_External Gills._--In young Crossopterygians and in the young
_Protopterus_ and _Lepidosiren_ true external gills occur of the same
morphological nature as those of Urodele amphibians. In Crossopterygians
a single one is present on each side on the hyoid arch; in the two
Dipnoans mentioned four are present on each side--on visceral arches
III., IV., V. and VI. (It may be recalled that in Urodeles they occur on
arches III., IV. and V., with vestiges[8] on arches I. and II.). Each
external gill develops as a projection of ectoderm with mesodermal core
near the upper end of its visceral arch; the main aortic arch is
prolonged into it as a loop. When fully developed it is pinnate, and is
provided with voluntary muscles by which it can be moved freely to renew
the water in contact with its respiratory surface. In the case of
_Polypterus_ a short rod of cartilage projects from the hyoid arch into
the base of the external gill. Their occurrence with identical main
features in the three groups mentioned indicates that the external gills
are important and archaic organs of the vertebrata. Their non-occurrence
in at least some of the groups where they are absent is to be explained
by the presence of a large vascular yolk sac, which necessarily fulfils
in a very efficient way the respiratory function.
_Alimentary Canal._--The alimentary canal forms a tube traversing the
body from mouth to cloacal opening. Corresponding with structural and
functional differences it is for descriptive purposes divided into the
following regions--(1) Buccal cavity or mouth cavity, (2) Pharynx, (3)
Oesophagus or gullet, (4) Stomach, (5) Intestine, and (6) Cloaca. The
buccal cavity or mouth cavity is morphologically a stomodaeum, i.e. it
represents an inpushing of the external surface. Its opening to the
exterior is wide and gaping in the embryo in certain groups (Selachians
and Crossopterygians), and even in the adult among the Cyclostomata, but
in the adult Gnathostome it can be voluntarily opened and shut in
correlation with the presence of a hinged jaw apparatus. The mouth
opening is less or more ventral in position in Cyclostomes and
Selachians, while in Dipnoans and Teleostomes it is usually terminal.
[Illustration: From Bridge, _Cambridge Natural History_, vol. vii.,
"Fishes, &c." (by permisson of Macmillan & Co., Ltd.). After Boas,
_Lehrbuch der Zoologie_ (by permission of Gustav Fischer).
FIG. 8.--Diagrams to illustrate the relations of branchial clefts and
pharynx in an Elasmobranch (A) and a Teleost (B); 1, 2, &c., Branchial
septa.
b.c, Opercular cavity.
b.l, Respiratory lamellae.
c, Coelom.
e.b.a, Opercular opening.
hy.a, Hyoid arch.
hy.c, Hyobranchial cleft.
l.s, Valvular outer edge of gill septum.
n, Nasal aperture.
oes, Oesophagus.
op, Operculum.
p.q, Palato quadrate cartilage.
Ph, Pharynx.
sp, Spiracle.]
In certain cases (e.g. _Lepidosiren_)[9] the buccal cavity arises by
secondary excavation without any actual pushing in of ectoderm.
It is highly characteristic of the vertebrata that the pharynx--the
portion of the alimentary canal immediately behind the buccal
cavity--communicates with the exterior by a series of paired clefts
associated with the function of respiration and known as the visceral
clefts. It is especially characteristic of fishes that a number of these
clefts remain open as functional breathing organs in the adult.
The visceral clefts arise as hollow pouches (or at first solid
projections) of the endoderm. Each pouch fuses with the ectoderm at its
outer end and then becomes perforated so as to form a free communication
between pharynx and exterior.
The mesenchymatous packing tissue between consecutive clefts forms the
visceral arches, and local condensation within each gives rise to
important skeletal elements--to which the name visceral arches is often
restricted. From the particular skeletal structures which develop in the
visceral arches bounding it the anterior cleft is known as the
hyomandibular cleft, the next one as hyobranchial. In common usage the
hyomandibular cleft is called the spiracle, and the series of clefts
behind it the branchial clefts.
The typical functional gill cleft forms a vertical slit, having on each
side a gill septum which separates it from its neighbours in the series.
The lining of the gill cleft possesses over a less or greater extent of
its area a richly developed network of capillary blood-vessels, through
the thin covering of which the respiratory exchange takes place between
the blood and the water which washes through the gill cleft. The area of
respiratory surface tends to become increased by the development of
outgrowths. Frequently these take the form of regular plate-like
structures known as gill lamellae. In the Selachians these lamellae are
strap-like structures (_Elasmobranch_) attached along nearly their
whole length to the gill septum as shown in fig. 8, A. In the
Holocephali and in the sturgeon the outer portions of the gill septa
have disappeared and this leads to the condition seen in the higher
Teleostomes (fig. 8, B), where the whole of the septum has disappeared
except its thick inner edge containing the skeletal arch. It follows
that in these higher Teleostomes--including the ordinary Teleosts--the
gill lamellae are attached only at their extreme inner end.
In the young of Selachians and certain Teleosts (e.g. _Gymnarchus_ and
_Heterotis_)[10] the gill lamellae are prolonged as filaments which
project freely to the exterior. These must not be confused with true
external gills.
The partial atrophy of the gill septa in the Teleostomes produces an
important change in their appearance. Whereas in the Selachian a series
of separate gill clefts is seen in external view each covered by a soft
valvular backgrowth of its anterior lip, in the Teleostean fish, on the
other hand, a single large opening is seen on each side (opercular
opening) covered over by the enormously enlarged valvular flap belonging
to the anterior lip of the hyobranchial cleft. This flap, an outgrowth
of the hyoid arch, is known as the operculum.
In the Teleostomi there are usually five functional clefts, but these
are the survivors of a formerly greater number. Evidence of reduction is
seen at both ends of the series. In front of the first functional cleft
(the hyobranchial) there is laid down in the embryo the rudiment of a
spiracular cleft. In the less highly organized fishes this survives in
many cases as an open cleft.
