is

probably of use in drawing water inwards to the pharynx, but it may also be

a sense-organ.

[Illustration: FIG. 81.—_Branchiostoma lanceolatum_. Anterior portion of

central nervous system from above, showing dorsal and ventral spinal

nerves. (From Willey, after Schneider.)]

The GONADS are segmentally arranged along the sides of the body, projecting

into the atrial cavity at the sides of the pharynx and intestine. In some

species the gonads are paired, but in others belonging to the genus

_Asymmetron_ (p. 137) only a single series, that of the right side, is

present. In the common Amphioxus (_Branchiostoma lanceolatum_) there are

about 26 pairs (Fig. 70, B), lying in somites 25 to 51; and ovaries and

testes are found in separate individuals in all other respects. Each gonad

is surrounded by a layer of coelomic epithelium. The gonad must therefore

be regarded as having grown down from a myotome of the body-wall into a

coelomic pouch, carrying before it the coelomic and then the atrial

epithelium (Figs. 72, and 74, A, _g_). Eventually the gonads, when ripe,

burst through the layers of epithelium, and the ova and sperms are shed

into the atrium and escape to the exterior by the atriopore, or it may be

in some cases by the mouth.

{130}EMBRYOLOGY AND LIFE-HISTORY.

Development takes place in the sea-water where the egg is

fertilised—apparently always about sunset, the embryonic stages being

passed through during the night, and the larva hatched in the early

morning.

[Illustration: FIG. 82.—Stages in the segmentation of Amphioxus. A

represents the eight-celled stage; B, the sixteen-celled; D, vertical

section of C; F, vertical section of the blastosphere or blastula stage

(E). (From Korschelt and Heider, after Hatschek.)]

The egg is small (0.105 mm. in diameter when shed) and contains very little

food-yolk. Segmentation is complete (Fig. 82, A), is nearly regular, and

results in the formation of a hollow blastosphere (Fig. 82, E, F), the wall

of which is one cell thick. The lower cells (Fig. 82, B, C, D) are slightly

larger than the upper. Invagination of the lower cells then takes place

(Fig. 83, A), resulting in the suppression of the blastocoele or

segmentation cavity and the formation of an archenteron, at first shallow

and opening widely to the exterior (Fig. 83, B), and then deeper and with

the opening narrowed to a small posterior blastopore (Fig. 83, C). This

"gastrula" stage differs from the blastosphere in having a mouth or

blastopore, and in being two cell-layers thick—epiblast (ectoderm) on the

outside and hypoblast (endoderm) within. It soon shows the future aspects

of the body by its {131}elongation and shape (Fig. 83, C), as the dorsal

surface becomes flat and the ventral convex, while the blastopore is at the

posterior end of the dorsal surface. The blastopore soon closes, and the

mouth and anus are formed independently later.

[Illustration: FIG. 83.—Three stages in the formation of the gastrula of

Amphioxus. In A the nuclei of the endoderm have been omitted; C has the

dorsal surface uppermost, and the posterior end to the right (From

Korschelt and Heider, after Hatschek.)]

The epiblast cells become ciliated all over the surface, so that the embryo

rotates within the thin covering which still surrounds it. And now all the

chief systems of the body begin to be marked out. The tubular nervous

system develops from a depression of the epiblast (the medullary plate) in

the middle line of the flattened dorsal surface (Fig. 84, A, _mp_). The

edges of the depressed area grow inwards and unite over the deeper layer of

epiblast, which becomes the wall of the neural canal or embryonic nervous

system (Fig. 84, D, _n_); and further back these edges of the medullary

plate join one another behind the blastopore, so that the latter comes to

open into the floor of the neural canal, thus forming the neurenteric canal

(Fig. 85, A, _cn_). Anteriorly the neural canal (_n_) opens to the exterior

for some time by the neuropore.

The hypoblastic walls of the archenteron give off a long median dorsal

groove which becomes the notochord (Fig. 84, C and D, _ch_); and also an

anterior pouch and certain lateral pairs {132}of diverticula which are the

enterocoeles or coelomic pouches, and give rise to the mesoblastic somites

(Fig. 84, B and C, _mk_). The notochord (Fig. 84, D, _ch_) is at first a

longitudinal cellular ridge, which becomes segmented off from the hypoblast

as a rod lying below the neural canal. It is seen in various stages of

development in Figs. 84 and 86, leading to the vacuolated condition of the

adult.

[Illustration: FIG. 84.—Four stages in the development of the notochord,

nervous system, and mesoderm of Amphioxus. _ak_, Ectoderm; _ch_, notochord;

_dh_, cavity of archenteron; _hb_, ridge of ectoderm growing over medullary

plate; _ik_, endoderm; _lh_, coelom; _mk_, coelomic pouch; _mk^1_, parietal

layer of mesoderm; _mk^2_, visceral layer; _mp_, medullary plate; _n_,

neural canal; _ns_, protovertebra. (From Korschelt and Heider, after

Hatschek.)]

The coelomic pouches are five in number—(1) one median, anterior, which

gives rise to the two head cavities, the left-hand one of which opens to

the exterior by means of the pre-oral pit; (2) a pair of small lateral

pouches, placed anteriorly and dorsally, which do not divide but give rise

to the first pair of myotomes only and their outgrowths which extend back

into the metapleural folds, where, however, they are later replaced by

lymph spaces; and {133}(3) a second pair of diverticula, more posteriorly

placed, which continue to grow back towards the blastopore, and have paired

mesoblastic somites, the cavities in which are the beginnings of the coelom

in the body, constricted off from them successively from before backwards

(Fig. 85, A, _ush_) to form all the remaining myotomes.[115] This is the

first sign of segmentation in the animal, and at this stage, when it has

about five pairs of mesoblastic somites, it breaks out of its covering and

becomes a free-swimming larva.

[Illustration: FIG. 85.—Embryo of Amphioxus. A, in vertical section,

slightly to the left of the middle line. B, in horizontal section. _ak_,

Ectoderm; _cn_, neurenteric canal; _dk_ and _ud_, archenteron; _ik_,

endoderm; _mk_, mesodermal folds; _n_, medullary canal; _us_, first

coelomic pouch; _ush_, coelomic cavity; _V_, anterior, _H_, posterior, end.

(From Korschelt and Heider, after Hatschek.)]

The mouth now appears, and soon grows to a large opening on the left side

of the now pointed anterior end (Fig. 86, A, _m_), and the first gill-slit

(_ks_) forms as a direct communication from the front of the mesenteron

(pharynx) to the exterior. It is ventral at first, and then shifts over to

the right side.

The anus forms posteriorly, and the neurenteric canal closes {134}up. A

depression on the floor of the enteron close to the mouth gives rise to the

"club-shaped gland" (Fig. 86, B, _k_), which is probably a gill-cleft in

its nature.

[Illustration: FIG. 86.—A, young larva of Amphioxus. B, anterior end

enlarged. _c_, Provisional tail-fin; _ch_, notochord; _cn_, neurenteric

canal; _d_, enteron; _h_, coelom of snout; _k_, club-shaped gland; _k_′,

its external aperture; _ks_, first gill-slit; _m_, mouth; _mr_, nerve-tube;

_np_, neuropore; _sv_, sub-intestinal vein; _w_, pre-oral pit. (After

Hatschek.)]

[Illustration: FIG. 87.—More advanced larva of Amphioxus. _an_, Anus; _au_,

eye-spot; _c_, larval tail-fin; _ch_, notochord; _d_, enteron; _fl_,

rudiment of endostyle; _k_, club-shaped gland; _k′_, its external aperture;

_m_, mouth; _np_, neuropore; _w_, pre-oral pit; _x_, provisional

nephridium; 1-4, gill-slits. (From Korschelt and Heider, after Lankester

and Willey.)]

The walls of the coelomic pouches, which have been extending both dorsally

and ventrally (Fig. 84, D), become the {135}mesoderm, the outer the somatic

and the inner the splanchnic layer; and the ventral parts of their cavities

unite to form the coelom. The cells of the dorsal parts become muscle

fibres, and constitute the myotomes internally and the connective tissue of

the skin externally.

The larva (Fig. 87) is now long and narrow with many segments, pointed

ends, and a caudal fin. The gill-slits all appear first in the mid-ventral

line and then shift over to the right side (Fig. 87, 1-4): they are

metamerically arranged. After fourteen have been so formed a series of

eight appear dorsally to those on the right side, and then the first set,

originally ventral, move over to the left side, and by the suppression of

some they become equal in number and segmentally arranged on the two sides

of the body. This is perhaps the stage at which Amphioxus shows the nearest

approach to the typical embryo of a higher Vertebrate. The gill-slits are

here seven to nine on each side, and the Vertebrate embryo has usually five

to seven on each side. These first gill-slits in Amphioxus are later

subdivided by the downgrowth of the tongue-bar from the dorsal edge.

[Illustration: FIG. 88.—Ventral aspect of three larvae of Amphioxus,

showing the metapleural folds and the formation of the atrium. _ap_,

Atriopore; _k_, gill-slits; _lf_ and _rf_, left and right metapleural

folds; _m_, mouth; _w_, pre-oral pit. (From Korschelt and Heider, after

Lankester and Willey.)]

The atrium is an ingrowth of the external space between the two ventral

metapleural or atrial folds (Figs. 88 and 89), paired lateral ridges of the

body-wall, and so is lined by ectoderm. This ingrowth is shut off from the

exterior by the {136}growth towards each other of sub-atrial ridges on the

inner sides of the metapleural folds (see Fig. 89, A, _sl_), and then

becomes greatly enlarged by the increased relative growth of the

ventro-lateral part of the body-wall (Fig. 89, B, C). The posterior opening

between the metapleural folds remains as the atriopore (Fig. 88, C, _ap_);

while the anterior end (Fig. 88) also remains open for some time, but

eventually closes. As the metapleural folds lie outside the gill-slits

(Fig. 88, A) when these folds close in (B and C), it comes about that the

gill-slits which formerly opened freely to the exterior now open into the

cavity of the atrium (compare Figs. 87 and 88).

[Illustration: FIG. 89.—Diagrammatic transverse sections of three larvae of

Amphioxus to show the development of the atrium. _ao_, Aorta; _c_, dermis;

_ch_, notochord; _d_, intestine; _f_, connective tissue; _fh_, cavity of

dorsal fin-ray; _m_, myotome; _n_, nerve-tube; _p_, atrium; _sf_,

metapleural folds; _sfh_, lymph space in metapleural folds; _si_,

sub-intestinal vein; _sk_, sheaths of notochord and nerve-tube; _sl_,

sub-atrial ridge; _sp_, coelom. (From Korschelt and Heider, after Lankester

and Willey.)]

The mouth now becomes median and ventral, and is reduced in size, the oral

hood (stomodaeum) is formed in front of it, the gill-slits become more

numerous and vertically elongated, the endostyle forms along the floor of

the pharynx, and the gonads grow as paired pouches from the body-wall. This

brings the animal to the young adult condition, reached at a period of

probably about three months after the fertilisation of the egg.

The development as a whole shows a very marked resemblance {137}to that of

the Tunicata (see p. 55), but lends no support to the view that Amphioxus

has degenerated from a higher group of the Vertebrata.

CLASSIFICATION OF THE CEPHALOCHORDATA.

The known species of Amphioxus may be classified as follows[116]:—

FAMILY BRANCHIOSTOMATIDAE.

Genus 1. _Branchiostoma_ (Costa, 1834).

Having biserial gonads and symmetrical metapleura.

_B. lanceolatum_ (Pallas)—Myotomes 36 + 14 + 12, gonads 23-29 pairs:

Mediterranean, N.W. Europe, Ceylon, E. of United States.

[_B. belcheri_, Gray—Myotomes 38 + 17 + 9: Torres Straits, Singapore,

Borneo, Ceylon.

[_B. nakagawae_, Jord. and S.—Myotomes 37 + 16 + 11: Japan.

[_B. caribbaeum_, Sundevall—Myotomes 37 + 14 + 9: West Indies, Atlantic,

N. and S. America.

_B. capense_, Gilchrist—Myotomes 47 + 19 + 9: S. Africa.

_B. californiense_, J. G. Cooper—Myotomes 45 + 17 + 9: California.

_B_. (_Dolichorhynchus_) _indicum_ (Willey)—Myotomes 42 + 14 + 15: India

and Ceylon.

(?) _B. elongatum_, Sundevall—Myotomes 49 + 18 + 12: Peru.

(?) _B. pelagicum_, Günther—Myotomes 36 + 16 + 15: Honolulu, Gulf of

Manaar, South Indian Ocean.

Genus 2. _Asymmetron_ (Andrews, 1893).

With uniserial (right) gonads and asymmetrical metapleura.

_A. lucayanum_, Andrews—Myotomes 44 + 9 + 13: Bahamas, Maldives,

Zanzibar.

_A. caudatum_ (Willey)—Myotomes 40 + 9 + 11: Louisiade Archipelago.

_A_. (_Heteropleuron_) _bassanum_ (Günther)—Myotomes 45 + 16 + 14: Bass

Straits, Australia.

" _cingalense_ (Kirkaldy)—Myotomes 39 + 16 + 8: Ceylon.

" _cultellum_ (Peters)—Myotomes 32 + 10 + 10: Torres

Straits, Australia, Ceylon.

" _maldivense_ (F. Cooper)—Myotomes 45 + 16 + 12: Maldive

Archipelago, Zanzibar.

" _hectori_ (Benham)—Myotomes 53 + 19 + 12: New Zealand.

{138}Thus sixteen species have been described, of which the three under

_Branchiostoma_ placed after square brackets, seem to be merely varieties

of _B. lanceolatum_, and _B. nakagawae_ is probably identical with _B.

belcheri_; while it is a question whether _Asymmetron caudatum_ is more

than a variety of _A. lucayanum_, thus leaving eleven or twelve species

that seem fairly well characterised. The exact positions of the two marked

(?), viz. _B. elongatum_ and _B. pelagicum_, cannot be determined in the

absence of fuller descriptions of these species.

[Illustration: FIG. 90.—Sketch-map showing geographical distribution of the

Cephalochordata. + indicates _Branchiostoma_; o indicates _Asymmetron_.]

The list above, and the map (Fig. 90), give some indication of the

geographical distribution of the group, and show that, although the few

species are widely distributed over the shallow waters of the globe, most

of the records lie between 40° N. and 40° S. latitudes. In fact the group

is mainly a tropical one, and is most abundant in the Indo-Pacific region.

