Chapter IV: Anatomy1 (1)

The special importance of a study of the anatomy of fishes lies in the fact that fishes are on the whole undoubtedly the most archaic of existing craniates, and it is therefore to them especially that we must look for evidence as to the evolutionary history of morphological features occurring in the higher groups of vertebrates.

In making a general survey of the morphology of fishes it is essential to take into consideration the structure of the young developing individual (embryology) as well as that of the adult (comparative anatomy in the narrow sense). Palaeontology is practically dumb excepting as regards external form and skeletal features, and even of these our knowledge must for long be in a hopelessly imperfect state. While it is of the utmost importance to pay due attention to embryological data it is equally important to consider them critically and in conjunction with broad morphological considerations. Taken by themselves they are apt to be extremely misleading.

_External Features._--The external features of a typical fish are intimately associated with its mode of life. Its shape is more or less that of a spindle; its surface is covered with a highly glandular epidermis, which is constantly producing lubricating mucus through the agency of which skin-friction is reduced to an extraordinary degree; and finally it possesses a set of remarkable propelling organs or fins.

The exact shape varies greatly from the typical spindle shape with
variations in the mode of life; e.g. bottom-living fishes may be much
flattened from above downwards as in the rays, or from side to side in
the Pleuronectids such as flounder, plaice or sole, or the shape may
be much elongated as in the eels.

_Head, Trunk and Tail._--In the body of the fish we may recognize the three main sub-divisions of the body--head, trunk and tail--as in the higher vertebrates, but there is no definite narrowing of the anterior region to form a neck such as occurs in the higher groups, though a suspicion of such a narrowing occurs in the young _Lepidosiren_.

The tail, or postanal region, is probably a secondary development--a prolongation of the hinder end of the body for motor purposes. This is indicated by the fact that it frequently develops late in ontogeny.

The vertebrate, in correlation perhaps with its extreme cephalization,
develops from before backwards (except the alimentary canal, which
develops more _en bloc_), there remaining at the hind end for a
prolonged period a mass of undifferentiated embryonic tissue from the
anterior side of which the definitive tissues are constantly being
developed. After development has reached the level of the anus it
still continues backwards and the tail region is formed, showing a
continuation of the same tissues as in front, notochord, nerve cord,
gut, myotomes. Of these the (postanal) gut soon undergoes atrophy.

_Fins._--The fins are extensions of the body surface which serve for propulsion. To give the necessary rigidity they are provided with special skeletal elements, while to give mobility they are provided with special muscles. These muscles, like the other voluntary muscles of the body, are derived from the primitive myotomes and are therefore segmental in origin. The fins are divisible into two main categories--the median or unpaired fins and the paired fins.

FIG. 2.--_Cladoselache._ (After Dean.)]

The median fins are to be regarded as the more primitive. The fundamental structure of the vertebrate, with its median skeletal axis and its great muscular mass divided into segments along each side of the body, indicates that its primitive method of movement was by waves of lateral flexure, as seen in an Amphioxus, a cyclostome or an eel. The system of median fins consists in the first instance of a continuous fin-fold extending round the posterior end of the body--as persists even in the adult in the existing Dipneusti. A continuous median fin-fold occurs also in various Teleosts (many deep-sea Teleosts, eels, &c.), though the highly specialized features in other respects make it probable that we have here to do with a secondary return to a condition like the primitive one. In the process of segmentation of the originally continuous fin-fold we notice first of all a separation of and an increase in size of that portion of the fin which from its position at the tip of the tail region is in the most advantageous position for producing movements of the body. There is thus formed the _caudal_ fin. In this region there is a greatly increased size of the fin-fold--both dorsally and ventrally. There is further developed a highly characteristic asymmetry. In the original symmetrical or _protocercal_ ( = _diphycercal_) type of tail (as seen in a cyclostome, a Dipnoan and in most fish embryos) the skeletal axis of the body runs straight out to its tip--the tail fold being equally developed above and below the axis. In the highly developed caudal fin of the majority of fishes, however, the fin-fold is developed to a much greater extent on the ventral side, and correlated with this the skeletal axis is turned upwards as in the _heterocercal_ tail of sharks and sturgeons. The highest stage in this evolution of the caudal fin is seen in the Teleostean fishes, where the ventral tail-fold becomes developed to such an extent as to produce a secondarily symmetrical appearance (_homocercal_ tail, fig. 4).

FIG. 3.--_Chlamydoselachus_. (After Günther.)]

The sharks have been referred to as possessing heterocercal tails,
but, though this is true of the majority, within the limits of the
group all three types of tail-fin occur, from the protocercal tail of
the fossil Pleuracanthids and the living _Chlamydoselachus_ to the
highly developed, practically homocercal tail of the ancient
_Cladoselache_(fig. 2).

The praecaudal portion of the fin-fold on the dorsal side of the body becomes broken into numerous finlets in living Crossopterygians, while in other fishes it disappears throughout part of its length, leaving only one, two or three enlarged portions--the _dorsal_ fins (fig. 4, d.f.). Similarly the praecaudal part of the fin-fold ventrally becomes reduced to a single _anal_ fin (a.f.), occasionally continued backwards by a series of finlets (_Scombridae_). In the sucker-fishes (_Remora_, _Eckeneis_) the anterior dorsal fin is metamorphosed into a sucker by which the creature attaches itself to larger fishes, turtles, &c.

FIG. 4.--_Tilapia dolloi_, a teleostean fish, to illustrate external features. (After Boulenger.)

A, Side view. g.r, Gill rakers.
B, First branchial arch. l.l, Lateral line organs.
a.f, Anal fin. n, Nasal opening.
c.f, Caudal fin. p.f, Pelvic fin.
d.f, Dorsal fin. p.op, Preoperculum.
g.f, Gill lamellae. pt.f, Pectoral fin.]