In many sharks and in sturgeons the spiracle forms a conspicuous
opening just behind the eye. In rays and skates, which are modified in
correlation with their ground feeding habit, the spiracle is a large
opening which during the great widening out of the body during
development comes to be situated on the dorsal side, while the
branchial clefts come to be ventral in position. In existing
Crossopterygians the spiracle is a slit-like opening on the dorsal
side of the head which can be opened or closed at will. In Dipneusti,
as in the higher Teleostomes, the spiracle is found as an embryonic
rudiment, but in this case it gives rise in the adult to a remarkable
sense organ of problematical function.[11]
Traces of what appear to be pre-spiracular clefts exist in the embryos
of various forms. Perhaps the most remarkable of these is to be found in
the larval Crossopterygian,[12] and apparently also in _Amia_[13] at
least, amongst the other ganoids, where a pair of entodermal pouches
become cut off from the main entoderm and, establishing an opening to
the exterior, give rise to the lining of the cement organs of the larva.
Posteriorily there is evidence that the extension backwards of the
series of gill clefts was much greater in the primitive fishes. In the
surviving sharks (_Chlamydoselachus_ and _Notidanus cinereus_), there
still exist in the adult respectively six and seven branchial clefts,
while in embryonic Selachians there are frequently to be seen pouch-like
outgrowths of entoderm apparently representing rudimentary gill pouches
but which never develop. Further evidence of the progressive reduction
in the series of clefts is seen in the reduction of their functional
activity at the two ends of the series. The spiracle, even where
persisting in the adult, has lost its gill lamellae either entirely or
excepting a few vestigial lamellae forming a "pseudobranch" on its
anterior wall (Selachians, sturgeons). A similar reduction affects the
lamellae on the anterior wall of the hyobranchial cleft (except in
Selachians) and on the posterior wall of the last branchial cleft.
A pseudobranch is frequently present in Teleostomes on the anterior
wall of the hyobranchial cleft, i.e. on the inner or posterior face of
the operculum. It is believed by some morphologists to belong really
to the cleft in front.[14]
_Phylogeny._--The phylogeny of the gill clefts or pouches is
uncertain. The only organs of vertebrates comparable with them
morphologically are the enterocoelic pouches of the entoderm which
give rise to the mesoderm. It is possible that the respiratory
significance of the wall of the gill cleft has been secondarily
acquired. This is indicated by the fact that they appear in some cases
to be lined by an ingrowth of ectoderm. This suggests that there may
have been a spreading inwards of respiratory surface from the external
gills. It is conceivable that before their walls became directly
respiratory the gill clefts served for the pumping of fresh water over
the external gills at the bases of which they lie.
_Lung._--As in the higher vertebrates, there develops in all the main
groups of gnathostomatous fishes, except the Selachians, an outgrowth of
the pharyngeal wall intimately associated with gaseous interchange. In
the Crossopterygians and Dipnoans this pharyngeal outgrowth agrees
exactly in its mid-ventral origin and in its blood-supply with the lungs
of the higher vertebrates, and there can be no question about its being
morphologically the same structure as it is also in function.
[Illustration: FIG. 9.--Lung of _Neoceratodus_, opened in its lower half
to show its cellular pouches. a, Right half; b, Left half; c, Cellular
pouches; e, Pulmonary vein; f, Arterial blood-vessel; oe, Oesophagus,
opened to show glottis (gl.)]
In the Crossopterygian the ventrally placed slit-like glottis leads
into a common chamber produced anteriorly into two horns and continued
backwards into two "lungs." These are smooth, thin-walled, saccular
structures, the right one small, the left very large and extending to
the hind end of the splanchnocoele. In the Dipnoans the lung has taken
a dorsal position close under the vertebral column and above the
splanchnocoele. Its walls are sacculated, almost spongy in
_Lepidosiren_ and _Protopterus_, so as to give increase to the
respiratory surface. In _Nexeratodus_ (fig. 9) an indication of
division into two halves is seen in the presence of two prominent
longitudinal ridges, one dorsal and one ventral. In _Lepidosiren_ and
_Protopterus_ the organ is completely divided except at its anterior
end into a right and a left lung. The anterior portion of the lung or
lungs is connected with the median ventral glottis by a short wide
vestibule which lies on the right side of the oesophagus.
In the Teleostei the representative of the lung, here termed the
swimbladder, has for its predominant function a hydrostatic one; it acts
as a float. It arises as a diverticulum of the gut-wall which may retain
a tubular connexion with the gut (_physostomatous_ condition) or may in
the adult completely lose such connexion (_physoclistic_). It shows two
conspicuous differences from the lung of other forms: (1) it arises in
the young fish as a dorsal instead of as a ventral diverticulum, and (2)
it derives its blood-supply not from the sixth aortic arch but from
branches of the dorsal aorta.
These differences are held by many to be sufficient to invalidate the
homologizing of the swimbladder with the lung. The following facts,
however, appear to do away with the force of such a contention. (1) In
the Dipneusti (e.g. _Neoceratodus_) the lung apparatus has acquired a
dorsal position, but its connexion with the mid-ventral glottis is
asymmetrical, passing round the right side of the gut. Were the
predominant function of the lung in such a form to become hydrostatic
we might expect the course of evolution to lead to a shifting of the
glottis dorsalwards so as to bring it nearer to the definitive
situation of the lung. (2) In _Erythrinus_ and other Characinids the
glottis is not mid-ventral but decidedly lateral in position,
suggesting either a retention of, or a return to, ancestral stages in
the dorsalward migration of the glottis. (3) The blood-supply of the
Teleostean swimbladder is from branches of the dorsal aorta, which may
be distributed over a long anteroposterior extent of that vessel.