The crosses indicate records of species of _Branchiostoma_, and the circles

those of _Asymmetron_ (including _Heteropleuron_); the latter are confined

to the Indo-Pacific seas, with the exception of _A. lucayanum_ from the

Bahamas—one of the numerous cases of interesting similarity between the

marine faunas of the East and West Indies.

FISHES

(EXCLUSIVE OF THE SYSTEMATIC ACCOUNT OF TELEOSTEI)

BY

T. W. BRIDGE, Sc.D., F.R.S.

Trinity College, Cambridge; Mason Professor of Zoology and Comparative

Anatomy in the University of Birmingham

{141}CHAPTER V

THE SYSTEMATIC POSITION AND CLASSIFICATION OF FISHES

In the first chapter of this volume it was pointed out that the Craniata,

of which the Fishes form a subordinate group, is the last of the four

principal divisions into which the Chordata are divided. The animals

included in the first three, viz. the Hemichordata, the Urochordata, and

the Cephalochordata, have already been dealt with in the earlier chapters,

and it now remains for us briefly to consider the diagnostic characters of

the Craniata, and then, more in detail, the organisation of the Fishes.

The Craniata, often termed Vertebrata, form one of the best defined and

most easily recognisable divisions of the animal kingdom. As the name

implies, they are distinguished from the more primitive Chordata by the

formation of a definite "head," as the result of the modification of the

anterior portion of the central nervous system to form a complex brain,

round which are concentrated the chief organs of special sense. This is

combined with the evolution of a skull, which, in addition to providing a

"cranium" for the enclosure and protection of the brain, and partial or

complete capsules for the sense-organs, is connected behind with a system

of bony or cartilaginous visceral arches, which loop round the pharynx

between the gill-clefts. Besides supporting the breathing organs (gills) in

the lower aquatic Craniata, or existing as embryonic vestiges in the higher

lung-breathing forms, these arches usually form the basis of jaws for the

mouth. The epidermal portion of the superficial skin is always composed of

several layers of cells. The notochord, which is always present in the

embryo, and in a few Craniates, both living and extinct, may even be

retained in its entirety in the adult, fails to reach {142}the anterior end

of the brain. In most Craniates, however, the notochord becomes more or

less completely replaced in the adult by the development round it of a

series of vertebrae, forming the backbone or vertebral column. Two pairs of

limbs, and cartilaginous or bony limb-girdles for their support, are very

generally present.

The segmentation, or serial repetition of certain organs of the body, which

is so marked a feature in the Cephalochordata, is also characteristic of

the Craniata. Examples of this may be seen in the division of the lateral

longitudinal muscles of the body wall into muscle-segments or myotomes by a

series of transverse fibrous septa; in the formation of the vertebral

column by a series of successive joints or vertebrae; in a similar serial

repetition of the cranial and spinal nerves, the gill-clefts and branchial

arches, certain blood-vessels, and the renal tubules. There is sometimes,

however, no precise regional or numerical correspondence between the

different organs which are successively repeated in this way, and hence it

is probable that, in at least some of the organs of the Craniate body, the

segmentation has been independently evolved in each case.

The pharynx is relatively much shorter than in other Chordata. The

gill-clefts are few in number, whether, as in the lower Craniata, they are

retained as the functional breathing organs, or are present, as vestiges

only, in the embryos of the higher members of the group. In no instance are

they subdivided by the growth of "tongue-bars" or "synapticula," nor do

they open externally into an atrial or peribranchial cavity. The liver is a

massive compound tubular gland, never, in the adult at all events, a simple

caecal sac; and usually there is a pancreas and a spleen.

A spacious epithelium-lined body cavity or coelom, which, as regards its

origin, may be regarded as a "syncoelom,"[117] surrounds the alimentary

canal and separates it from the body wall. From the epithelial walls of the

coelom are derived the gonads (ovaries and testes), which in the adult are

limited to a single pair; while paired and often segmentally-arranged

lateral tubular outgrowths from it (renal tubuli) acquire a glandular

character and form the basis of the excretory or kidney system. A special

portion {143}of the coelom also surrounds the heart and forms a pericardial

cavity, and in some Craniata the genital ducts may be formed from its

lining membrane.

There is always a muscular heart, consisting of at least three chambers, a

sinus venosus, an auricle and a ventricle, and formed by a modification of

the initial portion of the ventral or cardiac aorta of the Cephalochordata.

The disposition of the great blood-vessels is based on a common plan in all

Craniata, and the blood which circulates in them is red in colour owing to

the presence of red, haemoglobin-containing corpuscles in addition to the

colourless leucocytes which alone are present in the Cephalochordata.

Ductless blood-glands of various kinds (spleen, thyroid, thymus, inter- and

ad-renal bodies) are very generally present, and modify in different ways

the character of the blood as it circulates through them. Besides

blood-vessels there is also a somewhat similar system of lymphatic vessels

distributed throughout the organs and tissues of the body, which serves the

purpose of re-collecting the fluid portion of the blood that has diffused

from the blood-vessels for the nutrition of the tissues, and conveying it

back to the blood vascular system. These lymphatics contain lymph, a fluid

comparable to dilute blood plasma, in which leucocytes float. In addition

to their continuity with the blood-vessels at certain points, the lymphatic

vessels may also communicate with the coelom, and hence the Craniata must

be included among those somewhat rare exceptions to the general rule that

no connexion exists between the series of blood-containing channels and the

coelom.

In the excretory system the renal tubuli in the adult Craniata rarely

retain their primitive embryonic communication with the coelom, and in no

instance have they separate and independent external apertures; on the

contrary, by the union of their outer or distal extremities, common

efferent ducts are formed, which either open into a "cloaca," or directly

on to the exterior of the body near the anus.

In all Craniates the dorsally-placed and tubular central nervous system has

its anterior portion enlarged and otherwise modified to form a "brain,"

while the remaining portion, retaining a simpler and more uniform

structure, forms the spinal cord. In the embryo the brain always consists

of three successive sac-like enlargements known as the fore-, mid-, and

hind-brain, and from {144}these are developed the various parts of the

complex adult brain, which in the disposition and mutual relations of its

parts conforms to a common plan in all the members of the group. There are

at least ten pairs of cranial nerves having their origin from the brain,

and, in addition, a varying number of spinal nerves arising from the spinal

cord, and as a rule formed in each case by the union of a mainly sensory,

ganglionated, dorsal root with a mainly motor, non-ganglionated, ventral

root.

The median and usually vestigial, parietal, or pineal eye may sometimes be

retained as a functional organ, but there exist in all Craniates, in

addition, paired eyes, the sensory portion of which, the retina, is derived

as an outgrowth from the first of the primary embryonic brain-vesicles. To

these organs of special sense are added a pair of auditory organs, and a

pair of olfactory organs, besides, in the lower aquatic Craniates, the

peculiar sensory organs of the "lateral line."

The gonads are reduced to a single pair in the adult, although it is

possible that they may have a multiple origin in the embryo. Gonoducts for

the discharge of the sex-cells are almost invariably present, and may owe

their origin either to a change of function on the part of certain

kidney-ducts, or to independent evolution from the lining membrane of the

coelom. The ova are generally provided with a large amount of nutritive

reserve in the shape of food-yolk, and hence the process of segmentation is

frequently partial or "meroblastic," but in some groups, in which the ova

have less food-yolk, it is complete or "holoblastic." The typical

invaginate gastrula stage, which is so striking a feature in the embryonic

history of the lower Chordata, occurs also in a few of the lower Craniates,

but in most of them it is apt to become masked or modified in various ways

by the presence of a superabundant amount of food-yolk.

Functional hermaphroditism is of very rare occurrence in Craniates, and, as

in the Cephalochordata, reproduction by budding and the formation of

colonies are unknown.

Thus distinguished from other Chordata, the Craniata are divided into six

"classes," which may be variously grouped, as the following table shows:—

{ } {145}

{ I. CYCLOSTOMATA. } AGNATHOSTOMATA.

{ Lampreys and } Without biting

{ Hag-Fishes. } jaws.

ICHTHYOPSIDA. {

Breathing by gills at some { }

period of life. { }

{ II. PISCES. }

{ True Fishes. }

ANAMNIOTA { }

No embryonic covering { }

or amnion. { }

{ III. AMPHIBIA. }

{ Newts, Frogs, and }

{ Toads. }

ANALLANTOIDEA. { }

No embryonic respiratory { }

organ or allantois. { } GNATHOSTOMATA.

{ } With biting

} jaws.

{ IV. REPTILIA. }

{ Lizards, Snakes, }

{ Turtles, and }

{ Crocodiles. }

{ SAUROPSIDA. { }

AMNIOTA. { { }

Amnion present. { { V. AVES. }

{ { Birds. }

{ }

ALLANTOIDEA. { }

Allantois present.{ VI. MAMMALIA. }

{ Hairy Quadrupeds. }

Apart from the distinctive characters of the six "classes" into which the

Craniata are divided, two or three of these classes may possess important

structural features in common by which they are distinguished from others.

Thus, Cyclostomata, Fishes and Amphibia agree with one another, and differ

from all the remaining groups in breathing by gills and in possessing

lateral line sensory organs during part, or the whole, of life. Their

embryos have no investing amnion, neither does the sac-like outgrowth from

the hind-gut, which is known as the allantois, if present at all, ever

extend beyond the coelom to form an embryonic investment or to act as a

primitive breathing organ. Hence, therefore, the terms Ichthyopsida,

Anamniota, and Anallantoidea have been applied to these three classes.

Similarly, the term Sauropsida, as applied to Reptiles and Birds, is a

convenient means of giving expression to the fact that, underlying the most

striking diversity of outward form and habits, there is a community of

inward structure which justifies the conclusion that these animals are more

closely related to one another than either group is to any other class of

Craniates. And again, the application of the terms Agnathostomata and

Gnathostomata brings into sharp relief the fundamental distinction between

the Cyclostomata and all the remaining groups of Craniata which is

{146}only partially illustrated by the presence or absence of biting jaws.

In a general and popular sense the Cyclostomata are usually regarded as

"Fishes," but this usage rests on no better foundation than a certain

agreement between the Cyclostomata and the true Fishes in outward form and

habits, and in their method of respiration by gills. On the other hand, it

has been maintained that the distinctive features of the Cyclostomata are

of sufficient importance not merely to separate them from the true Fishes,

but possibly even (as is to some extent expressed by the use of the terms

Agnathostomata and Gnathostomata) to warrant their elevation to a group

equal in taxonomic value to all the remaining living Craniata taken

collectively. The organisms included in the Cyclostomata, the Lampreys, and

especially the Hag-Fishes, exhibit in many respects an extremely low grade

of Craniate structure; but how far the simplicity or archaic nature of some

of their organs is primitive, or has been acquired through degeneration, it

is difficult, and is sometimes impossible, to determine with any degree of

satisfaction. In other respects, such as the presence of a rasping

"tongue," it is obvious that the Cyclostomata have attained a high degree

of specialisation. As one of several illustrations which might be given of

difficulties of this kind, it may be mentioned that it is by no means

certain that the Cyclostomata are not the degenerate descendants of

primitive but now extinct Gnathostomata. At all events the presence of

paired cartilages in the skull of the Lamprey, which, with some show of

reason, may be regarded as representatives of the primitive upper and lower

jaws of the latter group, would seem to suggest this conclusion. If this be

correct, we must regard the formation of a suctorial buccal funnel, with

its complex system of supporting cartilages—one of the most striking

features in the structure of this animal—as a secondary and adaptive

specialisation of a mouth originally provided with biting jaws. But in

spite of such difficulties there can be no question that the Cyclostomata

are the most primitive of all existing Craniates, and so far differ from

the true Fishes and from all other classes of Craniate animals, that their

inclusion in a class by themselves is the least that can be done to give

graphic expression to their isolated position, even if we do not fully

accept the dictum of {147}Haeckel that "they are further removed from

Fishes than Fishes from Man."

Briefly stated, the Cyclostomata or Agnathostomata are distinguished from

"Fishes" and all the remaining Craniata (Gnathostomata) by the following

characters:—

The mouth is either nearly terminal, as in the Hag-Fishes (_Myxine_); or,

as in the Lampreys (_Petromyzon_), it opens out of a spacious, pre-oral,

suctorial, buccal funnel, which, in its relations to the hypophysis or

pituitary body, recalls the pre-oral buccal cavity of the Cephalochordata.

As in Amphioxus, the hypophysis[118] is displaced dorsally by the forward

growth of the pre-oral portion of the head in the embryo, and consequently

it only attains its normal relations to the infundibular downgrowth[119]

from the ventral surface of the fore-brain by perforating the floor of the

skull from above instead of from below as in all other Craniates. In one

section of the group (e.g. _Myxine_) the hypophysis opens into the oral

cavity, and serves as a tubular passage for the inspiratory water-current

to the gill-sacs, a feature in which these Cyclostomes are unique. The

apparently median olfactory organ is carried inwards with the hypophysial

involution, and communicates with the latter throughout life. A primitive

upper jaw (palato-quadrate cartilages or sub-ocular arches) is present, and

in at least some Cyclostomes (_e.g._ the Lampreys), and possibly in all,

there are structures which very probably represent a primitive lower jaw

(Meckel's cartilages); but such structures are always non-biting, and

merely form skeletal supports for other portions of the skull. In place of

biting jaws the mouth is provided with a complex rasping lingual apparatus

supported by special cartilages, the so-called tongue, which bears horny

teeth and is capable of protrusion and retraction. Paired limbs are

entirely wanting.

In the Gnathostomata, on the contrary, there is no buccal funnel, and the

mouth, whether terminal or ventral in position, opens directly outwards.

The hypophysis is usually carried inwards with the stomatodaeal

invagination which in the embryo gives rise to the mouth, and is therefore

from the first in relation with the ventral surface of the brain. Biting

jaws (palato-quadrate and Meckelian cartilages), formed by the modification

of an anterior and primitively gill-bearing visceral arch, are

{148}invariably present. The olfactory organs are obviously paired, and

they are distinct from the hypophysis. Paired limbs are present.

