Chapter VII: , is termed the endoskeleton. The latter consists (i.) of an (2)

Numerous dermal bones invest the dorsal surface of the chondrocranium, and only to a limited extent correspond with the less numerous membrane bones of the Salmon. To these are added a series of circum-orbital bones and a large parasphenoid. Undoubtedly the most striking feature in these Fishes is the primitive character of the upper jaw. In _Polyodon_ (Fig. 131) the palato-quadrates are wholly cartilaginous, and, as in the Dog-Fish, they meet in front beneath the basis cranii, where the two are connected by ligament. The secondary upper jaw is but feebly developed, and is represented on each side by a thin splint-like maxilla in relation with the outer surface of each palato-quadrate cartilage, which meets its fellow in front. There are no premaxillae. The lower jaw is also very primitive. Meckel's cartilages are persistent, and except for a mento-Meckelian bone on each side, they are unossified, although membrane bones representing dentary and splenial elements are present. The skull is hyostylic. The hyoid and branchial arches are only partially ossified. Each opercular fold is supported by an operculum and an interoperculum, and both of these retain somewhat the shape of the cartilaginous hyoidean rays which they have replaced. In the Sturgeon (Fig. 130) the upper jaw is greatly modified in relation with the singular mouth of this Fish. The palato-quadrate cartilages meet not only in front, but also along their dorsal margins, and, with the help of the similarly opposed and somewhat fragmentary metapterygoid {226}cartilages, they form a complete concave roof for the protrusible spout-like mouth. Palatine, mesopterygoid, and pterygoid bones invest, and in some measure replace these cartilages. In brief, the skull of the Chondrostei occupies an interesting intermediate position between the purely cartilaginous and mainly bony types. While retaining a well-developed and unossified primary cranium, it has acquired a complete secondary cranium of dermal bones. Equally notable is the condition of the jaws. Unique among the Teleostomi in possessing the typical Elasmobranch union of the palato-quadrate cartilages beneath the basis cranii, the Chondrostei are so far specialised that they have acquired certain of the membrane bones which constitute the secondary jaws of the more typical bony Fishes.

As regards the general structure of the skull and the nature and disposition of its cartilage- and membrane-bones, the remaining living Teleostomi have much in common with the Salmon. In all the skull is hyostylic, and, unlike the Chondrostei, each half of the primitive upper jaw remains distinct from its fellow, and is separately articulated in front with the lateral ethmoid of the same side by its palatine element. The palato-quadrate cartilage is always more or less completely replaced by bones similar to those of the Salmon, and although they often carry teeth, as a rule they do little more than constitute a rigid buttress for the fixation of the quadrate condyle for the lower jaw. The secondary upper jaw is nearly always well developed, and includes a premaxilla as well as a maxilla on each side. There are, however, certain features in each of the minor groups which are either distinctive or highly characteristic.

In the surviving Crossopterygii (e.g. _Polypterus_[199]) the chondro-cranium is complete in the ethmoidal and post-orbital regions, except where it has been partially replaced by cartilage bones, but in the inter-orbital region the continuity of the roof is interrupted by a large fontanelle, which is only closed by the investing frontal bones (Fig. 132, C). There is also a large basi-cranial fontanelle in the sphenethmoid, closed, however, by the underlying parasphenoid. A large "occipital" bone continuously ossifies in the occipital cartilage and completely surrounds the foramen magnum.

{227}

Prootics and pterotics are absent, and the opisthotics seem to be confluent with their respective epiotics. The floor and side walls of the inter-orbital section of the cranium are formed by a remarkable "sphenethmoid" bone which occupies the position of the paired ali- and orbito-sphenoids in other bony Fishes; and in one species, _P. lapradei_,[200] it forms in front distinct tubular investments round the olfactory nerves. In many respects this bone is singularly like the sphenethmoid bone of the Frog and other tailless Amphibia. A median ethmoid as well as lateral ethmoids are present. In addition to the ordinary dermal bones which invest the cranial roof there is a transverse row of supra-temporal plates crossing the cranial roof behind the paired parietals (Fig. 132, A). Fringing the outer margins of the frontals and parietals a row of pre- and post-spiracular ossicles extends nearly to the orbits, and between two of them, which form a {228}valve, is the spiracular aperture itself. There is a dentigerous splenial on the inner surface of the lower jaw. The hyoid arch has no separate symplectic bone. An operculum and a suboperculum are present, but no inter-operculum; and unless the hinder part of the large cheek-plate, which is traversed by the mandibulo-hyoid sensory canal, represents a pre-operculum, the latter is wanting. Branchiostegal rays are absent, but there is a single pair of large jugular plates.

Very little is certainly known about the cranial cartilage-bones in the fossil members of the group, but the investing dermal bones, which bear a general resemblance to those of _Polypterus_, are often somewhat more numerous, and they form a very complete dermal armature for the entire head. There is a very complete ring of circum-orbital bones, and very often a ring of sclerotic plates. Two large cheek-plates are often present. Nothing comparable to pre- and post-spiracular ossicles is known, but squamosal and supra-temporals can often be identified. To the ordinary bones of the lower jaw there may be added a series of infra-dentary plates, and besides the paired principal jugular plates there may also be present a small anterior median plate and a series of small lateral jugular plates on each side, as in the Carboniferous _Rhizodopsis_ (Fig. 274). Most of the superficial dermal bones, both in the living and extinct Crossopterygii, are invested externally by a granulated or rugose layer of enamel-like ganoin.