Embryology, however, shows that the swimbladder arises as a localized
diverticulum. It follows that the blood-supply from a long stretch of
the aorta can hardly be primitive. We should rather expect the
primitive blood-supply to be from the main arteries of the pharyngeal
wall, i.e. from the hinder aortic arch as is the case with the lungs
of other forms. Now in _Amia_ at least we actually find such a
blood-supply, there being here a pulmonary artery corresponding with
that in lung-possessing forms. Taking these points into consideration
there seems no valid reason for doubting that in lung and swimbladder
we are dealing with the same morphological structure.
_Function._--In the Crossopterygians and Dipnoans the lung is used for
respiration, while at the same time fulfilling a hydrostatic function.
Amongst the Actinopterygians a few forms still use it for respiration,
but its main function is that of a float. In connexion with this
function there exists an interesting compensatory mechanism whereby the
amount of gas in the swimbladder may be diminished (by absorption), or,
on the other hand, increased, so as to counteract alterations in
specific gravity produced, e.g. by change of pressure with change of
depth. This mechanism is specially developed in physoclistic forms,
where there occur certain glandular patches ("red glands") in the lining
epithelium of the swimbladder richly stuffed with capillary
blood-vessels and serving apparently to secrete gas into the
swimbladder. That the gas in the swimbladder is produced by some vital
process, such as secretion, is already indicated by its composition, as
it may contain nearly 90% of oxygen in deep-sea forms or a similar
proportion of nitrogen in fishes from deep lakes, i.e. its composition
is quite different from what it would be were it accumulated within the
swimbladder by mere ordinary diffusion processes. Further, the formation
of gas is shown by experiment to be controlled by branches of the vagus
and sympathetic nerves in an exactly similar fashion to the secretion of
saliva in a salivary gland. (See below for relations of swimbladder to
ear).
Of the important non-respiratory derivatives of the pharyngeal wall
(thyroid, thymus, postbranchial bodies, &c.), only the thyroid calls for
special mention, as important clues to its evolutionary history are
afforded by the lampreys. In the larval lamprey the thyroid develops as
a longitudinal groove on the pharyngeal floor. From the anterior end of
this groove there pass a pair of peripharyngeal ciliated tracts to the
dorsal side of the pharynx where they pass backwards to the hind end of
the pharynx. Morphologically the whole apparatus corresponds closely
with the endostyle and peripharyngeal and dorsal ciliated tracts of the
pharynx of _Amphioxus_. The correspondence extends to function, as the
open thyroid groove secretes a sticky mucus which passes into the
pharyngeal cavity for the entanglement of food particles exactly as in
_Amphioxus_. Later on the thyroid groove becomes shut off from the
pharynx; its secretion now accumulates in the lumina of its interior and
it functions as a ductless gland as in the Gnathostomata. The only
conceivable explanation of this developmental history of the thyroid in
the lamprey is that it is a repetition of phylogenetic history.
Behind the pharynx comes the main portion of the alimentary canal
concerned with the digestion and absorption of the food. This forms a
tube varying greatly in length, more elongated and coiled in the higher
Teleostomes, shorter and straighter in the Selachians, Dipnoans and
lower Teleostomes. The oesophagus or gullet, usually forming a short,
wide tube, leads into the glandular, more or less dilated stomach. This
is frequently in the form of a letter J, the longer limb being
continuous with the gullet, the shorter with the intestine. The curve of
the J may be as in _Polypterus_ and the perch produced backwards into a
large pocket. The intestine is usually marked off from the stomach by a
ring-like sphincter muscle forming the pyloric valve. In the lower
gnathostomatous fishes (Selachians, Crossopterygians, Dipnoans,
sturgeons) the intestine possesses the highly characteristic spiral
valve, a shelf-like projection into its lumen which pursues a spiral
course, and along the turns of which the food passes during the course
of digestion. From its universal occurrence in the groups mentioned we
conclude that it is a structure of a very archaic type, once
characteristic of ancestral Gnathostomata; a hint as to its
morphological significance is given by its method of development.[15] In
an early stage of development the intestinal rudiment is coiled into a
spiral and it is by the fusion together of the turns that the spiral
valve arises. The only feasible explanation of this peculiar method of
development seems to lie in the assumption that the ancestral
gnathostome possessed an elongated, coiled intestine which subsequently
became shortened with a fusion of its coils. In the higher fishes the
spiral valve has disappeared--being still found, however, in a reduced
condition in _Amia_ and _Lepidosteus_, and possibly as a faint vestige
in one or two Teleosts (certain _Clupeidae_[16] and _Salmonidae_[17]).
In the majority of the Teleosts the absence of spiral valves is coupled
with a secondary elongation of the intestinal region, which in extreme
cases (_Loricariidae_) may be accompanied by a secondary spiral coiling.
The terminal part of the alimentary canal--the cloaca--is characterized
by the fact that into it open the two kidney ducts. In Teleostomes the
cloaca is commonly flattened out, so that the kidney ducts and the
alimentary canal come to open independently on the outer surface.
The lining of the alimentary canal is throughout the greater part of its
extent richly glandular. And at certain points local enlargements of the
secretory surface take place so as to form glandular diverticula. The
most ancient of these as indicated by its occurrence even in _Amphioxus_
appears to be the _liver_, which, originally--as we may assume--mainly a
digestive gland, has in the existing Craniates developed important
excretory and glycogen-storing functions. Arising in the embryo as a
simple caecum, the liver becomes in the adult a compact gland of very
large size, usually bi-lobed in shape and lying in the front portion of
the splanchnocoele. The stalk of the liver rudiment becomes drawn out
into a tubular bile duct, which may become subdivided into branches, and
as a rule develops on its course a pocket-like expansion, the
gall-bladder. This may hang freely in the splanchnocoele or may be, as
in many Selachians, imbedded in the liver substance.