As previously stated, the true Fishes form the second of the six "classes"

into which the Craniata are divided. As compared with the higher Craniata,

their distinctive characters may be concisely stated as follows:—

Fresh water or marine Gnathostomata, which in their shape and in method of

breathing are adapted for an aquatic life. Throughout life their

respiratory organs are in the form of vascular processes (gills) derived

from the walls of the branchial clefts, and supported by a series of

branchial arches. The principal organ of locomotion is the powerful

muscular tail; in addition, however, there are paired fins, pectoral and

pelvic, corresponding to the fore- and hind-limbs of the terrestrial

Craniata, and possessing a supporting cartilaginous or bony skeleton

("ichthyopterygium") which cannot readily be compared with the

limb-skeleton of the latter. Fishes also possess a system of median fins,

supported by a special skeleton of their own. An exoskeleton of dermal

spines or denticles, scales or bony plates, is usually present. Except in

one group, the Dipnoi, the heart has but one auricle, and receives only

venous blood, which it forces, first, through the blood-vessels of the

gills, and thence, as arterial blood, through the vessels of the body

generally. An air-bladder is frequently present, and serves as a

hydrostatic organ or float, but in a few cases it may act as a lung, and

helps the gills in the work of respiration. The paired olfactory organs

rarely communicate with the oral cavity by internal nostrils. Peculiar

cutaneous sense-organs are disposed in linear tracts along the sides of the

body (lateral line sensory organs), and on the head, and appear to be

specially associated with a life in water.

Fishes may be divided into the following "sub-classes," and these in turn

may be subdivided into various "orders" and "sub-orders":—

(i.) ELASMOBRANCHII; _e.g_. Sharks, Dog-Fishes, Skates, and Rays.

(1) Pleuropterygii†; e.g. _Cladoselache_.

(2) Ichthyotomi†; e.g. _Pleuracanthus_.

(3) Acanthodei†; e.g. _Acanthodes_.

(4) Plagiostomi.

(_a_) Selachii; _e.g_. many extinct and all living Sharks and

Dog-Fishes.

(_b_) Batoidei; _e.g_. Skates and Rays.

(5) Holocephali; e.g. _Chimaera_ and _Callorhynchus_.

(ii.) TELEOSTOMI; _e.g_. such well-known Fishes as the Perch, Cod,{149}

Salmon, and Herring, and also the less familiar "Ganoids,"

living and extinct.

(1) Crossopterygii; e.g. _Polypterus_.

(2) Chondrostei; _e.g_. the Sturgeons (_Acipenser_).

(3) Holostei; _e.g_. the Bow-fin (_Amia_), and the Gar Pike

(_Lepidosteus_).

(4) Teleostei; _e.g_. the Perch, Cod, Salmon, etc.

(iii.) DIPNOI; e.g. _Neoceratodus, Protopterus_, and _Lepidosiren_.

_Appendix to the Class Pisces_.

(i.) PALAEOSPONDYLIDAE†; e.g. _Palaeospondylus_.

(ii.) OSTRACODERMI†.

(1) Heterostraci; e.g. _Pteraspis_.

(2) Osteostraci; e.g. _Cephalaspis_.

(3) Anaspida; e.g. _Birkenia_.

(iii.) ANTIARCHI†; e.g. _Pterichthys_.

(iv.) ARTHRODIRA†; e.g. _Coccosteus, Dinichthys_.

† Entirely extinct.

The Fishes included in the Teleostomi were formerly arranged in two groups:

the Ganoidei, including the Crossopterygii, Chondrostei, and the Holostei,

with their numerous fossil allies; and the Teleostei. Living Ganoids agree

with one another, and differ from Teleosts in possessing an intestinal

spiral valve and a conus arteriosus. It is difficult, however, to separate

the two groups, inasmuch as in each group there are living forms which tend

to approximate to the other; and numerous fossil genera, of whose soft

parts nothing is known, are in many respects intermediate between the two.

The position and relationships of the Palaeospondylidae, Ostracodermi,

Antiarchi, and Arthrodira are very uncertain. The Palaeospondylidae have

been included in the Cyclostomata, or at all events have been regarded as

more or less closely related to that group, while the absence of paired

fins and the apparent want of jaws have suggested that the Ostracodermi

occupy an intermediate position between the Cyclostomata and the

Gnathostomata.[120] On the other hand, the Arthrodira are either regarded

as an independent group of Fishes, or are included amongst the Dipnoi. In

the latter case, the Dipnoi are divided into the Arthrodira and the

Sirenoidei, the last mentioned group including _Neoceratodus, Protopterus_

and _Lepidosiren_, and their extinct allies.

{150}CHAPTER VI

EXTERNAL CHARACTERS OF CYCLOSTOMATA AND OF FISHES

EXTERNAL CHARACTERS—COLORATION—POISON GLANDS AND POISON SPINES—

PHOSPHORESCENT ORGANS.

[Illustration: FIG. 91.—_Petromyzon marinus_. A, ventral; B, lateral; and

C, dorsal, view of the head. _br.cl.1_, First branchial cleft; _buc.f_,

buccal funnel; _eye_, the eye; _mth_, mouth; _na.ap_, nasal aperture; _p_,

papillae; _pn_, pineal area; _t^1_, _t^2_, _t^3_, teeth of buccal funnel;

_t^4_, teeth on the tongue. (From Parker and Haswell, after W. K. Parker.)]

In all the Cyclostomata the body is Eel-like in shape, the head and trunk

being nearly cylindrical, and the tail somewhat flattened from side to

side. In _Petromyzon_ the head terminates in a ventrally-directed,

funnel-like cavity—the buccal funnel—in the roof of which the relatively

small mouth is situated (Fig. 91, A.). The margin of the funnel is fringed

by a series of short papillae, {151}but in the Hag-Fishes (_Myxine_ and

_Bdellostoma_), where a buccal funnel is not developed, longer

tentacle-like structures are present on each side of the mouth. On the

upper surface of the head is the single median nostril, or naso-pituitary

aperture, placed between the eyes in the Lampreys (Fig. 91, B, C), but at

the anterior margin of the head in _Myxine_ and its allies (Fig. 92). In

the living Lampreys a semi-transparent area of skin may be noticed behind

the nasal organ, which coincides with the position of the more

deeply-seated parietal eye. On each side of the body, commencing a short

distance behind the eye, is a series of small and almost circular branchial

clefts (_Petromyzon_, _Bdellostoma_). In _Myxine_, however, the clefts of

each side have a single common external aperture, situated on the ventral

side of the body and some distance behind the head (Fig. 92, A). At the

junction of the trunk with the tail is the anus, behind which is the

papilla which carries the urino-genital aperture at its extremity. There

are no paired limbs or vestiges of such organs. Median fins are represented

in the Lampreys by an anterior dorsal fin and a posterior dorsal fin, the

latter being continuous with the caudal fin which fringes the upper and

lower margins of the protocercal tail. In _Myxine_ a caudal fin only is

present, surrounding the extremity of the tail.

[Illustration: FIG. 92.—Head of _Myxine glutinosa_ (A), and of _Bdellostoma

forsteri_ (B), from beneath. _br.ap_, Left external branchial aperture;

_br.cl.1_, first branchial cleft; _mth_, mouth; _na.ap_, nasal aperture;

_oes.ct.d_, oesophageo-cutaneous duct. The smaller openings in A are those

of mucous glands. (From Parker and Haswell, after W. K. Parker.)]

{152}[Illustration: FIG. 93.—_Tilapia dolloi_. To show the external

characters of an Acanthopterygian Teleost. A, side view; B, the first

branchial arch. _a.f_, Spinose part of the anal fin; _a.f^1_, soft rays;

_c.f_, caudal fin; _d.f_, spinose portion of the dorsal fin; _d.f^1_, soft

rays; _g.f_, gill filaments; _g.r_, gill rakers; _i.l.l_, inferior lateral

line; _n_, nostril; _p.f_, pelvic fin; _p.op_, preoperculum; _pt.f_,

pectoral fin; _s.l.l_, superior lateral line; _t.s_, transverse row of

scales. (From Boulenger.)]

In Fishes the characteristic shape of the body is more or less that of a

spindle, tapering at each end and somewhat flattened from side to side;

and, as a rule, the three regions of the body—head, trunk, and tail—pass

almost imperceptibly into one another (Fig. 93, A). Nevertheless, there is

great diversity of form in different Fishes. Compare, for example, the

elongated, cylindrical shape of the Eels (which is perhaps associated with

their habit of insinuating themselves into holes and crevices, and their

undulatory, snake-like movements when swimming); the compressed, band-like

shape of the Ribbon-Fishes (Trachypteridae); the flattened bodies of those

Fishes which habitually live and move on the bottom, like the Skates and

Rays; the thin, laterally-compressed bodies, often nearly as high as long,

of the Flat-Fishes (Pleuronectidae), which always swim and rest on either

the right or left side; the almost spherical Globe-Fishes (_Tetrodon_)

which often float passively in the water; and the singular rectangular,

coffin-like Coffer-Fishes (_Ostracion_). There is also much difference in

the relative proportions of the three regions of the body in different

Fishes, as witness the enormous size and {153}grotesque appearance of the

head of the Angler-Fish (_Lophius_); the huge high trunk and abbreviated

tail of the Sun-Fish (_Orthagoriscus_); and the short high trunk and long

tail of _Notopterus_ (Fig. 334).

In its external appearance the head perhaps differs more in different

Fishes than any other part of the body. Long and flattened in the Skates

and Rays, the head becomes short and high in most Holocephali and in many

Teleosts, or is shaped like a blunt cone, as in such Dipnoi as

_Protopterus_ and _Lepidosiren_; or becomes long and pointed, as in the

North American "Gar Pike" (_Lepidosteus_); or, finally, as in the

Hammer-head Shark (_Sphyrna_), the head may be produced into great lateral

extensions, carrying the eyes at their extremities (Fig. 256, B). Apart

from its relative shape and size, the appearance of the head may be further

modified by the thinness of the investing scaleless skin, which readily

allows the surface and contour lines of the bones of the skull to be seen

through it, as in the Crossopterygii, and in such Teleosts as the Siluroid

genera _Clarias_ and _Callichthys_; or the skin, even if devoid of scales,

may be so thick that scarcely any of the bones are visible externally. The

exoskeleton, whether in the form of scales or bony plates, may extend to a

varying degree on to the surface of the head in different Teleosts, or may

even invest nearly the whole of the head. When, as is not infrequently the

case (_e.g._ many Scorpaenidae) certain of the bones of the skull are

produced into projecting spines, the head assumes a singularly formidable

appearance (Fig. 424).

The mouth differs greatly in size and position. In existing Elasmobranchs

it is generally crescentic in shape and always ventral in position, but in

certain primitive fossil members of the group, as in the Palaeozoic

_Cladoselache_, it is anterior and terminal. The Sturgeon and other living

Chondrostei have the mouth ventral. In the Dipnoi also the mouth is

ventral, but is near the extremity of the snout. As a rule, the mouth is

terminal or nearly so in the living Crossopterygii and Holostei, and in the

great majority of Teleosts, although in the latter group it is occasionally

distinctly ventral, especially when a snout is developed, and it may

sometimes look upwards by reason of the projection of the lower jaw in

front of the upper. A pronounced "beak" is sometimes formed by the forward

prolongation of both jaws, as in the Gar Pike (_Lepidosteus_), with the

result that the {154}vertical gape of the mouth is greatly increased, but

in a few Teleosts a beak may result from a forward extension of one jaw

only, the upper in the Sword-Fish (_Xiphias_) and the lower in the

"Half-Beak" (_Hemirhamphus_). A further modification is to be noted in many

Teleosts, in which, owing to the forward prolongation and inclination of

the skeletal supports of the jaws, the mouth is at the extremity of a

longer or shorter spout-like beak, and is then usually very small. This is

the case in the "Sea-Horse" (_Hippocampus_), the Pipe-Fishes

(_Syngnathus_), the "Flute-mouths" (_Fistularia_), and the Trumpet-Fish

(_Centriscus_), and especially in certain species of the African family

Mormyridae, where the pore-like mouth is at the extremity of a long,

tapering, downwardly-curved proboscis (Fig. 330). In many Teleosts the

mouth can be protruded and withdrawn at will by a sliding motion of the

bones of the upper jaw (premaxillae) on the anterior skull bones by which

they are supported. From this point of view the toothless mouth of the

Sturgeon is even more remarkable. By a forward or a backward swing of the

elements which form the upper half of the hyoid arch (hyomandibular and

symplectic) the mouth can be thrust downwards from the under side of the

head like a spout, when the Fish is feeding, and subsequently retracted. In

not a few Fishes the forepart of the head is prolonged forwards over the

mouth and jaws in the form of a rostrum or "snout"; it is, in fact, to the

growth of a snout that the ventral position of the mouth in Fishes is

generally due. This feature is more or less characteristic of most

Elasmobranchs, in which the snout forms a cut-water overhanging the mouth.

In the Holocephali the snout is short and blunt, except in _Harriotta_,

where it is pointed and unusually long. Among the Chondrostei the Sturgeon

has an exceptionally massive snout, the length and shape of which differs

in different species. In the allied _Polyodon_ the thin, flattened,

spoon-like snout is scarcely less than one-fourth the length of the body

(Fig. 289).

Simple or branched tactile filaments or "barbels" are present on different

parts of the head in many Teleostomi, sometimes at or near the chin, as in

certain Gadidae, like the Haddock and Cod, or on the under surface of the

snout, in front of the mouth, as in the Sturgeon. In the Siluridae (Fig.

356), where they are found in relation with the upper and lower jaws, and

even between the nostrils, these structures are often remarkably developed.

{155}The eyes of Fishes are usually very large. They are generally situated

on the sides of the head, but in the "Star-gazers" (_Uranoscopus_) they are

on the upper surface and close together. In the goggle-eyed

_Periophthalmus_ the eyes seem to protrude from their orbits, and in a

variety of a species of Carp, the Gold-Fish (_Cyprinus auratus_), the

protrusion is so marked that the eyes seem as if on stalks. In a few

species, which live either in caves or at very great oceanic depths, the

eyes become vestigial, and are hidden beneath the skin, or are even covered

by scales (Fig. 430).

In the Elasmobranchs and Dipnoi the olfactory organs retain their primitive

position as pit-like sacs on the ventral surface of the snout, just in

front of the mouth. In the Dipnoi (e.g. _Protopterus_) each olfactory sac

has two apertures, of which one, the external nostril, is placed on the

under surface of the snout, while the other, the internal nostril, opens

within the upper lip into the oral cavity—a feature which is unique among

Fishes. In nearly all Teleostomi, also, each sac has two nostrils, which,

however, are situated either on the upper surface or on the sides of the

fore-part of the head, and have no communication with the mouth.