The paired fins--though more recent developments than the median--are yet of very great morphological interest, as in them we are compelled to recognize the homologues of the paired limbs of the higher vertebrates. We accordingly distinguish the two pairs of fins as pectoral or anterior and pelvic ( = "ventral") or posterior. There are two main types of paired fin--the _archipterygial_ type, a paddle-like structure supported by a jointed axis which bears lateral rays and exists in an unmodified form in _Neoceratodus_ alone amongst living fishes, and the _actinopterygial_ type, supported by fine raylike structures as seen in the fins of any ordinary fish. The relatively less efficiency of the archipterygium and its predominance amongst the more ancient forms of fishes point to its being the more archaic of these two types.

In the less highly specialized groups of fishes the pectoral fins are close behind the head, the pelvic fins in the region of the cloacal opening. In the more specialized forms the pelvic fins frequently show a more or less extensive shifting towards the head, so that their position is described as thoracic (fig. 4) or jugular (_Gadus_--cod, haddock, &c., fig. 5).

The median fin, especially in its caudal section, is the main
propelling organ: the paired fins in the majority of fishes serve for
balancing. In the Dipneusti the paired fins are used for clambering
about amidst vegetation, much in the same fashion as the limbs of
Urodeles. In _Ceratodus_ they also function as paddles. In various
Teleosts the pectoral fins have acquired secondarily a leg-like
function, being used for creeping or skipping over the mud
(_Periophthalmus_; cf. also Trigloids, Scorpaenids and Pediculati). In
the "flying" fishes the pectoral fins are greatly enlarged and are
used as aeroplanes, their quivering movements frequently giving a
(probably erroneous) impression of voluntary flapping movements. In
the gobies and lumpsuckers (_Cyclopteridae_) the pelvic fins are fused
to form an adhesive sucker; in the _Gobiesocidae_ they take part in
the formation of a somewhat similar sucker.

The evolutionary history of the paired limbs forms a fascinating
chapter in vertebrate morphology. As regards their origin two
hypotheses have attracted special attention: (1) that enunciated by
Gegenbaur, according to which the limb is a modified gill septum, and
(2) that supported by James K. Thacher, F. M. Balfour, St George
Mivart and others, that the paired fins are persisting and modified
portions of a once continuous fin-fold on each side of the body. The
majority of morphologists are now inclined to accept the second of
these views. Each has been supported by plausible arguments, for which
reference must be made to the literature of the subject.[2] Both views
rest upon the assumed occurrence of stages for the existence of which
there is no direct evidence, viz. in the case of (1) transitional
stages between gill septum and limb, and in the case of (2) a
continuous lateral fin-fold. (There is no evidence that the lateral
row of spines in the acanthodian _Climatius_ has any other than a
defensive significance.) In the opinion of the writer of this article,
such assumptions are without justification, now that our knowledge of
Dipnoan and Crossopterygian and Urodele embryology points towards the
former possession by the primitive vertebrate of a series of
projecting, voluntarily movable, and hence potentially motor structure
on each side of the body. It must be emphasized that these--the true
external gills--are the _only_ organs known actually to exist in
vertebrates which might readily be transformed into limbs. When
insuperable objections are adduced to this having actually taken place
in the course of evolution, it will be time enough to fall back upon
purely hypothetical ancestral structures on which to base the
evolutionary history of the limbs.

The ectoderm covering the general surface is highly glandular. In the case of the Dipneusti, flask-shaped multicellular glands like those of Amphibians occur in addition to the scattered gland cells.

A characteristic feature of glandular activity is the production of a
slight electrical disturbance. In the case of _Malopterurus_ this
elsewhere subsidiary function of the skin has become so exaggerated as
to lead to the conversion of the skin of each side of the body into a
powerful electrical organ.[3] Each of these consists of some two
million small chambers, each containing an electric disk and all
deriving their nerve supply from the branches of a single enormous
axis cylinder. This takes its origin from a gigantic ganglion cell
situated latero-dorsally in the spinal cord between the levels of the
first and second spinal nerves.

_Cement Organs._--The larvae of certain Teleostomes and Dipnoans possess special glandular organs in the head region for the secretion of a sticky cement by which the young fish is able to attach itself to water-plants or other objects. As a rule these are ectodermal in origin; e.g. in _Lepidosiren_ and _Protopterus_[4] the crescentic cement organ lying ventrally behind the mouth consists of a glandular thickening of the deep layer of the ectoderm. In young ganoid fishes preoral cement organs occur. In Crossopterygians there is one cup-shaped structure on each side immediately in front of the mouth. Here the glandular epithelium is endodermal, developed[5] as an outgrowth from the wall of the alimentary canal, closely resembling a gill pouch. In _Amia_[6] the same appears to be the case. In a few Teleosts similar organs occur, e.g. _Sarcodaces_, _Hyperopisus_,[7] where so far as is known they are ectodermal.

_Photogenic Organs._--The slimy secretion produced by the epidermal glands of fishes contains in some cases substances which apparently readily undergo a slow process of oxidation, giving out light of low wave-length in the process and so giving rise to a phosphorescent appearance. In many deep-sea fishes this property of producing light-emitting secretion has undergone great development, leading to the existence of definite photogenic organs. These vary much in character, and much remains to be done in working out their minute structure. Good examples are seen in the Teleostean family _Scopelidae_, where they form brightly shining eye-like spots scattered about the surface of the body, especially towards the ventral side.

FIG. 6.--Larva of Polypterus. (After Budgett.)]

FIG. 7.--Thirty Days' Larval Lepidosiren. (After Graham Kerr.)]

_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.

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.

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.

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.

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.

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_).

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_).