The pancreas also arises by localized bulging outwards of the intestinal
lining--there being commonly three distinct rudiments in the embryo. In
the Selachians the whitish compact pancreas of the adult opens into the
intestine some little distance behind the opening of the bile duct, but
in the Teleostomes it becomes involved in the liver outgrowth and mixed
with its tissue, being frequently recognizable only by the study of
microscopic sections. In the Dipnoans the pancreatic rudiment remains
imbedded in the wall of the intestine: its duct is united with that of
the liver.
_Pyloric Caeca._--In the Teleostomi one or more glandular diverticula
commonly occur at the commencement of the intestine and are known as the
pyloric caeca. There may be a single caecum (crossopterygians,
_Ammodytes_ amongst Teleosts) or there may be nearly two hundred
(mackerel). In the sturgeons the numerous caeca form a compact gland. In
several families of Teleosts, on the other hand, there is no trace of
these pyloric caeca.
In Selachians a small glandular diverticulum known as the _rectal gland_
opens into the terminal part of the intestine on its dorsal side.
_Coelomic Organs._--The development of the mesoderm in the restricted
sense (mesothelium) as seen in the fishes (lamprey, _Lepidosiren_,
_Protopterus_, _Polypterus_) appears to indicate beyond doubt that the
mesoderm segments of vertebrates are really enterocoelic pouches in
which the development of the lumen is delayed. Either the inner, or both
inner and outer (e.g. _Lepidosiren_) walls of the mesoderm segment pass
through a myoepithelial condition and give rise eventually to the great
muscle segments (myomeres, or myotomes) which lie in series on each side
of the trunk. In the fishes these remain distinct throughout life. The
fins, both median and paired, obtain their musculature by the ingrowth
into them of muscle buds from the adjoining myotomes.
[Illustration: From Gegenbaur, _Untersuchungen zur vergleich. Anat. der
Wirbeltiere_, by permission of Wilhelm Engelmann.
FIG. 10.--View of _Torpedo_ from the dorsal side: the electric organs are
exposed.
I, Fore-brain.
II, Mesencephalon.
III, Cerebellum.
IV, Electric lobe.
br, Common muscular sheath covering branchial clefts (on the left side
this has been removed so as to expose the series of branchial sacs).
f, Spiracle.
o.e, Electric organ, on the left side the nerve-supply is shown.
o, Eye.
t, Sensory tubes of lateral line system.]
_Electrical Organs._[18]--It is characteristic of muscle that at the
moment of contraction it produces a slight electrical disturbance. In
certain fishes definite tracts of the musculature show a reduction of
their previously predominant function of contraction and an increase of
their previously subsidiary function of producing electrical
disturbance; so that the latter function is now predominant.
In the skates (_Raia_) the electrical organ is a fusiform structure
derived from the lateral musculature of the tail; in _Gymnotus_--the
electric eel--and in _Mormyrus_ it forms an enormous structure
occupying the place of the ventral halves of the myotomes along nearly
the whole length of the body; in _Torpedo_ it forms a large, somewhat
kidney-shaped structure as viewed from above lying on each side of the
head and derived from the musculature of the anterior visceral arches.
In _Torpedo_ the nerve-supply is derived from cranial nerves VII. IX.
and the anterior branchial branches of X.
The electric organ is composed of prismatic columns each built up of a
row of compartments. Each compartment contains a lamellated electric
disc representing the shortened-up and otherwise metamorphosed muscle
fibre. On one face (ventral in _Torpedo_, anterior in _Raia_) of the
electric disc is a gigantic end-plate supplied by a beautiful,
dichotomously branched, terminal nervous arborization.
The development of the mesoderm of the head region is too obscure for
treatment here.[19] The ventral portion of the trunk mesoderm gives rise
to the splanchnocoel or general coelom. Except in the Myxinoids the
anterior part of the splanchnocoel becomes separated off as a
pericardiac cavity, though in adult Selachians the separation becomes
incomplete, the two cavities being in communication by a
pericardio-peritoneal canal.
_Nephridial System._---The kidney system in fishes consists of
segmentally arranged tubes leading from the coelom into a longitudinal
duct which opens within the hinder end of the enteron--the whole forming
what is known as the _archinephros_ (Lankester) or _holonephros_
(Price). Like the other segmented organs of the vertebrate the
archinephros develops from before backwards. The sequence is, however,
not regular. A small number of tubules at the head end of the series
become specially enlarged and are able to meet the excretory needs
during larval existence (_Pronephros_): the immediately succeeding
tubules remain undeveloped, and then come the tubules of the rest of the
series which form the functional kidney of the adult (_Mesonephros_).
The kidney tubules subserve the excretory function in two different
ways. The wall of the tubule, bathed in blood from the posterior
cardinal vein, serves to extract nitrogenous products of excretion from
the blood and pass them into the lumen of the tubule. The open ciliated
funnel or nephrostome at the coelomic end of the tubule serves for the
passage outwards of coelomic fluid to flush the cavity of the tubule.
The secretory activity of the coelomic lining is specially concentrated
in certain limited areas in the neighbourhood of the nephrostomes, each
such area ensheathing a rounded mass depending into the coelom and
formed of a blood-vessel coiled into a kind of skein--a glomerulus. In
the case of the pronephros the glomeruli are as a rule fused together
into a single glomus. In the mesonephros they remain separate and in
this case the portion of coelom surrounding the glomerulus tends to be
nipped off from the general coelom--to form a Malpighian body. The
separation may be incomplete--the Malpighian coelom remaining in
connexion with the general coelom by a narrow peritoneal canal. The
splanchnocoelic end of this is usually ciliated and is termed a
peritoneal funnel: it is frequently confused with the nephrostome.