Directly behind the head in Elasmobranchs, or beneath its hinder part in

all other Fishes, are placed the external apertures of the branchial

clefts. In the former group these apertures are visible externally in the

form of a series of narrow vertical slits, but in the latter they

communicate with the exterior by opening on each side into a common

branchial cavity, the outer wall of which is formed by a movable flap-like

fold with a free hinder margin and a special internal skeleton of

cartilaginous rays or of bony plates and rods, the gill-cover or operculum

(Fig. 161, B). Behind the free margin of the operculum there is a slit-like

orifice, the gill-opening or external branchial aperture, which curves from

above downward and forward toward the chin, and places the branchial cavity

in communication with the exterior. Through this aperture the water, which

has entered through the mouth, traversed the gill-clefts, and bathed the

gills, finds its exit from the body. The space on the ventral side of the

head between the two halves of the lower jaw, and between the two external

branchial apertures, is termed the "isthmus." The size of the external

branchial aperture differs greatly in different Fishes, according to the

extent to which the {156}free opercular margin fuses below with the

isthmus, or behind with the side of the head. Thus the aperture may extend

from the chin in front upward and backward to near the dorsal surface of

the head, or it may be reduced to little more than a mere pore situated on

any part of the opercular edge (e.g. _Hippocampus_); or, as in

_Symbranchus_, the reduced pores of opposite sides may coalesce in the

floor of the throat in a common median opening.

In the Elasmobranchs and in the Dipnoi the cloacal aperture is always

situated at the junction of the trunk with the tail. In the Teleostomi,

however, where the intestine has a separate external orifice or anus,

distinct from, and placed in front of, the separate or combined

urino-genital ducts, the anus may either retain its primitive position near

the union of the trunk and tail, or occupy almost any intermediate position

between this point and the throat.

Most Fishes possess both median and paired fins (Fig. 93, A). From an

evolutionary point of view the median fins have a far greater antiquity

than the paired fins. They appear before the latter in embryonic

development, and in the Cephalochordata, and such lower Craniates as the

Cyclostomata, they are the only fins which exist. The isolated median fins

of most Fishes are discontinuous remnants of a primitively continuous

structure, which, extending like a fringe along the median line of the

back, was thence continued round the end of the tail and forward along the

ventral surface as far as the cloacal or anal orifice. This primitive

condition, which, as we have seen, is characteristic of Amphioxus, is also

very general in the embryos and larvae of Fishes (Figs. 238 and 309), and

is more or less completely retained in the Dipnoi and in many adult

Teleosts, notably in those species in which the body is greatly elongated

and locomotion is effected by serpentine lateral undulations, as in the

Eels (Anguillidae), and in others which, either through their

quasi-parasitic or commensal habit (e.g. _Fierasfer acus_), or by reason of

a peculiar environment, as in certain deep-sea Fishes (Fig. 430) are

distinguished by the retention of many larval features. More generally,

however, the continuity of the fin becomes interrupted, and that portion of

it which surrounds the extremity of the tail is the first to become

separated from the rest as a caudal fin (Fig. 429). By further

interruptions the remaining dorsal portion may become divided into two or

three isolated {157}dorsal fins (Fig. 398), or even into a series of

isolated finlets; and similarly also with the ventral portion or anal fin;

or, without undergoing subdivision, both fins may become reduced in length

to an extent which differs greatly in different Fishes, and persist as

single dorsal or anal fins. But even when a median fin is reduced in length

by atrophy, or becomes subdivided by breaches in its continuity, the

externally invisible supporting radial elements frequently remain to prove

the originally greater length of the fin, or the continuity of its detached

remnants.

Like the median fins, the paired fins may also be regarded as discontinuous

remnants of an originally continuous _lateral_ fin which extended along

each side of the body from the head to the vent, and of which only the

anterior and posterior portions now remain as the pectoral and pelvic fins.

Pectoral fins are rarely absent in existing Fishes, and when present they

are always situated just behind the branchial clefts, where, as in most

Teleostomi, the outline of their supporting pectoral girdle can often be

seen. They vary greatly in form and size in different Fishes, and in the

Elasmobranchs are larger than in most others. In certain members of the

latter group, the Skates and Rays, in which the feebly-developed tail is

probably useless as a locomotor organ, the pectoral fins are exceptionally

large, forming broad triangular lobes, the broad bases of which are

continuous with the sides of the body from the anterior part of the head to

near the origin of the pelvic fins, and thus in outward form, if not in

inward structure, simulate re-acquired continuous lateral fins. Except in a

few instances, the Teleostomi have relatively small fan-shaped or

paddle-like pectoral fins, and usually only that portion of each fin which

is supported by the dermal fin-rays is visible externally. In the

Crossopterygii, however, each fin appears to consist of a central lobe

invested by scales and encircled by a peripheral fringe of fin-rays, and is

hence described as a "lobate" fin (Fig. 279). When the central lobe is much

increased in length but reduced in width the fin becomes acutely lobate. A

similar type of fin is present in the Dipnoi, but in _Protopterus_ and

_Lepidosiren_, owing to the length and narrowness of the central lobe, and

the reduction or suppression of the marginal fringe, the pectoral members

assume the condition of long tapering filaments (Fig. 304).

{158}Although as a rule smaller in size, the pelvic fins bear a general

resemblance to the pectoral fins, but in certain groups, especially in

Teleosts, they are liable to undergo extraordinary changes in position,

and, as will be seen presently, are much more prone to exhibit the effects

of adaptive modification and degeneration. They are present in all existing

Fishes, with the exception of the Crossopterygian _Calamichthys_ and some

Teleosts, and, except in the Teleostei, they invariably retain their

primitive position near the junction of the trunk with the tail, and

directly in front of the cloacal or the anal aperture; in this position

they are said to be "abdominal." In other Teleostei the fins undergo

forward displacement and come to lie directly beneath the pectorals (Fig.

415), when they are said to be "thoracic," as in the Mackerels (Scombridae)

and the Horse-Mackerels (Carangidae); or even in front of the pectoral fins

on the under surface of the throat, when they are described as "jugular,"

as in the Cod and other Gadidae (Fig. 398).

Both the median and the paired fins are supported by an internal skeleton,

consisting (i.) of a series of cartilaginous or bony, rod-like radial

elements or pterygiophores, for the support of the inner or proximal

portion of the fins, and (ii.) of a series of horny fibres, or bony dermal

fin-rays, which fulfil a like function for the outer or distal portion. The

radial elements, however, are never visible externally, even when, as in

most Elasmobranchs, they support the greater part of the fins, inasmuch as

they are invested by the fin-muscles and the skin; and in the same group,

where horny fibres complete the fin-skeleton, they too are covered by the

spinose skin, and hence offer no external evidence of their existence. In

the Teleostomi a marked reduction in the number and length of the radial

elements of the paired fins, and the insinking of those pertaining to the

median fins into the adjacent muscles of the body-wall, leaves the dermal

fin-rays, with their thin covering of transparent and usually scaleless

skin, as obvious features in the external appearance of the Fish, and

apparently as the sole support of the fins.

The dermal fin-rays of the Teleostomi exhibit an obvious distinction into

spines and soft rays (Fig. 93, A). The former are stout, rigid, and

unbranched structures, pointed at their free distal ends, which, in numbers

differing in different genera and species, support the anterior portions of

the dorsal, anal, and pelvic fins. {159}Soft rays are flexible, branched

distally, and generally exhibit a transversely-jointed structure; when

present in conjunction with spines they invariably lie behind the latter.

The presence of both kinds of fin-rays, or of soft rays only, is one of the

more obvious distinctions between the Teleostean groups of the

Acanthopterygii and the Malacopterygii, of which the Perch and the Salmon

respectively are well-known examples. Powerful spines are frequently

developed in front of the dorsal fin in many living and extinct

Elasmobranchs, and, under the general term of "ichthyodorulites,"

constitute the sole fossil remains of many extinct Devonian and

Carboniferous genera.

The caudal fin and the terminal portion of the tail exhibit interesting

modifications which are highly characteristic of particular groups of

Fishes. In the embryonic and early larval stages of most Fishes the

tapering caudal extremity retains its coincidence with the axis of the

body, and divides the caudal fin into two equal portions, a dorsal and a

ventral lobe, the two being continuous round the tip of the tail; and this

condition, which is certainly the most primitive, is termed "protocercal"

or "diphycercal" (Figs. 238 and 309). Such a symmetrical tail, as we have

seen, is retained in the Cyclostomata, and was also present in certain

extinct palaeozoic Sharks (e.g. _Pleuracanthus_), but it may be doubted if

any existing Fish has a tail which is truly and primitively diphycercal.

The Dipnoi (Fig. 304) and the Crossopterygii, including fossil

representatives of both groups, and perhaps a few Teleosts, seem to

approach this condition; but it is by no means certain that the apparent

symmetry is primitive, and has not been secondarily acquired. In other

Fishes the terminal part of the tail, including also its section of the

vertebral column, is bent upwards, and is fringed along its upper border by

the reduced dorsal lobe of the caudal fin, which, nevertheless, retains its

continuity with the ventral lobe round the tip of the tail. The latter, or

rather its hinder portion, is strongly developed, but, owing to the

prolongation of the up-tilted caudal axis beyond it, the dorsal lobe

appears longer than the ventral, and hence there is a marked want of

symmetry between the upper and lower division of the caudal fin (Fig. 253,

A). The Ostracodermi, all living and nearly all extinct Elasmobranchs, the

Acanthodei, Holocephali, some extinct Dipnoi, and amongst the Teleostomi,

{160}the living Chondrostei and certain extinct Crossopterygii, afford

examples of this unsymmetrical or heterocercal type of tail. A third type

is the "homocercal." In this type the caudal fin appears externally as if

perfectly symmetrical, the supporting fin-rays radiating from the blunt

extremity of the tail in such a way that a prolongation of the axis of the

body appears to divide the fin into equal-sized and continuous upper and

lower lobes (Fig. 343). Dissection, however, reveals the fact that the

terminal portion of the vertebral column is bent upwards as in the

heterocercal tail, and that while the dorsal lobe is almost vestigial, the

ventral lobe is enormously developed, and its supporting rays so inclined

backwards parallel to the axis of the body as to form practically the whole

of the caudal fin, with the exception of the dorsal border, which is formed

by the few remaining fin-rays of the dorsal lobe (Fig. 140). A homocercal

tail, therefore, is a disguised or masked heterocercal tail. It is

specially characteristic of Teleosts, and is closely approached in the

Holostean genera _Lepidosteus_ (Fig. 299) and _Amia_, which offer an

interesting transition from the heterocercal to the homocercal types; and,

singularly enough, even the heterocercal tail of the Palaeozoic Shark

_Cladoselache_ (Fig. 249), seems as if it had undergone some degree of

independent specialisation in the same direction. The homocercal tail

exhibits much diversity of form in different Teleosts, sometimes being

rounded or lancet-shaped, and sometimes having a deeply-forked hinder

margin. One of the Ribbon-Fishes, _Trachypterus taenia_, is singular in

having the caudal fin on the dorsal side of the tip of the tail, and

directed upwards like a fan. In some Teleosts, again, there is no

recognisable upward deflection of the terminal portion of the vertebral

axis, and the caudal fin-rays seem to be derived in equal proportions from

the dorsal and ventral lobes of the fin (Fig. 414). This apparently

diphycercal tail is probably a secondary acquisition, and may be considered

due to the atrophy of the terminal portion of the vertebral column, and the

subsequent coalescence of the dorsal and ventral lobes of the caudal fin

round the extremity of a more or less abbreviated tail. It is even possible

that in some Fishes the proper caudal fin has completely atrophied, and

that the apparent caudal fin has really been formed by a similar

modification affecting the hinder portions of the dorsal and anal fins. In

the extinct Crossopterygian genera, _Coelacanthus_, {161}_Diplurus_, and

_Undina_ (Fig. 278), there is evidence that the latter modification has

actually taken place, for the atrophying terminal part of the tail, with a

vestige of the original caudal fin, is still retained as an axial

prolongation between and even beyond the secondarily formed caudal fin. To

this secondary diphycercal tail the term "gephyrocercal" has been applied.

The apparent diphycercal tail of many Fishes, and especially of Teleosts,

is really a gephyrocercal structure. The ancestral evolution of the

different types of caudal fin is recapitulated in the embryonic histories

of their possessors. The heterocercal condition of an adult Fish is always

preceded by a transitory embryonic diphycercal stage: from the same

starting-point the homocercal condition is attained after passing through a

heterocercal stage; while the gephyrocercal may perhaps be derived by

degeneration from any one of the others.

The normal function of the fins, both median and paired, has reference to

locomotion in the form of progression, steering or balancing, but in not a

few Fishes the fins may be variously modified and adapted for quite

different purposes; and especially is this the case in the dominant group

of existing Fishes—the Teleostei. Thus, to quote a few examples, the first

dorsal fin of the Sucker-Fishes (_Remora_, _Echeneis_) forms a cephalic

sucker, by means of which the Fish attaches itself to Sharks and Turtles

(Fig. 421); or, as in the Angler-Fish (_Lophius_), its anterior rays are

much elongated, and terminate in lobes which serve as a bait to attract the

prey on which the animal feeds; again, in some of the deep-sea Fishes the

dorsal fin, like the pectoral and caudal fins in others of a similar

habitat, is produced into long trailing filaments whose use is probably

tactile. The pelagic young of many Teleosts, such as some of the

Ribbon-Fishes and the Horse-Mackerels (_Caranx_), also have certain of

their fin-rays prolonged into similar filaments. The pectoral fins are

enormously elongated and wing-like in the Flying-Fishes (_Exocoetus_), and,

after the fashion of a parachute, serve to sustain the Fish in its flying

leaps through the air. They are also similarly modified for a like purpose

in the so-called Flying-Gurnard (_Dactylopterus volitans_). The pectoral

fins may also be used for progression on land, as in the African and East

Indian Goby (_Periophthalmus_), where the fins are large and muscular and

are applied to the ground like feet, enabling {162}the Fish to hop about

the muddy or sandy flats left bare by the retreating tide, in pursuit of

the small Crustaceans on which it feeds. In other Teleosts certain of the

rays of the pectoral fin separate from the rest and from one another, and

form free tentacle-like structures the use of which is probably tactile. In

the Gurnards these organs are relatively short and stout, but in other

Fishes they may form long slender filaments twice as long as the animal,

and capable of being moved independently of the fin, as in the West African

and West Indian species of Polynemidae (_Pentanemus quinquarius_). Similar

free rays are also present in some deep-sea Scopelidae, as in _Bathypterois

dubius_, where they are nearly as long as the Fish itself (Fig. 371, B). A

familiar modification of the pelvic fins in several Teleosts is their

coalescence and more or less complete conversion into a ventrally-placed

sucker-like organ of attachment, as in the common Lump-Sucker

(_Cyclopterus_) and the Gobies (_Gobius_). In the gaudy Chilian Fish,

_Sicyases sanguineus_ (Fig. 428), the anterior part of a huge ventral

sucker is supported by the jugular pelvic fins, and the hinder part by

prolongations from the pectoral girdle. Certain Cyprinidae (e.g.