_Mesonephros._--The kidney of the adult fish is usually a compact gland
extending over a considerable distance in an anteroposterior direction
and lying immediately dorsal to the coelomic cavity.
Peritoneal funnels are present in the adult of certain Selachians (e.g.
_Acanthias_, _Squatina_), though apparently in at least some of these
forms they no longer communicate with the Malpighian bodies or tubules.
The kidneys of the two sides become fused together posteriorly in
_Protopterus_ and in some Teleosts. The mesonephric ducts undergo fusion
posteriorly in many cases to form a median urinary or urinogenital
sinus. In the Selachians this median sinus is prolonged forwards into a
pair of horn-like continuations--the sperm sacs. In Dipnoans the sinus
becomes greatly dilated and forms a large, rounded, dorsally placed
cloacal caecum. In Actinopterygians a urinary bladder of similar
morphological import is commonly present.
_Gonads._--The portion of coelomic lining which gives rise to the
reproductive cells retains its primitive relations most nearly in the
female, where, as a rule, the genital cells are still shed into the
splanchnocoele. Only in Teleostomes (_Lepidosteus_ and most Teleosts)
the modification occurs that the ovary is shut off from the
splanchnocoele as a closed cavity continuous with its duct.
In a few Teleosts (_Salmonidae_, _Muraenidae_, _Cobitis_) the ovary is
not a closed sac, its eggs being shed into the coelom as in other
groups.
The appearance of the ovary naturally varies greatly with the character
of the eggs.
The portion of coelomic lining which gives rise to the male genital
cells (testis) is in nearly, if not quite, all cases, shut off from the
splanchnocoele. The testes are commonly elongated in form. In
Dipneusti[20] (_Lepidosiren_ and _Protopterus_) the hinder portion of
the elongated testis has lost its sperm-producing function, though the
spermatozoa produced in the anterior portion have to traverse it in
order to reach the kidney. In _Polypterus_[21] the testis is continued
backwards as a "testis ridge," which appears to correspond with the
posterior vesicular region of the testis in _Lepidosiren_ and
_Protopterus_. Here also the spermatozoa pass back through the cavities
of the testis ridge to reach the kidney duct. In the young Teleost[22]
the rudiment of the duct forms a backward continuation of the testis
containing a network of cavities and opening as a rule posteriorly into
the kidney duct. It is difficult to avoid the conclusion that the testis
duct of the Teleost is for the most part the equivalent morphologically
of the posterior vesicular region of the testis of _Polypterus_ and the
Dipneusti.
_Relations of Renal and Reproductive Organs._ (1) _Female._--In the
Selachians and Dipnoans the oviduct is of the type (Müllerian duct)
present in the higher vertebrates and apparently representing a
split-off portion of the archinephric duct. At its anterior end is a
wide funnel-like coelomic opening. Its walls are glandular and secrete
accessory coverings for the eggs. In the great majority of Teleosts and
in _Lepidosteus_ the oviduct possesses no coelomic funnel, its walls
being in structural continuity with the wall of the ovary. In most of
the more primitive Teleostomes (Crossopterygians, sturgeons, _Amia_) the
oviduct has at its front end an open coelomic funnel, and it is
difficult to find adequate reason for refusing to regard such oviducts
as true Müllerian ducts. On this interpretation the condition
characteristic of Teleosts would be due to the lips of the oviduct
becoming fused with the ovarian wall, and the duct itself would be a
Müllerian duct as elsewhere.
[Illustration: From _Arch. zool, expérimentale_, by permission of
Schleicher Frères.
FIG. 11.--Urino-Genital Organs of the right side in a male _Scyllium_.
(After Borcea.)
m.n. 1, Anterior (genital) portion of mesonephros
with its coiled duct.
m.n. 2, Posterior (renal) portion of mesonephros.
s.s, Sperm sac.
T, Testis.
u, "Ureter" formed by fusion of collecting tubes of renal portion of
mesonephros.
u.g.s, Urino-genital sinus;
v.s, Vesicula seminalis.]
A departure from the normal arrangement is found in those Teleosts
which shed their eggs into the splanchnocoele, e.g. amongst
_Salmonidae_, the smelt (_Osmerus_) and capelin (_Mallotus_) possess a
pair of oviducts resembling Müllerian ducts while the salmon possesses
merely a pair of genital pores opening together behind the anus. It
seems most probable that the latter condition has been derived from
the former by reduction of the Müllerian ducts, though it has been
argued that the converse process has taken place. The genital pores
mentioned must not be confused with the _abdominal pores_, which in
many adult fishes, particularly in those without open peritoneal
funnels, lead from coelom directly to the exterior in the region of
the cloacal opening. These appear to be recent developments, and to
have nothing to do morphologically with the genitourinary system.[23]
(2) _Male._--It seems that primitively the male reproductive elements
like the female were shed into the coelom and passed thence through the
nephridial tubules. In correlation probably with the greatly reduced
size of these elements they are commonly no longer shed into the
splanchnocoele, but are conveyed from the testis through covered-in
canals to the Malpighian bodies or kidney tubules. The system of
covered-in canals forms the testicular network, the individual canals
being termed vasa efferentia. In all probability the series of vasa
efferentia was originally spread over the whole length of the elongated
testis (cf. _Lepidosteus_), but in existing fishes the series is as a
rule restricted to a comparatively short anteroposterior extent. In
Selachians the vasa efferentia are restricted to the anterior end of
testis and kidney, and are connected by a longitudinal canal ending
blindly in front and behind. The number of vasa efferentia varies and in
the rays (_Raia_, _Torpedo_) may be reduced to a single one opening
directly into the front end of the mesonephric duct. The anterior
portion of the mesonephros is much reduced in size in correlation with
the fact that it has lost its renal function. The hinder part, which is
the functional kidney, is considerably enlarged. The primary tubules of
this region of the kidney have undergone a modification of high
morphological interest. Their distal portions have become much
elongated, they are more or less fused, and their openings into the
mesonephric duct have undergone backward migration until they open
together either into the mesonephric duct at its posterior end or into
the urinogenital sinus independently of the mesonephric duct. The
mesonephric duct is now connected only with the anterior part of the
kidney, and serves merely as a vas deferens or sperm duct. In
correlation with this it is somewhat enlarged, especially in its
posterior portion, to form a vesicula seminalis.
The morphological interest of these features lies in the fact that
they represent a stage in evolution which carried a little farther
would lead to a complete separation of the definitive kidney
(_metanephros_) from the purely genital anterior section of the
mesonephros (_epididymis_), as occurs so characteristically in the
Amniota.
Dipneusti.--In _Lepidosiren_[24] a small number (about half a dozen) of
vasa efferentia occur towards the hind end of the vesicular part of the
testis and open into Malpighian bodies. In _Protopterus_ the vasa
efferentia are reduced to a single one on each side at the extreme hind
end of the testis.
[Illustration: Graham Kerr, _Proc. Zool. Soc. London_.
FIG. 12.--Diagram illustrating Connexion between Kidney and Testis in
Various Groups of Fishes.
A, Distributed condition of _vasa efferentia_ (_Acipenser_,
_Lepidosteus_).
B, _Vasa efferentia_ reduced to a few at the hind end (_Lepidosiren_).
C, Reduction of vasa efferentia to a single one posteriorly
(_Protopterus_).
D, Direct communication between testis and kidney duct (_Polypterus_,
Teleosts).
c.f, Nephrostome leading from Malpighian coelom into kidney tubule.
T1, Functional region of testis.
T2, Vesicular region of testis.
WD, Mesonephric duct.]
Teleostomi.--In the actinopterygian Ganoids a well-developed testicular
network is present; e.g. in _Lepidosteus_[25] numerous vasa efferentia
arise from the testis along nearly its whole length and pass to a
longitudinal canal lying on the surface of the kidney, from which in
turn transverse canals lead to the Malpighian bodies. (In the case of
_Amia_ they open into the tubules or even directly into the mesonephric
duct.) In the Teleosts and in _Polypterus_ there is no obvious connexion
between testis and kidney, the wall of the testis being continuous with
that of its duct, much as is the case with the ovary and its duct in the
female. In all probability this peculiar condition is to be
explained[26] by the reduction of the testicular network to a single vas
efferens (much as in _Protopterus_ or as in _Raia_ and various anurous
Amphibians at the front end of the series) which has come to open
directly into the mesonephric duct (cf. fig. 12).
_Organs of the Mesenchyme._--In vertebrates as in all other Metazoa,
except the very lowest, there are numerous cell elements which no longer
form part of the regularly arranged epithelial layers, but which take
part in the formation of the packing tissue of the body. Much of this
forms the various kinds of connective tissue which fill up many of the
spaces between the various epithelial layers; other and very important
parts of the general mesenchyme become specialized in two definite
directions and give rise to two special systems of organs. One of these
is characterized by the fact that the intercellular substance or matrix
assumes a more or less rigid character--it may be infiltrated with salts
of lime--giving rise to the supporting tissues of the skeletal system.
The other is characterized by the intercellular matrix becoming fluid,
and by the cell elements losing their connexion with one another and
forming the characteristic fluid tissue, the blood, which with its
well-marked containing walls forms the blood vascular system.
_Skeletal System._--The skeletal system may be considered under three
headings--(1) the chordal skeleton, (2) the cartilaginous skeleton and
(3) the osseous skeleton.
1. _Chordal Skeleton._--The most ancient element of the skeleton appears
to be the _notochord_--a cylindrical rod composed of highly vacuolated
cells lying ventral to the central nervous system and dorsal to the gut.
Except in _Amphioxus_--where the condition may probably be secondary,
due to degenerative shortening of the central nervous system--the
notochord extends from a point just behind the infundibulum of the brain
(see below) to nearly the tip of the tail. In ontogeny the notochord is
a derivative of the dorsal wall of the archenteron. The outer layer of
cells, which are commonly less vacuolated and form a "chordal
epithelium," soon secretes a thin cuticle which ensheaths the notochord
and is known as the primary sheath. Within this there is formed later a
secondary sheath, like the primary, cuticular in nature. This secondary
sheath attains a considerable thickness and plays an important part in
strengthening the notochord. The notochord with its sheaths is in
existing fishes essentially the skeleton of early life (embryonic or
larval). In the adult it may, in the more primitive forms (Cyclostomata,
Dipneusti), persist as an important part of the skeleton, but as a rule
it merely forms the foundation on which the cartilaginous or bony
vertebral column is laid down.
2. _Cartilaginous or Chondral Skeleton._--(A) Vertebral column.[27] In
the embryonic connective tissue or mesenchyme lying just outside the
primary sheath of the notochord there are developed a dorsal and a
ventral series of paired nodules of cartilage known as _arcualia_ (fig.
13, d.a, v.a). The dorsal arcualia are commonly prolonged upwards by
supradorsal cartilages which complete the _neural arches_ and serve to
protect the spinal cord. The ventral arcualia become, in the tail region
only, also incorporated in complete arches--the _haemal arches_. In
correlation with the flattening of the body of the fish from side to
side the arches are commonly prolonged into elongated neural or haemal
spines.
The relations of the arcualia to the segmentation of the body, as
shown by myotomes and spinal nerves, is somewhat obscure. The
mesenchyme in which they arise is segmental in origin (sclerotom),
which suggests that they too may have been primitively segmental, but
in existing fishes there are commonly two sets of arcualia to each
body segment.