_Gastromyzon_, which frequents the rapidly-flowing mountain streams of

Borneo), have the whole ventral surface of the trunk, in conjunction with

the outwardly and horizontally directed pectoral and pelvic fins, modified

to form an efficient adhesive surface for attaching the Fish to the stones

and rocks of the river bottom[121] (Fig. 355). In the males of

Elasmobranchs, except in the Palaeozoic Shark _Cladoselache_, and of

Holocephali, the hinder portions of the pelvic fins are modified to form

copulatory organs, the claspers, mixipterygia, or pterygopodia. Lastly, it

may be mentioned that the spines, often long, pointed, and sometimes

serrated, with which the paired and median fins of many Fishes are

provided, furnish formidable offensive or defensive organs, especially when

they are associated with poison glands, and also that in by no means an

inconsiderable number of Teleosts the spines may form part of a

stridulating vocal mechanism.

In different Fishes the pectoral and pelvic fins and the {163}median fins

may, individually, all be absent through atrophy. The pectoral fins are

rarely absent: nevertheless, in certain species of Syngnathidae, and in

most Muraenidae, for example, these fins are entirely wanting. The pelvic

fins are much less constant and are often absent in entire families, as in

the Pipe-Fishes (Syngnathidae), the "Electric Eels" (Gymnotidae), and the

true Eels (Anguillidae), and in the Globe-Fishes and Porcupine-Fishes

(_Tetrodon_, _Diodon_), as well as in certain genera of families where they

are usually present, as in some of the Blennies (Blenniidae) and in the

Ophidiidae. Even when present the pelvic fins are often reduced to mere

vestiges in the shape of filaments, as in some of the Gadoids (Gadidae) and

Ribbon-Fishes, or are represented only by a pair of defensive spines, as in

some Sticklebacks (_Gastrosteus_), or even by a single spine (Balistidae).

Complete suppression of the pelvic fins, or their reduction to vestigial

remnants, seems to be of frequent occurrence in Fishes which live in the

mud, or are able to pass a longer or shorter time in soil periodically

dried during the hot season, as in some Cyprinodontidae, and in species of

such tropical Teleostean families as the Ophiocephalidae, Galaxiidae, and

Siluridae. Suppression of the dorsal fin is apparent in the Gymnotidae, and

of the anal fin in the Ribbon-Fishes. In some of the latter family, as in

the rare British visitor the Oar-Fish (_Regalecus banksii_), and in the

Sea-Horse (_Hippocampus_), where the tail becomes a prehensile organ for

coiling round seaweeds when the animal is not swimming, the otherwise

remarkably constant caudal fin is absent.

An initial stage in the degeneration of median fins is to be seen in many

of the Salmonidae and Siluridae, in which a posterior division of the

dorsal fin becomes reduced in size, loses its fin rays, acquires much fat

in its substance, and becomes an "adipose fin."

The "lateral line" is a notable feature in the external appearance of most

Fishes. Originally developed in the superficial epidermis of the skin in

the form of linear tracts of isolated and often segmentally arranged masses

of sense-cells, these organs subsequently become imbedded for protection in

the epidermic lining of either an open groove or a closed canal extending

along each side of the trunk and tail, and prolonged on to the more exposed

parts of each side of the head in the shape of a more {164}complex system

of branching grooves or of deeply-seated and externally inconspicuous

canals. The course of the lateral line can, as a rule, readily be detected

by the naked eye, and, even when not otherwise distinguishable, may be

traced by the series of simple or multiple pores through which, at

intervals, the canal communicates with the exterior (Fig. 93, A), and often

also, in the trunk and tail, by a band of coloration different to that of

the rest of the body.

COLORATION.

Contrary to popular opinion, it may be doubted if any animals, even Insects

or Birds, can vie with living Fishes in the brilliancy and changeability of

their colours. The nature of their habitat, the rapid fading of the natural

tints after death, and the fact that museum specimens, however carefully

preserved, afford but a ghostly resemblance to the colours of the living

animal, account, no doubt, for much of the prevalent ignorance of the

extraordinary extent to which colour-development may proceed in a

considerable number of Fishes. Like the generality of northern forms of

life, the Fishes of our own seas, rivers and lakes, are less conspicuous

for vivid and striking coloration than those of tropical or subtropical

climes, although such familiar Teleostean Fishes of our seas and fresh

waters as the Mackerel, the Salmon and Trout, the males of the Stickleback

and Dragonet, some of the Gurnards (Triglidae) and Wrasses (Labridae), the

Opah or King-Fish (_Lampris luna_), and many others, are notable

exceptions. Brilliancy of coloration is most conspicuous in the Teleostei:

in nearly all other Fishes the colours are more uniform, usually sober and

often sombre, with no more variety than is afforded by the presence of dark

spots or bands on a lighter ground, or _vice versâ_, or by the lighter

colour of the ventral as compared with the dorsal surface. In Teleosts all

the resources of colour-formation, pigmentation, reflection, and

iridescence through optical interference, in diverse combinations, are

employed in the production of the various tints, while the dominant ground

colour is often diversified by the presence of stripes, bands or bars,

longitudinal or transverse, or of spots of different hues, frequently

arranged in striking and intricate patterns.

The possibilities of coloration in these Fishes may be briefly illustrated

by a few examples:—

{165}In an Australian Fish (_Plectropoma richardsoni_) the prevalent ground

colour of the body is a brilliant carmine, with a tendency to yellow

beneath, and diversified on the back and sides with ultramarine spots of

almost sapphire-like intensity.[122] Certain Australian species of _Beryx_

(_B. affinis_ and _B. mülleri_)[123] have a similar ground-colour when

freshly caught, but with various opalescent tints, chiefly blue and lilac

reflections. In _Polynemus vereker_[124] the ground colour is chrome

yellow, with darker markings, the pectoral and caudal fins are bright

orange, the remaining fins being a lighter shade of the same tint, and by

contrast the long free filaments of the pectoral fins are a bright

vermilion red. The Velvet-Fish (_Holoxenus cutaneus_), also a denizen of

Australian seas, has a dominant colour of brilliant scarlet vermilion, or a

mixture of vermilion and orange. The skin has no scales and presents a

singular pilose or velvety appearance.[125] It is, however, in some of the

Pacific Trigger-Fishes (e.g. _Monacanthus_) and Coffer-Fishes (species of

_Ostracion_) that the eccentricities of coloration are perhaps most

strikingly manifest, for not only are the prevailing colours of the most

brilliant description, but the presence of differently coloured bands or

stripes, often arranged in complex patterns, adds greatly to the gorgeous

and singularly bizarre appearance of these Fishes. To quote one

illustration, the male of the Tasmanian Coffer-Fish (_Ostracion

ornatus_)[126] has the back and sides of its body grass-green and its belly

pale lemon: the caudal fin is orange-yellow, and the remaining fins a

neutral transparent tint. The sides of the trunk and head are traversed by

broad, irregular, and somewhat interrupted bands of the most brilliant

ultramarine blue, the edges of which are sharply defined by dark

chocolate-brown lines. Two or three of the blue body-bands are continued on

to the caudal fin, where they curl into characteristic loop-like patterns.

The lemon-yellow of the belly is further variegated by a reticulated

pattern in pale blue. In the female, formerly regarded as a distinct

species, the ground colour is not green but a pale pinkish-grey, or

dove-colour, with local flushes of a more decided pink, and the belly is a

pure yellow. The blue stripes of the male are represented in the female by

comparatively unbroken bands of a rich reddish-brown which, at the bases of

the pectoral and dorsal fins, {166}form an irregular spiral pattern. In

both sexes the pattern of the longitudinal bands is never precisely the

same in any two individuals. Scarcely less brilliant is the coloration of

those Teleosts, notably species of Pomacentridae and Chaetodontidae, which

frequent the coral reefs of the East Indian Archipelago and the Pacific and

feed on the coral polypes, and of many of the Wrasses (Labridae). Many

other groups, such, for example, as the Percidae, Cirrhitinae, and the

Pipe-Fishes (Syngnathidae), include species in which the coloration is

vivid and often beautiful, although less striking than is the case with the

Fishes mentioned above. As illustrating the opposite extreme in the scale

of coloration, between which and the brilliant tints just described every

conceivable gradation exists, mention may be made of the colourless

appearance of those Fishes which, like the Kentuckian Blind-Fish

(_Amblyopsis spelaea_), are denizens of subterranean rivers; and, omitting

a few species in which the coloration is almost brilliant, the prevalent

sombre tints, dark brown or black, rarely relieved by spots, bands, or

other distinctive markings, of the Fishes inhabiting the abyssal waters of

the deep sea.

The coloration of Fishes is due to the presence in the dermic portion of

the skin of (_a_) special pigment-containing cells (colour-sacs,

chromoblasts or chromatophores), and (_b_) a peculiar reflecting tissue

composed of iridocytes.[127] Chromatophores are probably branched

connective-tissue cells in which pigments of various colours are deposited.

The colouring matter present in different chromatophores is red, orange,

and yellow, all of which belong to the lipochrome group of pigments, or

black (melanin group), but by the combination or blending of

differently-coloured chromatophores other colours may be produced. Thus,

green results from the mixing of yellow and black in suitable proportions;

brown from the blending of yellow and black; and other shades or tints from

an appropriate mixture of chromatophores of various colours. As a rule the

muscles of Fishes contain but little haemoglobin, but, when visible through

the skin, the occasional presence of this substance in localised patches

may contribute a few red spots to the general coloration, as is the case in

the British Flat-Fish _Lepidorhombus megastoma_.

{167}Iridocytes consist of guanin, which, in its chemical reactions,

closely resembles the guanin obtained from guano, and therefore is to be

regarded as a further illustration of the utilisation of waste excretion

products for the production of colour in animals. In forming iridocytes the

guanin is deposited in the shape of granules, or of rounded, polygonal, or

stellate bodies, or in flattened plates. Considered as an agent in the

production of colour, the chief feature in the iridocytes is their opacity

and great reflecting power; and according to the way in which light is

reflected from them, the result may be a chalky white or a bright silvery

appearance. By interference these colour elements are also responsible for

the prismatic colours and brilliant iridescence which so many Fishes

exhibit. The optical properties of guanin has led to its use in the

manufacture of artificial pearls. "Essence d'orient," or "blanc

d'ablette,"[128] from which these pearls are made, principally in Paris, is

obtained from the scales of the Bleak (_Alburnus lucidus_), and is really

the guanin of which the iridocytes of this Cyprinoid are composed. It is

also to the presence of crystals of guanin that the beautiful metallic

lustre of the iris in many Fishes is due.[129]

[Illustration: FIG. 94.—The coloration elements in the skin of the upper

side of a freshly-killed normal Flounder (_Pleuronectes flesus_), seen by

transmitted light. The stellate black bodies are the black chromatophores;

the grey bodies of similar shape represent the yellow chromatophores; and

the small grey plates the iridocytes. (From Cunningham and MacMunn.)]

The chromatophores and iridocytes are chiefly disposed in two layers in the

skin, one outside the scales and the other on the inner surface of the

scales, between the latter and the underlying muscles; and although the two

kinds of coloration elements may be present in both layers, their relative

{168}abundance varies in different Fishes, and in different parts of the

surface of the same Fish. Where chromatophores are most abundant, usually

on the back, the reflecting tissue is relatively scanty, and _vice versâ_.

On the sides and belly of a Fish the place of the inner layer of the dorsal

surface may be taken by the "argenteum." This layer is devoid of

chromatophores, and consists of reflecting tissue in which the iridocytes

form a continuous stratum, either in the form of granules, or as a close

network of rod-like bodies or of polygonal plates in contact with one

another, instead of being less numerous and more scattered as on the back.

When iridescence is produced, it is due to the iridocytes of the outer

layer of the skin; the dead whiteness and silvery lustre, on the other

hand, have their origin in the different ways in which incident light is

reflected from the inner layer or argenteum.

To the relative abundance of chromatophores, the kind of pigment they

contain, and the manner in which they are distributed and blended, combined

with the different reflecting properties, or the iridescence, of the

iridocytes, are due the extraordinary wealth and variety of colour in

Fishes.

The part played by the different coloration elements in the production of

the characteristic colours of different Fishes may be illustrated by two

examples.[130]

In the common Whiting (_Gadus merlangus_) the back of the Fish is a dark

bluish-grey; the sides have a beautiful iridescence and silvery glitter,

while the belly is very nearly a dead white. Briefly, these appearances are

due to the fact that chromatophores (black and deep yellow) are most

abundant on the back, less numerous on the sides, and wanting altogether on

the belly; while the iridescence and silvery appearance of the sides are

due to the iridescence of the iridocytes external to the scales, combined

with the non-iridescent but highly reflective property of a layer of

iridocytes internal to the scales; and the dead white of the belly to the

different reflecting power of the argenteum, and the absence of

chromatophores in that region.

In the Mackerel (_Scomber scombrus_) the distribution of coloration

elements is different, inasmuch as they are mainly situated in the deeper

part of the skin, internal to the deciduous scales. The back is marked by

the well-known alternating wavy bands {169}of black and green; the sides

gleam with the most brilliant iridescence, changing from silver to yellow

or red gold, according to the angle at which the Fish is viewed. The black

bands of the back are produced by the crowding together of black

chromatophores and the diminished number of yellow; the green bands by an

equal blending of yellow and black. Over the dorsal surface and sides of

the Fish, where the coloured bands extend, there is also a reflecting layer

external to the chromatophores, and to this layer is due the silvery

reflection and iridescence. On the belly the disappearance of the

chromatophores and the greater thickness and opacity of the argenteum

account for the lighter colour and the diminished iridescence and silvery

glitter of this part of the skin.