In gnathostomatous fishes the arcualia play a most important part in
that cartilaginous tissue derived from them comes into special
relationships with the notochord and gives rise to the vertebral column
which functionally replaces this notochord in most of the fishes. This
replacement occurs according to two different methods, giving rise to
the different types of vertebral column known as chordacentrous and
arcicentrous.
(a) Chordacentrous type. An incipient stage in the evolution of a
chordacentrous vertebral column occurs in the Dipneusti, where cartilage
cells from the arcualia become amoeboid and migrate into the substance
of the secondary sheath, boring their way through the primary sheath
(fig. 13, C). They wander throughout the whole extent of the secondary
sheath, colonizing it as it were, and settle down as typical stationary
cartilage cells. The secondary sheath is thus converted into a cylinder
of cartilage. In Selachians exactly the same thing takes place, but in
recent forms development goes a step further, as the cartilage cylinder
becomes broken into a series of segments, known as vertebral centra. The
wall of each segment becomes much thickened in the middle so that the
notochord becomes constricted within each centrum and the space occupied
by it is shaped like the cavity of a dice-box. When free from notochord
and surrounding tissues such a cartilaginous centrum presents a deep
conical cavity at each end (_amphicoelous_).
[Illustration: From Wiedersheim, _Grundriss der vergleichenden
Anatomie_, by permission of Gustav Fischer.
FIG. 13.--Diagrammatic transverse sections to illustrate the morphology
of the vertebral column.
A, Primitive conditions as seen in any young embryo.
B, Condition as it occurs in Cyclostomata, sturgeons, embryos of bony
Actinopterygians.
C, Condition found in Selachians and Dipnoans.
D and E, Illustrating the developmental process in bony
Actinopterygians and higher vertebrates.
c, Centrum.
d.a, Dorsal arcualia.
n.a, Neural arch.
nc, Notochord.
nc.ep, Chordal epithelium.
n.sp, Neural spine.
sh.1, Primary sheath.
sh.2, Secondary sheath.
sk.l, Connective tissue.
tr.p, Transverse process.
v.a, Ventral arcualia.]
A secondary modification of the centrum consists in the calcification
of certain zones of the cartilaginous matrix. The precise arrangement
of these calcified zones varies in different families and affords
characters which are of taxonomic importance in palaeontology where
only skeletal structures are available (see SELACHIANS).
(b) Arcicentrous type. Already in the Selachians the vertebral column is
to a certain extent strengthened by the broadening of the basis of the
arcualia so as partially to surround the centra. In the Teleostomes,
with the exceptions of those ganoids mentioned, the expanded bases of
the arcualia undergo complete fusion to form cartilaginous centra which,
unlike the chordacentrous centra, lie outside the primary sheath (figs.
13, D and E). In these forms no invasion of the secondary sheath by
cartilage cells takes place. The composition of the groups of arcualia
which give rise to the individual centrum is different in different
groups. The end result is an amphicoelous or biconcave centrum in
general appearance much like that of the Selachian.
In _Lepidosteus_ the spaces between adjacent centra become filled by a
secondary development of intervertebral cartilage which then splits in
such a way that the definitive vertebrae are _opisthocoelous_, i.e.
concave behind, convex in front.
_Ribs._--In the Crossopterygians a double set of "ribs" is present on
each side of the vertebral column, a ventral set lying immediately
outside the splanchnocoelic lining and apparently serially homologous
with the haemal arches of the caudal region, and a second set passing
outwards in the thickness of the body wall at a more dorsal level. In
the Teleostomes and Dipnoans only the first type is present; in the
Selachians only the second. It would appear that it is the latter which
is homologous with the ribs of vertebrates above fishes.
_Median Fin Skeleton._--the foundation of the skeleton of the median
fins consists of a series of rod-like elements, the radialia, each of
which frequently is segmented into three portions. In a few cases the
radialia correspond segmentally with the neural and haemal arches
(living Dipnoans, _Pleuracanthus_ tail region) and this suggests that
they represent morphologically prolongations of the neural and haemal
spines. That this is so is rendered probable by the fact that we must
regard the evolution of the system of median fins as commencing with a
simple flattening of the posterior part of the body. It is only natural
to suppose that the edges of the flattened region would be at first
supported merely by prolongations of the already existing spinous
processes. In the Cyclostomes (where they are branched) and in the
Selachians, the radialia form the main supports of the fin, though
already in the latter they are reinforced by a new set of fin rays
apparently related morphologically to the osseous or placoid skeleton
(see below).
The series of radialia tends to undergo the same process of local
concentration which characterizes the fin-fold as a whole. In its
extreme form this leads to complete fusion of the basal portions of a
number of radialia (dorsal fins of _Holoptychius_ and various
Selachians, and anal fin of _Pleuracanthus_). In view of the identity
in function it is not surprising that a remarkable resemblance exists
between the mechanical arrangements (of skeleton, muscles, &c.), of
the paired and unpaired fins. The resemblance to paired fins becomes
very striking in some of the cases where the basal fusion mentioned
above takes place (_Pleuracanthus_).
[Illustration: _Trans. Roy. Soc. Edinburgh._
FIG. 14.--Chondrocranium of a young Lepidosiren, showing the suspension
of the lower jaw by the upper portion of the mandibular arch. (After
Agar.)
H, Hyoid arch.
M, Mandibular arch.
o.a, Occipital arch.
ot, Auditory capsule.
q, Quadrate = upper end of mandibular arch.
tr, Trabecula.
The palatopterygoid bar (p.pt) is represented by a faint vestige which
disappears before the stage figured.]
(B) _Chondrocranium[28]._--In front of the vertebral column lies the
cartilaginous trough, the chondrocranium, which protects the brain. This
consists of a praechordal portion--developed out of a pair of lateral
cartilaginous rods--the _trabeculae cranii_--and a parachordal portion
lying on either side of the anterior end of the notochord. This arises
in development from a cartilaginous rod (parachordal cartilage) lying
on each side of the notochord and possibly representing a fused row of
dorsal arcualia. The originally separate parachordals and trabeculae
become connected to form a trough-like, primitive cranium, complete or
nearly so laterally and ventrally but open dorsally. With the primitive
cranium there are also connected cartilaginous capsules developed round
the olfactory and auditory organs. There also become fused with the
hinder end of the cranium a varying number of originally distinct neural
arches.
[Illustration: _A._ After W. K. Parker, _Trans. Zool. Soc. London_.
_B._ After Gegenbaur, _Untersuchungen zur verg. Anat. der Wirbeltiere_,
by permission of Wilhelm Engelmann.
_C._ After Hubrecht, Brown's _Tierreich_, by permission of Gustav
Fischer.
FIG. 15.--Chondrocranium, &c. of _Scyllium_ (A), _Notidanus cinereus_
(B) and _Chimaera_ (C).
Br.A, Branchial arches. olf, Olfactory capsule.
c.h, Ceratohyal. ot, Auditory capsule.
e.p.l, Ethmopalatine ligament. p.pt, Palato-pterygoid bar.
Hm, Hyomandibular. p.s.l, Prespiracular ligament.
M, Meckel's cartilage. r, Rostrum.]
o, Orbit.
(C) _Visceral Arches._--The skeleton of the visceral arches consists
essentially of a series of half-hoops of cartilage, each divided in the
adult into a number of segments and connected with its fellow by a median
ventral cartilage. The skeleton of arches I. and II. (mandibular and
hyoidean) undergoes modifications of special interest (figs. 14 and 15).
The lower portion of the mandibular arch becomes greatly thickened to
support the lower or hinder edge of the mouth. It forms the primitive
lower jaw or "Meckel's cartilage." Dorsal to this an outgrowth arises from
the anterior face of the arch which supports the upper or anterior margin
of the mouth: it is the primitive upper jaw or palato-pterygoquadrate
cartilage. The portion of the arch dorsal to the palato-pterygo-quadrate
outgrowth may form the suspensorial apparatus of the lower jaw, being
fused with the cranium at its upper end. This relatively primitive
con-arrangement (_protostylic_, as it may be termed) occurs in Dipneusti
among fishes (cf. fig. 14). More usually this dorsal part of the
mandibular arch becomes reduced, its place being occupied by a ligament
(pre-spiracular) uniting the jaw apparatus to the chondrocranium, the
upper jaw being also attached to the chondrocranium by the ethmopalatine
ligament situated more anteriorly. The main attachment, however, of the
jaws to the chondrocranium in such a case, as holds for the majority of
fishes, is through the enlarged dorsal segment of the hyoid arch
(hyomandibular) which articulates at its dorsal end with the
chondrocranium, while its ventral end is attached to the hinge region of
the jaw by stout ligamentous bands. A skull in which the jaws are
suspended in this manner is termed a hyostylic skull (e.g. _Scyllium_ in
fig. 15).
[Illustration: FIG. 16.--Fore-limb of _Ceratodus_.]
In _Notidanus_ (fig. 15, B) there is a large direct articulation of
the upper jaw to the chondrocranium in addition to the indirect one
through the hyomandibular: such a skull is amphistylic. In
_Heterodontus_ the upper jaw is firmly bound to the cranium throughout
its length, while in Holocephali (fig. 15, C) complete fusion has
taken place, so that the lower jaw appears to articulate directly with
the cranium ("auto stylic" condition). In Dipneusti[29] (_Lepidosiren_
and _Protopterus_) the cartilaginous upper jaw never develops (except
in its hinder quadrate portion) beyond the condition of a faint
rudiment, owing doubtless to its being replaced functionally by
precociously developed bone.
(D) _Appendicular Skeleton._--The skeleton of the free part of the limb
is attached to the limb girdle which lies embedded in the musculature of
the body. Each limb girdle is probably to be looked upon as consisting,
like the skeleton of the visceral arches, of a pair of lateral
half-hoops of cartilage. While in _Pleuracanthus_ the lateral halves are
distinct (and segmented like the branchial arches), in living Selachians
generally the two halves are completely fused ventrally with one
another. The part of the girdle lying dorsal to the articulation of the
limb is termed scapular in the case of the pectoral limb, iliac in the
case of the pelvic, while the ventral portions are known respectively as
coracoid and ischio-pubic.
[Illustration: FIG. 17.--a, Skeleton of pectoral limb of
_Pleuracanthus_. (From Gegenbauer, after Frisch.) b, Skeleton of
pectoral limb of _Acanthias_. (After Gegenbauer.)]
In most Teleostomes the primitive pelvic girdle does not develop; in
the Dipneusti it is represented by a median unpaired cartilage.
The skeleton of the free limb is probably seen in its most archaic form
amongst existing fishes in the biserial archipterygium of _Ceratodus_
(fig. 16). This is indicated by the relative predominance of this type
of fin amongst the geologically more ancient fishes. The biserial
archipterygium consists of a segmented axial rod, bearing a praeaxial
and a postaxial series of jointed rays.
In _Protopterus_ and _Lepidosiren_ the limbs are reduced and the
lateral rays have less (_Protopterus_) or more (_Lepidosiren_)
completely disappeared.
[Illustration: From Budgett, _Trans. Zool. Soc. London_, xvi, part vii.
From Wiedersheim's _Verg. Anat. der Wirbeltiere_, by permission of
Gustav Fischer.
FIG. 18.--Skeleton of Pectoral Limb of _Polypterus_. a, 30 mm. larva. b,
Adult.]
[Illustration: From Wiesdersheim's _Verg. Anat. der Wirbeltiere_, by
permission of Gustav Fischer.