Many Fishes are known to have the power of changing their colours, and in

some the change is rapid. Such changes are due to incident light reflected

from surrounding surfaces, acting through the visual organs and the nervous

system on the differently coloured chromatophores. The latter are capable

of contraction and expansion. Expansion of any particular kind of

chromatophores is accompanied by a diffusion of their pigment—black, red,

orange, or other colour as the case may be—and, according to the number and

distribution of the chromatophores affected, the prevailing tint or tints

of the whole body will be intensified, or only spots, bands, patches, or

flushes of colour will be produced. Conversely, when chromatophores

contract, they may shrink up to mere dots and bring about a diminution in

the vividness of their respective colours, or even an alteration of colour,

seeing that yellow chromatophores become orange when contracted, while

orange or red appear brown or black. Colour changes of this kind may be

artificially brought about. Experiments with Sticklebacks

(_Gastrosteus_)[131], kept in glass dishes with a bottom of black or white

tiles, have shown that the Fishes over the white tiles became partially

bleached, while others with a background of black tiles retained their

original coloration. Bleached Fishes exposed to the white tiles for a

relatively short period (three to ten days) tend to regain their original

colour when subsequently removed to a background of black tiles, but

prolonged exposure to the former conditions (five to six weeks) seems to

render the acquired light colour more or less permanent. {170}The interior

of a Minnow-can is sometimes painted white, so that the bait may assume a

lighter colour, and thus become more conspicuous in the deeper and darker

water where Perch and Pike abound. Hence the colour of a Fish may vary with

its surroundings; and, as will shortly be shown, the capacity for producing

such changes under natural conditions is of the greatest importance to

Fishes in various ways.

Change of coloration may take place through the development of new

chromatophores under the influence of new conditions, and is then

comparatively slow. Artificial illumination of the unpigmented white side

of a Flounder (_Pleuronectes flesus_), by means of a mirror, induces the

formation of chromatophores, and produces a coloration more or less closely

resembling the upper pigmented side.[132] A similar change sometimes occurs

as a natural variation, and is then usually associated with structural

deformity in other respects.

Coloration also varies with age, sex, ill-health, and even with the

emotions. Young or immature Fishes are often marked by bands, bars, or

blotches of colour (_e.g._ the Parr-marks of young Salmonidae), which, as

they disappear when the Fish approaches the adult state, are perhaps

residual traces of ancestral coloration; although, no doubt, change of

habits and surroundings are sometimes responsible for such colour changes

as are observable during growth. Conspicuous coloration is one of the most

frequent of secondary sexual characters, the colours of the male being

brighter than those of the female, particularly during the breeding season.

A diminution of colour has been noticed in the artificially-induced

pigmentation of the lower side of a Flounder when the Fish was suffering

from partial suffocation owing to the temporary failure in the supply of

fresh water, the normal colour returning when the deficiency had been

remedied. A similar pallor was caused by fright when the same Fish was

disturbed.[133] A nocturnal colour-change has been recorded in the

Tasmanian Trumpeter (_Latris hecateia_).[134] In addition to the

olive-green longitudinal bands which are always apparent, there are visible

at night five broad, transversely-arranged, blackish bands. As illustrating

the importance of vision in colour-changes, it may be mentioned that in a

specimen of this Fish, living in a tank, which had been blinded,

{171}probably by a rat or a cat, the dark bands were permanently retained.

Changes of coloration sometimes take place, which either have no

discernible relation to age, condition, or surroundings, or are brought

about by domestication; and in individuals of the same species there is

often a wide range of colour-variation, which is sometimes, but not always,

associated with particular localities. In some fresh-water Fishes a yellow

colour may replace the original tint (xanthochroism). The usually dull

greenish Tench (_Tinca vulgaris_) occasionally becomes a bright

orange-yellow. Another Cyprinoid, the common Gold Fish (_Cyprinus

auratus_), in its wild state in China is also a dull brown or green, but,

when domesticated, assumes in the first year of its life a black colour

(melanism), then a silvery hue, and finally the vivid ruddy golden colour

of the adult; occasionally, but rarely, the Fish is an albino.

The value of a particular coloration in Fishes, either as an aid to

concealment and protection from enemies, or by enabling them to secure

their prey, may now be illustrated by a few examples.

As previously shown, the colours of Fishes may be artificially varied

according to their surroundings. Changes of a similar kind occur naturally,

and when they tend to assimilate the tints of the Fish to the prevalent

hues of its surroundings, and consequently aid concealment, we have

examples of what has been termed variable protective resemblance.

Individuals of the same species vary in colour according to the opacity of

the water they live in, becoming darker in muddy or peaty water, and

brighter and lighter in shallower or clearer water. Trout caught in a

stream with a gravelly or sandy bottom are lighter in colour than those

obtained from a muddy stream, and it is well known that the same Fish

changes colour as it passes from the one background to the other.[135] In a

lake in County Monaghan, Ireland, the Trout are darker on that side which

is bounded by a bog, but are of the beautiful and sprightly variety

generally inhabiting rapid and sandy streams on the opposite side where the

bottom is gravelly; and narrow as the lake is, the two kinds of Trout

appear to confine themselves to their respective areas.[136] Trout obtained

from a {172}stream near Ivy Bridge have become much lighter since the

pollution of the water by white china clay.[137] As an illustration of the

necessity of vision to such colour-changes, it may be mentioned that blind

Fishes cannot vary their tint in this protective fashion. A blind Turbot

living upon a light sandy bottom differed from its fellows in being much

darker and more conspicuous. Dark Trout have been observed among their

light-coloured brethren in a chalk stream in Hampshire, but the former were

invariably blind, probably, as their larger size indicated, through

age.[138]

Of other forms of protective resemblance, reference may be made to the

bottom-feeding Flat-Fishes (Pleuronectidae), many of which have the upper

surface of the body coloured with various shades of brown, speckled with

black or light specks or blotches, in harmony with the prevailing tints of

the sandy banks which usually form their feeding-ground. When disturbed

these Fishes court concealment, and render themselves still less

conspicuous by partially burying themselves in the sand. Many of the Skates

and Rays, which have a white ventral surface, have the back mottled and

coloured in accordance with the colour of the sea-bottom, but in this case

it is possible that the advantage lies in enabling the Fish to secure

passing prey by concealing its own whereabouts.

Striking examples of protective coloration occur among the Pipe-Fishes and

Sea-Horses (Syngnathidae), which usually frequent groves of _Zostera_,

Fucoids, and other sea-weeds. A British species of Pipe-Fish (_Siphonostoma

typhle_),[139] which lives among the blades of the sea-grass, _Zostera_, is

olive-green in colour, and is a typical example of protective resemblance

both in colour and in the slender elongated shape of the body. Similar

protective resemblances are noticeable among the Sea-Horses, the coloration

varying with the general hue of their environment of sea-weed; but the

climax is certainly reached by the singular Australian species,

_Phyllopteryx eques_ (Fig. 388).[140] In this Fish the skin is produced

into numerous long, flattened, branched filaments, which are prolonged from

the extremities of spine-like outgrowths of the dermal skeleton, and marked

by alternate bands of brown and orange,[141] {173}thus resembling both in

shape and colour the fronds of the surrounding fucoids and other marine

algae amongst which the Fish lives.

Many of the Fishes frequenting the coral reefs of the East Indian and

Pacific areas, especially those belonging to the Teleostean families

Chaetodontidae and Pomacentridae, have a most brilliant and vivid

coloration, frequently marked by bands or stripes of different tint. So far

from rendering these Fishes unduly conspicuous, there can be little doubt

that, by harmonising with the striking and varied colours of the

anemone-like coral polypes, their coloration is distinctly protective; and

it is interesting to note that similar colour-patterns have been

independently reproduced in both families.[142] Even the reef-frequenting

Flat-Fishes (Pleuronectidae) have the usually sombre upper surface

ornamented by vivid colours and striking patterns.

Pelagic Fishes, like the Herring, Mackerel, Flying-Fish (_Exocoetus_), and

many others, often have the belly and sides silvery or white, and the back

dark green, black, or steely blue. Seen from below against the light sky,

or viewed from above against the background of the dark water, these Fishes

would seem to be practically invisible to their predatory foes, whether

Fishes or Birds, or at all events not easily detected.

Coloration may not only be protective, but also aggressive, by helping to

conceal the proximity of an animal from its prey; add to this some device

for deceiving and attracting the prey, and we have an example of "alluring"

coloration.[143]

As an example of coloration which is both aggressive and alluring, the

Angler-Fish or Fishing-Frog (_Lophius piscatorius_) of our own coasts may

be quoted. Naturally sluggish and inactive in its habits, and often using

its muscular pectoral fins for crawling about the sea-bottom, the

Angler-Fish usually hides itself in the sand or amongst sea-weeds, which it

closely resembles in general colour. Curious branched tag-like processes of

soft skin fringe the sides of the head and body, and in appearance and

colour resemble the smaller fronds of the surrounding sea-weed. So far the

coloration is simply aggressive, and helps to conceal the Fish from its

prey, but in addition the animal is provided with a special device for

luring its prey within the reach of its capacious and Frog-like mouth. The

first three spines {174}of the dorsal fin are detached from one another and

greatly elongated, and moreover extend along the middle of the dorsal

surface of the head. The first, which is the longest, terminates in lobes

or lappets of skin, and can be freely moved in every direction by the

muscles inserted into its base. By the agitation of this lure or bait

smaller Fishes, probably mistaking the disturbance for the presence of a

wriggling worm, are tempted to their fate, and soon find themselves

engulfed in the enormous mouth of the artful angler.[144] In some allied

forms (e.g. _Ceratias bispinosus_ and _Oneirodes eschrichtii_)[145] which

live in the abyssal darkness of the deep sea, use is made of the attraction

which light has to aquatic animals, and the fishing-rod spine terminates in

a phosphorescent organ, which is probably used for enticing smaller Fishes

within the reach of the jaws of these singularly modified

Angler-Fishes.[146]

It is by no means improbable that examples of "warning" coloration occur

amongst Fishes. The brilliant colours of some of the Trigger-Fishes

(_Balistes_, _Monacanthus_), Coffer-Fishes (_Ostracion_), and Globe-Fishes

(_Tetrodon_) are perhaps of this nature. They are often associated with the

presence of strong spines, defensive and often erectile, either in

connexion with the dorsal fin or on the general surface of the body, and

may therefore serve the purpose of a danger signal to such predatory foes

of these Fishes as might otherwise be tempted to attack them—to the mutual

advantage of the Fishes themselves and their would-be enemies. The British

Weever-Fish (_Trachinus_) may perhaps offer another example of warning

coloration.[147] The Fish is armed with poisonous spines on its opercula,

and, in addition, has a conspicuous black dorsal fin. When the body of the

Fish is buried in the sand, only its projecting dorsal fin remains to

indicate its whereabouts to predatory Gurnards, which might otherwise

mistake the Weever for harmless Fishes of similar size and habits. The

existence of "recognition" colours or markings peculiar to the species, to

enable individuals of the same species to recognise one another and to keep

together in shoals, has not yet been proved. It is probable that the

relatively {175}limited range of vision, even in the clearest water, would

render coloration unsuitable for this purpose. Recognition sounds are

likely to be far more effective, and there is evidence of their production

by a special vocal mechanism in many Fishes.[148]

The examples given above show how natural selection may lead to the

evolution of distinctive forms of coloration which are advantageous to the

Fish either for concealment, aggression, or protection, and in conclusion

it may be pointed out that by the same cause colour may be eliminated or

its development checked if in any way harmful to the animal; and further,

that if a particular coloration becomes useless to the Fish by reason of a

change in its habits or environment, natural selection ceasing to act where

its intervention is no longer necessary to maintain the coloration, the

latter will sooner or later tend to disappear.

The absence of pigment is sometimes protective. The surface-swimming larvae

of many Teleosts have no chromatophores, and therefore no obvious

pigmentary colours. Their bodies are so translucent that they can be seen

through, and hence are visible only with difficulty. The transparency of

the body may even be increased by the absence of the red haemoglobin of the

blood, as is the case with the pelagic _Leptocephalus_-larvae of the

Eel.[149] The iridocytes of the reflecting tissue may also disappear under

the influence of changed surroundings. The larvae of various species of

_Onus_ (Gadidae) are silvery in hue during their pelagic career, owing to

the presence of iridocytes in the skin, but on becoming mature they change

to a dull dark colour, and live under stones or in holes and crevices in

the rocks. During the change of habit the reflecting tissue (argenteum) is

lost, and the needful chromatophores are acquired.[150]

Instances of the loss of pigmentary colours, owing to the cessation of the

controlling influence of natural selection, are to be found in the absence

of chromatophores on the white under surface of the Flat-Fishes, where such

colours are useless but not necessarily harmful, and in the colourless,

cave-inhabiting Fishes, of which the Blind-Fish (_Amblyopsis_) of North

America may be taken as an example.

{176}POISON GLANDS OF FISHES.

A few Teleosts are provided with weapons of offence or defence in the shape

of poison-glands, probably derived from the epidermis, and associated with

spines on the gill-covers, or in connexion with the dorsal fin, or with

both.

[Illustration: FIG. 95.—The opercular spine of _Trachinus draco_ and its

poison-glands. _ar_, Articulation of the opercular bone with the

hyomandibular; _gl.gl_, the two poison-glands; _op.m_, opercular membrane;

_op.s_, opercular spine; _r_, outer ridge of the spine; _sh_, sheath of the

spine. (From W. Newton Parker.)]

The two British species of "Weever" (_Trachinus draco_ and _T. vipera_) are

both provided with poison-organs in connexion with a spine on the operculum

and with the five or six spiny rays of the anterior dorsal fin.[151] The

first of these spines is a structure projecting backwards from the hinder

margin of the opercular bone of the gill-cover, and is traversed along both

its upper and lower margins, from base to point, by a deep groove. Except

at its protruding naked point the spine is ensheathed in an extension of

the external skin. Along each of the grooves there extends a solid

pear-shaped mass of gland-cells, the broad base of which coincides with the

base of the spine, while the gradually tapering, narrower portion is

continued as far as the sharp point. The glands enclose no cavity, and

there is no duct, so that whatever poisonous fluid their cells secrete is

probably set free by the rupture of the cells and discharged into the

grooves, along which it passes to the point of the spine, somewhat after

the fashion of a hypodermic syringe. The origin of the gland-cells from an

in-pushing of the epidermis is indicated by the continuity of the two

structures near the point of the spine. Both in structure and in their

relation to poison-glands each of the spines of the {177}dorsal fin is

almost precisely similar to the opercular spine. There is no evidence as to

how the poison is ejected into a wound, and it can only be conjectured that

it may be caused by the pressure exerted on the gland when the spine is

forcibly thrust for some distance into the flesh. Certain it is that these

structures are capable of inflicting painful and troublesome wounds when

the Fish is incautiously handled and the skin accidentally punctured, and

no doubt they can be used with great effect as offensive organs.

A similar poison apparatus exists in certain species of Batrachidae, such

as _Thalassophryne reticulata_,[152] which is by no means uncommon at

Panama. This apparatus is formed by a spinous outgrowth from the opercular

bone and by the first two dorsal spines. Instead, however, of having two

grooves, the opercular spine resembles the fang of a venomous snake, and is

perforated by a complete canal which is only open at the base and point of

the spine. A poison-sac at the base of the spine discharges its contents

into the canal. The nature of the glands which secrete the poison has yet

to be discovered, but it is probable either that there are glands in

connexion with the poison-sac, or that the latter is lined by a glandular

epithelium. The structure of the dorsal spines is similar. In some species

of the Scorpaenoid genus _Synancia_[153] (e.g. _S. verrucosa_, from the

Indian Ocean), the terminal portions of the dorsal spines are deeply

grooved on each side, and at the origin of each groove there is a

pear-shaped bag containing a milky poison. The bag is prolonged into a duct

which, after traversing the groove, opens at the extremity of the spine.

Many Siluridae are armed with powerful and often serrated dorsal and

pectoral spines which are certainly capable of inflicting dangerous wounds,

and not a few of them possess a sac-like organ with an external opening in

the axilla of the pectoral fin. It is possible that the sac secretes a

poison for anointing the spine, but at present there is no evidence that

such is the case, or that the sac produces any poisonous secretion at

all.[154]

Among the Elasmobranchs the Eagle-Rays (_Aëtobatis_),[155] and the

Sting-Rays (_Trygon_), have barbed or serrated spines on the tail, which

inflict wounds far more severe than those caused by {178}mere mechanical

laceration; but, except the mucus secreted by the gland cells of the skin,

which may possess venomous properties, no special poison-forming glands in

connexion with the spines are at present known.

PHOSPHORESCENT ORGANS.[156]

In common with many other animals of similar habitat, phosphorescent organs

(photophores) are highly characteristic structures in many deep-sea

Teleosts belonging to widely different families (e.g. _Stomiatidae_,

_Scopelidae_, _Halosauridae_, and _Anomalopidae_). These organs probably

had their origin in local aggregations of the gland cells of the epidermis,

which had acquired the power of secreting a luminous slime. Luminous organs

vary greatly in number and in their mode of distribution in the skin.

Usually they are found on the sides and ventral surface of the body and

head, very rarely on the dorsal surface, and they often present the

appearance of brightly glistening jewels set in the skin. A very frequent

method of arrangement is in one or two longitudinal lines along the lateral

and ventral surfaces, sometimes extending continuously from the head to the

end of the tail (Fig. 371, A, and Fig. 379), but occasionally interrupted

and limited to portions of the body and tail; and in a few a distinctly

metameric disposition is obvious. On the other hand, the very numerous and

simple organs of _Opostomias_ are disposed in many transverse bands along

the sides of the Fish. In addition to these organs, which are usually

numerous, and whose arrangement is linear, specially large and often

structurally complex luminous organs are present on different parts of the

head and body. In _Opostomias micripnus_ there is a phosphorescent organ on

a median barbel depending from the chin. _Sternoptyx diaphana_ has one on

the lower jaw. The presence of one or two organs beneath the eyes (Fig. 96)

is characteristic of several species (e.g. _Opostomias micripnus_,

_Astronesthes niger_, _Pachystomias microdon_, _Scopelus benoitii_,

_Malacosteus indicus_). _Opostomias micripnus_ has a luminous organ on the

isolated and elongated first fin ray of the pectoral fin, while in certain

deep-sea Angler-Fishes (e.g. _Ceratias_) there is one on the anterior

cephalic fin-ray of the dorsal fin. The Scopelid _Ipnops {179}murrayi_[157]

(Fig. 371, C) has a singular organ, probably luminous, beneath the

transparent superficial bones of each side of the roof of the skull.

Another member of the same family (_Scopelus benoitii_) is interesting in

having a phosphorescent organ in the middle of the back, which is directed

backwards. An American genus of Batrachidae (_Porichthys_) has about 350

photophores in relation with the lateral sense-organs of each side of the

head and body.[158] The existence of luminous organs has also been noticed

in the Haddock (_Gadidae_).[159] A primitive form of photophore,

distributed in considerable numbers on the head and trunk, either in lines

or diffused over the surface, exists in eleven species of Selachii

(_Spinacidae_), of which some are known to be luminous.[160]

[Illustration: FIG. 96.—_Pachystomias microdon_, showing the two rows of

phosphorescent organs along the side of the body, and the anterior and

posterior suborbital luminous organs. (After Günther.)]

[Illustration: FIG. 97.—_Opostomias micripnus_. Median section of a simple

phosphorescent organ. _g_, Radial gland tubes. (After Lendenfeld.)]

Diversity of structure is equally marked. The essential part of each

luminous organ is always a collection of gland cells, usually disposed so

as to form the lining of a series of radially arranged gland-tubules in the

deeper part of the organ, which also contains ganglion cells, and is

supplied with nerves from contiguous spinal or cranial nerves. The simplest

form of phosphorescent organ consists of little more than these essential

elements. In the more complex organs an investing pigment-sheath,

reflecting and lens-like structures, and an iris diaphragm, either singly

or in combination, may be added. Fig. 97 represents one of the simplest

types of phosphorescent organ, which, in groups of {180}50 to 100, are

arranged in transverse bands on the sides of _Opostomias micripnus_, and

appear as small white spots on the otherwise black skin of this Fish.

Each organ has the shape of a biconvex lens, sunk to about half its

thickness in the skin. The inner half is formed of radially-arranged gland

tubes filled with small granular cells, and converging towards the centre

of the organ. Into the connective-tissue walls of the tubes extend

blood-vessels and nerves. External to the gland tubes there is a layer of

long slender cells arranged perpendicularly to the surface, and more

externally still a layer of ganglion cells. There is evidence that these

organs multiply by division. Such simple phosphorescent organs as these

differ little from the groups of epidermic gland cells, which probably

formed the evolutionary starting-point in the development of these singular

structures.

[Illustration: FIG. 98.—_Pachystomias microdon_. Section of the anterior

suborbital organ. _g_, Irregular gland tubes; _g^1_, radial gland tubes;

_i_, iris-like diaphragm; _l_, lens-like body; _p.s_, pigment sheath; _s_,

layer of light-reflecting spicules. (After Lendenfeld.)]

A much more complex type of luminous organ is to be found in the suborbital

organs of _Pachystomias microdon_, of which there are two on each side,

appearing as conspicuous white masses, one in front of the other, and

situated just below the eye. The more anterior of the two organs is

somewhat pouch-shaped in section, its walls consisting of several

concentric layers (Fig. 98). Externally there is a layer of black pigment,

within which is a stratum of irregular gland tubes. More internally still

there is a thick layer of light-reflecting spicules, probably derived from

an inverted and modified dermal scale. The axial part of the organ is

occupied by a number of radial-disposed structures, probably similar to the

gland tubes of the simple organs of _Opostomias_, and continuous with a

lens-like structure which, as it were, closes the expanded mouth of the

pouch. The superficial skin which forms the margin of the aperture

partially projects over the outer surface {181}of the lens-like body,

somewhat after the fashion of an iris-diaphragm. The organ is supplied by a

branch of the fifth cranial nerve. Between such simple and complex organs

as those above described there are various other types which are more or

less intermediate in character.

A particular type of phosphorescent organ is not necessarily restricted to

the same species; both the simplest and one or more of the more complex

types may be represented in the same Fish. Thus, _Opostomias micripnus_,

which frequents depths of over 2000 fathoms, has not only the simple organs

described above, but also others differing from the former in having an

external pigmentary sheath, which are scattered all over the body at

intervals of 1 to 3 mm. There are also larger and still more complex organs

which are disposed in two parallel rows along each side of the body; and

finally, the same species has special luminous organs on a median

chin-barbel, and also on an elongated fin-ray pertaining to the pectoral

fin.

The light emitted by phosphorescent organs is probably of use to deep-sea

Fishes in enabling them to seek and detect their prey in the sunless depths

which they frequent. The position of the organs on the sides and ventral

surface of the body, and the frequent presence of special luminous organs

in the vicinity of the mouth, render them admirably adapted to light up the

water in front of and beneath the Fish, while the existence of optical

accessories for intensifying the luminous beams, and for regulating their

distribution, combined with an abundant nervous supply, suggests that the

emission of light is under the control of the Fish, and may be varied as

the occasion requires. That these organs may also be defensive, in some

instances at all events, seems not improbable. A flash-light from the

dorsal luminous organ or "stern-chaser" of _Scopelus benoitii_ would

probably dazzle and frighten an enemy in hot pursuit of the _Scopelus_. The

use of phosphorescent organs as baits or lures for enticing prey has

already been alluded to. There is some evidence that the colour of the

emitted light differs in different Fishes; and as there is considerable

variety in the precise disposition of the organs, it seems probable that in

deep-sea Fishes recognition lights may take the place of the recognition

colours and sounds of those whose lot is cast in a sunnier habitat.

{182}CHAPTER VII

THE SKIN AND SCALES

The skin of the Cyclostomata and Fishes consists (1) of the epidermis,

formed of several layers of epidermic cells, which are constantly being

recruited by the division of the cells of the basal layer; and (2) of a

stratum of connective tissue with intermingled unstriped muscle-fibres,

blood-vessels and nerves, which constitutes the deeper layer or dermis.

From the epidermis are formed the various unicellular or multicellular

glands with which the skin is provided; and from one or both of the skin

layers originate the different calcareous structures which constitute the

hard exoskeleton.

In the Cyclostomata the epidermis is particularly rich in goblet-shaped,

mucus-secreting, gland-cells. The Myxinoids also possess numerous pockets

of so-called "thread-cells." In each of these cells the protoplasm secretes

a long spirally-coiled thread, and under the influence of appropriate

stimuli the thread is shot out and unwound to a great length. The threads

and the mucus are so abundant that one of these animals will convert a

bucket of water into a thick mass of jelly. No scales or other hard

exoskeletal structures are present in any of the Cyclostomata.

In Fishes mucus-glands are also abundant in the epidermis, and to their

activity is due the slimy mucus which lubricates the surface of the body.

They are specially numerous in the Dipnoi (e.g. _Protopterus_), where, in

addition, there are many simple multicellular glands which secrete the

"cocoon" or capsule in which the Fish is enclosed during the dry season.

From the epidermis are derived the poison-glands of some Teleosts, and also

the "glandula pterygopodia" in relation with the claspers of the male

Elasmobranchs. The glandular structures in connexion with {183}the

phosphorescent organs of the deep-sea Fishes will no doubt be traced to the

same source.

In the great majority of Fishes the skin becomes the seat of calcareous

deposit, and gives rise to such diverse exoskeletal structures as the

varied forms of spines and scales with which the surface of a Fish is

invested.[161] These structures, probably the most ancient form of

Vertebrate skeleton owing its existence to the presence of lime salts in

the tissues of the body, present highly characteristic modifications in the

different groups.

Exoskeletal structures are of two kinds: (1) those which owe their

formation to the secretory activity of cells belonging both to the

epidermis and the dermis, and (2) those which are derived solely from the

dermis. To the first belong the dermal denticles or so-called placoid

scales of most Elasmobranchs, and to the second the scales which form the

skin-skeleton of living and extinct Teleostomi and Dipnoi. With the

exception of enamel, which is always formed by the cells of the epidermis,

the hard exoskeletal tissues owe their existence to the secretion of

certain cells of the dermis (scleroblasts),[162] the inclusion of which in

a growing calcifying tissue is the cause of whatever cellular structure the

tissue may present. It will shortly be apparent that the dermic

scleroblasts are by no means uniform in their products, and that in

different Fishes they give rise to widely different hard tissues.

The dermal denticles or "shagreen" of the ordinary Sharks and Dog-Fishes

(Elasmobranchii) probably represent the most primitive form of exoskeleton.

In the development of a dermal denticle a papilla of the dermis grows up

into the overlying epidermis, pushing before it the basal layer of

epidermic cells, which forms an investment to the papilla and constitutes

the so-called "enamel organ" (Fig. 100). The papilla itself subsequently

becomes converted into dentine, leaving, however, a central pulp-cavity,

while the apex of the papilla is invested by a cap of enamel formed by the

enamel organ. Ultimately the base of the papilla widens out into a more or

less rhomboidal basal plate formed of bone. In this way there is formed a

{184}pointed, enamel-tipped spine of dentine which protrudes through the

epidermis, and projects backwards on the surface of the body, but is firmly

fixed in the skin by the basal plate with which it is continuous. The

centre of the under surface of the basal plate is perforated for the

entrance of the blood-vessels which pass to the cellular pulp in the axis

of the spine. In the adult Fish the denticles form a fairly close-set

covering to the whole body, including the head and even the surfaces of the

fins, and are larger on the dorsal than on the ventral surface (Fig. 99).

In the Rays (_Raia_) they are more sparsely scattered, and in different

parts of the body may form spines of considerable size for offensive or

defensive purposes. The spines vary greatly in shape in different members

of the group, sometimes being acutely pointed, and sometimes flattened or

depressed, and often they are furnished with smaller accessory spines

developed at their bases or from the surface of the basal plate. An

arrangement of the denticles in oblique transverse rows is observable in

some genera (e.g. _Scyllium_). In the Saw-Fishes (e.g. _Pristis_) the

denticles which fringe the lateral margins of the long flattened rostrum

are not only enormously enlarged, but are implanted in sockets and form the

teeth of the saw (Fig. 262). In the Holocephali the smooth skin is almost

entirely devoid of exoskeletal structures, but dermal denticles are present

on the frontal and anterior claspers, and in the young there may be a

double row of small denticles along the back.

[Illustration: FIG. 99.—Surface view of the dermal denticles of _Scyllium_

sp., showing their arrangement in oblique transverse rows. _b_, Basal

plate; _c_, canal which perforates the basal plate and becomes the axial

pulp-cavity of the spine; _f.b_, intersecting fibrous bands of the dermis;

_s_, spine; in the spine of one scale the dentinal tubules are shown. The

smaller denticles are those most recently formed. (After Klaatsch.)]

{185}[Illustration: FIG. 100.—Vertical section through the skin of an

embryo Shark. _C_, Dermis; _c.c.c.d_, layers of the dermis; _E_, epidermis;

_e_, enamel organ; _o_, enamel layer; _p_, papilla of the dermis. (From

Wiedersheim, after Gegenbaur.)]

In the remaining groups of Fishes, the Teleostomi and the Dipnoi, the spine

of the primitive dermal denticle is either evanescent or entirely wanting,

while the equivalent of the basal plate remains to form the unit of a scaly

armature. Evidence of this may be found in the presence of transitory

evanescent spines, provided with an enamel-cap, secreted by the basal

epidermis, on the developing rhomboidal scales, as in the young

_Lepidosteus_[163] (Fig. 101); while the entrance of blood-vessels into the

scales through perforations on their inner surfaces, as in _Polypterus_ and

_Lepidosteus_, obviously recalls the perforated base of a dermal denticle

(Fig. 99). The epidermis now ceases to take any part in the formation of

the scales, and hence enamel no longer enters into their structure. A more

regular and definite arrangement of the scales is noticeable, and whether

distinct, or articulating with one another, or overlapping like the slates

on the roof of a house, they are usually disposed in a series of successive

oblique transverse rows. In some of these Fishes the embryonic epidermic

covering of the scales becomes lost, and their outer surfaces are naked.

More frequently, as in the generality of Teleosts, and in the Dipnoi, the

reverse is the case, and the scales are more or less completely invested

both by the dermis and the epidermis. As regards their shape, size, and

minute structure there is much variation. In some Teleostomi {186}the

primitive rhomboidal shape of the dermal denticle is retained; in others a

rounded or cycloid scale supplants the earlier rhombic type. Within the

limits of the same group (_e.g._ Crossopterygii) there are examples of the

independent evolution of a cycloid from a pre-existing rhombic squamation;

and with the introduction of the cycloid type an overlapping or imbricated

disposition of the scales always takes the place of the marginal

articulation of the rhombic type.

[Illustration: FIG. 101.—Development of a scale in _Lepidosteus osseus_ ×

330. _b.p_, Basal plate, with included bone cells, at first distinct from

the spine; _e_, enamel; _e.o_, enamel organ; _ep_, epidermis, with large

gland cells; _p_, dermic papilla which forms the vestigial spine; _Scl_,

scleroblasts. (From Klaatsch.)]

As to the causes which may have determined the shape and mutual relations

of scales interesting suggestions have been made.[164] Scales bear a

segmental relation to the subjacent muscle-segments or myotomes, sometimes

being disposed in oblique transverse rows coinciding with the latter, or

the rows may be so far increased as to be multiples of the myotomes. From

mechanical considerations depending on the sigmoid shape and

interdigitating relations of the myotomes and their separating fibrous

septa or myocommata, and the attachment of the myocommata to the dermis,

the contraction of the myotomes during the lateral flexions of the trunk in

swimming has a tendency to wrinkle the skin into definitely circumscribed

rhombic areas, thus determining the shape, limits, and disposition of the

scales which are developed in those areas. The rhombic was probably the

primitive shape of scales, and is certainly characteristic of the

palaeontologically older types of scaly Fishes. Generally the rhombic

condition is associated with a peg-and-socket articulation between the

upper and lower margins of adjacent scales. But a rhombic squamation is not

without {187}disadvantages, and would certainly impose some restriction on

the lateral flexures of the body in swimming, and hence in the different

groups of Fishes it may happen that, in the more specialised forms, an

imbricated cycloid squamation supersedes a rhombic condition, and with the

change the Fish acquires greater lateral mobility. Even in the same Fish

the gradual substitution of the cycloid for the rhombic type may be

observed. In the Australian _Aetheolepis_,[165] a fossil genus related to

the European Liassic _Dapedius_, there is a gradual transition along the

sides of the body between the articulated rhombic scales of the relatively

immobile trunk and the cycloid overlapping scales of the flexible tail; and

it may be mentioned that, even where a typical rhombic squamation exists,

the peg-and-socket articulation may be wanting in the caudal region, so as

to ensure greater freedom of movement. Mechanical considerations may also

explain the overlapping of cycloid scales. From the mode of attachment of

the myocommata to the dermis, the contractions of the myotomes, through the

pull which they exert on the former, tend to deflect or depress the

scale-areas, particularly at their anterior margins.

[Illustration: FIG. 102.—_Acipenser ruthenus_. A, Side view of the trunk of

a specimen 30 cm. in length (nat. size); _d_, dorsal row of plates; _l_,

_l′_, lateral rows; between the rows of large scutes may be seen the

numerous small denticles which are represented (× 10) in B; C, one of the

large scutes (× 10). (From Hertwig.)]

In the surviving Crossopterygii, as in _Polypterus_, the scales are

rhomboidal and thick, and they only slightly overlap. They articulate with

one another by means of marginal peg-and-socket articulations (Fig. 106,

B). A thick layer of hard, glistening, enamel-like substance or "ganoin"

forms the outer layer of the scale; the inner layer consisting of bone in

which dentinal tubules as well as bone-cells are present. In the numerous

fossil members of the group the scales are either rhomboidal or cycloid.

{188}The oldest representatives of the Chondrostei, the Palaeoniscidae

(Fig. 283) possessed a complete armature of rhombic scales, but in all the

surviving members of the group the scales have undergone considerable

modification in some respects, and in others are degenerate. In the

Sturgeon (_Acipenser_)[166] the primitive rhombic squamation is retained

only on the sides of the terminal part of the tail, and there they are in

close apposition in oblique rows. The rest of the body is traversed by five

widely-separated longitudinal rows of large bony scutes, which, like the

rhombic scales, are furnished with ridges and projecting spines (Fig. 102).

Between the rows of large scales there are numerous denticle-like

structures arranged in oblique rows. Each of these consists of a basal

plate imbedded in the dermis, and of one or more projecting spines which

perforate the epidermis. All the scales have the same minute structure,

consisting mainly of bone; but the surface layer and the spines seem to be

composed of a hard laminated substance from which bone-cells are absent

(ganoin). In _Polyodon_ the scutes are wanting, but vestigial denticles are

retained.

[Illustration: FIG. 103.—Surface view of the rhombic scales of a young

_Lepidosteus_. In two scales the parts which are overlapped by adjacent

scales are shaded. _c_, Position of the central canal which perforates the

inner surface of each scale; _f.b_, intersecting fibrous bands of the

dermis; _s_, vestigial spines. (After Klaatsch.)]

Among the Holostei the scales are very different in the two surviving

members of the group. In _Lepidosteus_ (Fig. 103) the {189}scales are

rhombic, and both in arrangement and structure, as well as in their method

of articulating with one another, they closely resemble those of

_Polypterus_. In _Amia_,[167] on the contrary, the relatively thin scales

are cycloid in shape, and in their imbricated arrangement, in their

enclosure in pouches of the dermis, and in the absence of any superficial

covering of ganoin, they are very similar to the scales of the more typical

Teleosts (Fig. 295). The resemblance extends even to histological

structure, for each scale consists of an outer layer of bone, which

gradually passes into an inner fibrous stratum.

[Illustration: FIG. 104.—Diagrammatic longitudinal section through the skin

of a Teleost to show the position of the scales. _d_, Dermis; _ep_,

epidermis; _s_, scale. (After Boas.)]

In Teleosts the usually thin and flexible scales are primarily developed

from dermic papillae, but subsequently they come to lie in pockets or

pouches in the dermis. As a rule no spines are developed, and so far no

trace of an enamel organ has been detected during their development. The

scales in their dermal pouches are disposed obliquely to the surface of the

body, so that the hinder part of one scale overlaps the anterior portion of

the scale next behind it (Fig. 104). Only the free hinder part of each

scale has an epidermic investment (Fig. 105). In minute structure each

scale consists of an outer layer of bone, which, like the bone of the

endoskeleton, may either be homogeneous except for a feeble lamination, or

it may contain bone-cells arranged in successive layers, parallel to the

surface of the scale. In addition, there is an inner fibrous stratum in

which the fibrous bundles in any one plane cross those in planes above or

below them. The scales are either "cycloid," that is, they have smooth,

unbroken margins (Fig. 105), or the free margin of each scale is produced

into a series of tooth-like spines, and the scale is said to be "ctenoid"

(Fig. 106, A).

Some Teleosts, however, have scales which are neither cycloid nor ctenoid,

and in certain features seem to be intermediate between ordinary Teleostean

scales and dermal denticles. Thus, on certain parts of the body of

_Centriscus_,[168] each scale consists {190}of a rhombic basal plate,

produced into a curved, backwardly-inclined spine, the axis of which

contains a pulp-cavity opening on the inner surface of the basal plate

(Fig. 107). Some Malthidae (e.g. _Malthe_[169]) have similar scales, but

with round basal plates and solid spines (Fig. 108, B). Similar scales

(Fig. 109), sometimes rhombic in shape, with one or more spines, which may

be simple or branched, are also found in the Sclerodermi (e.g. _Balistes_,

_Monacanthus_, _Triacanthus_).[170]

[Illustration: FIG. 105.—Cycloid scale of _Salmo fario_. _a_, Anterior

portion covered by overlap of preceding scales; _b_, free portion covered

only by pigmented epidermis. (From Parker and Haswell.)

FIG. 106.—A, Ctenoid scale; B, "Ganoid" scale. (After Günther; from Parker

and Haswell.)]

[Illustration: FIG. 107.—_Centriscus scolopax_. A, Scale from the orbital

region, × 50; B, scale from the base of the pectoral fin, × 100. (From

Hertwig.)]

Amongst some of the usually scaleless Siluroid Fishes the scales assume a

very peculiar structure. In _Hypostoma_[171] (_Plecostomus_) the sides and

back of the Fish are covered by large bony plates, but on the under surface

and on the fins these are replaced by much smaller ones. Both kinds,

however, carry numerous small movable spines implanted in sockets (Fig.

110), a fact which suggests comparison with a stage in the development of

the scales of _Lepidosteus_, when the independently formed and evanescent

spines have not yet fused with the basal plates.

In other Teleosts, as in the Agonidae and some Triglidae, the body is

completely cuirassed with large keeled bony plates. The singular appearance

of many of the Plectognathi is largely {191}the result of the curious

modifications which their scales undergo. In some of the Coffer-Fishes

(_Ostracion_) these structures assume the form of polygonal bony plates

which suturally articulate with one another and enclose the trunk in a

rigid cuirass, from which the scaleless tail protrudes behind (Fig. 438);

while in some Globe-Fishes and Porcupine-Fishes (e.g. _Tetrodon_, _Diodon_)

the prolongation of the scales into strong erectile spines equally well

serves the purpose of protection (Fig. 439).

Most Teleostomi have the scales along the "lateral line" perforated by

single or multiple apertures, through which the sensory canal communicates

with the exterior.

[Illustration: FIG. 108.—A, Scale of _Antennarius hispidus_, × 100; B,

scale of a young _Malthe vespertilio_, × 100. (After Hertwig.)]

In a few Teleosts scales are entirely absent, as in most Siluridae; or they

exist only as microscopic vestiges hidden in the skin, as in Eels; or, as

in such naked forms as _Antennarius marmoratus_ and _Lepadogaster_, and in

some Siluridae, they become reduced to mere papillae of the dermis.

[Illustration: FIG. 109.—A, Scale of _Balistes capriscus_, × 20; B, scale

of _Monacanthus scopas_, × 20. (After Hertwig.)]

The concentric rings observable (Fig. 105) on the surface of many

Teleostean scales are an index to the age of the {192}Fish.[172] The

formation of these rings depends on the fact that the lines of growth on

the surface of the scale are more widely separated from one another on that

portion of the scale formed during summer, and relatively closer together

on that part which is formed during the winter; the more rapid growth in

the warmer season probably being due to favourable conditions as to food

and temperature, and the retarded growth of the colder season to the

reverse. Hence, by counting the alternating zones of close-set winter lines

and less closely approximated summer lines of growth, a reliable clue may

be gained as to the age of the Fish.

In the Dipnoi,[173] as in Teleosts, the scales are enclosed in dermal

pockets, and exhibit a regular, imbricated disposition in oblique rows

(Fig. 304, A). In shape they are nearly cycloid, or slightly oval, with the

long axis coinciding with that of the body. Structurally, also, they bear

some resemblance to Teleostean scales, although differing in details. On

the outer surface of the scales there are numerous small conical spines. No

significance, other than as an example of evolutionary convergence, can be

attached to the resemblance between the scales of Fishes so widely

separated as the Dipnoi and the Teleosts.

[Illustration: FIG. 110.—_Hypostoma commersonii_. A scale from the

periphery of the caudal fin, × 50; one of the spines (_s_) is implanted in

its socket (_s′_). (From Hertwig.)]

All known fossil Dipnoi had scales of a similar character, although

differing greatly in size in different genera. In some (e.g. _Dipterus_) a

layer of enamel-like substance invests the exposed portions of the scales.

{193}CHAPTER VIII

THE SKELETON

All Fishes possess an internal skeleton which, in order that it may be

distinguished from the more superficial scaly exoskeleton described in

Chapter VII., is termed the endoskeleton. The latter consists (i.) of an

axial part, including the vertebral column and the skull; and (ii.) of an

appendicular portion, consisting of the skeleton of the limbs and their

supporting pectoral and pelvic girdles.

THE VERTEBRAL COLUMN.[174]—The individual segments or vertebrae which,

arranged in a linear series, collectively form the vertebral column, are

highly complex structures, each being formed by a number of vertebral

elements, the sum total of which constitutes a vertebra. Perhaps the best

conception of the nature of vertebral elements is to be gleaned from the

study of such primitive Fishes as the Elasmobranchs, in which not only are

all the vertebral components present, but they are less modified by

suppression and fusion than in most other Fishes, and on this account they

afford a convenient introduction to the study of the puzzling

eccentricities of vertebral structure in other groups. Selecting any common

Dog-Fish, such as _Scyllium canicula_, and starting with an early embryonic

stage, it may be stated that the first indication of a vertebral column is

the formation of the notochord, which, invested by its chordal sheath,

extends from the tip of the tail to a point on the under surface of the

brain just behind the hypophysis or pituitary body.

{194}[Illustration: FIG. 111.—A, side view of precaudal vertebrae of

_Scyllium canicula_; B, similar view of caudal vertebrae. _b.d_,

Basi-dorsal; _c_, centrum; _h_, basi-ventral; _h.s_, haemal spine; _i.d_,

inter-dorsal; _p_, parapophysis; _r_, rib; _s.d_, supra-dorsals. The

vertical dotted lines indicate the limits of neuromeres and myotomes. The

small circles represent the exits of the dorsal and ventral roots of spinal

nerves. (After Ridewood.)]