In the Holostei, and especially in _Amia_, the skull approximates more closely to the normal Teleostean type as represented by the Salmon's skull. In _Amia_[201] all the occipital cartilage-bones are present—a basi-occipital, two exoccipitals, and a supra-occipital; and, except for the absence of a pterotic, the periotic series of bones is also complete. Paired ali- and orbito-sphenoids form the lateral walls of the inter-orbital portion of the cranial cavity. Above, the complete cartilaginous roof of the cranial cavity is invested by a shield of suturally united and ganoin-covered dermal plates. The hyomandibular element has a symplectic bone at its distal extremity. There is a complete series of opercular bones, and the branchiostegal rays are numerous. A single median jugular plate is present. The lower jaw has on each side five dentigerous splenial bones in addition to dentary and angular bones, while cartilage-bones are {229}represented by articular and mento-Meckelian elements. In its essential structure the skull of _Lepidosteus_[202] resembles that of _Amia_, but it has obviously undergone much specialisation. In some species (_e.g._ _L. osseus_) its appearance is greatly modified by the exceptional length and tapering shape of the beak, due to the elongation of that part of the skull which lies between the orbital and nasal regions; but in _L. platycephalus_ the reduced length and greater width of the beak, combined with its somewhat flattened condition, impart an almost Crocodilian aspect to the head. Amongst other points of difference it may be mentioned that in _Lepidosteus_ the continuity of the chondro-cranial roof is interrupted by a large superior fontanelle. There is no supra-occipital, and there are no lateral ethmoids, at all events in the usual position. The inter-orbital portion of the cranial cavity is largely obliterated by the formation of an inter-orbital septum, consisting of a thin vertical plate of bone, which either represents a pair of fused orbito-sphenoids or a pair of similarly modified lateral ethmoids. In addition to the ordinary investing dermal bones, including circum-orbitals, squamosal, and supra-temporals, there are numerous scale-like ossicles which take the place of the cheek-plates of _Polypterus_. The maxillae are segmented into numerous dentigerous bones fringing the margins of the upper jaw. The lower jaw has no mento-Meckelian bones, but there is a very complete series of dermal elements, including dentary, coronary, splenial, angular, and supra-angular bones in addition to an articular cartilage-bone. One of the most remarkable features in the skull of _Lepidosteus_ is the existence of a secondary articulation between the metapterygoid bones and a pair of transversely elongated condyles formed on each side by a lateral outgrowth from the parasphenoid and alisphenoid bones. By a horizontal sliding movement of the former on the latter, provision is made for the lateral expansion and contraction of the walls of the oral cavity and the separation and approximation of the lateral halves of the upper jaw.[203]

The generality of Teleosts[204] more or less closely agree with _Amia_ in the main features of their cranial structure. There are, however, certain minor features which are characteristic if not {230}always distinctive of the group. As a rule, to which, nevertheless, there are notable exceptions, there is little of the primary cartilaginous cranium in the adult, nearly the whole of it having become absorbed or converted into cartilage-bones. A supraoccipital is invariably present, and usually a mesethmoid and a basisphenoid. An additional bone is added to the periotic series, viz. a pterotic. Supra-temporal bones and jugular plates are always absent, and it may be doubted if mento-Meckelian bones and dentigerous splenials are ever developed in the lower jaw. Within the group itself the skull exhibits many notable modifications, of which only a few can here be mentioned. The shape, size, and character of the mouth and jaws, the extent to which they can be protruded and retracted, and the nature of the dentition, are the source of many characteristic modifications in the structure and appearance of the fore-part of the skull, and these again largely depend upon differences of habit and food. A protrusible mouth, or a mouth which is projected forwards, is usually associated with a suspensorium (hyomandibular) of considerable length, and so greatly inclined forwards as to make a more or less acute angle with the forepart of the cranium.

The presence or absence of an inter-orbital septum is also a feature in which considerable variation occurs. In some Teleosts there is no septum, and the cranial cavity is prolonged forwards between the orbits, where its lateral walls are formed by well-developed, paired ali- and orbito-sphenoid bones, as, for example, in the Carp and other Cyprinidae. In others the fusion of the cranial walls is accompanied by the median union of the orbito-sphenoids, so that a partly bony and partly cartilaginous inter-orbital septum is found, and the cranial cavity becomes largely obliterated in this region, as in the Salmon; or the orbito-sphenoids may be non-existent, the cartilage may undergo absorption, and the inter-orbital septum may become reduced to a vertical fibrous sheath extending between the frontals above and the parasphenoid below, as is the case in the Cod (_Gadus_).

An interesting modification of certain of the bones of the primary and secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae are very small and edentulous, and serve no other purpose than forming basal supports for the maxillary barbels, while the rod-like palatine bone, losing its connexion with the pterygoid portion of the primitive upper jaw, {231}but retaining its articulation with the lateral ethmoid, serves to support the maxilla, and at the same time receives the insertion of the muscles by which the barbel is moved in various directions.

In the Plectognathi the premaxillae are co-ossified with the maxillae. Many other interesting cranial modifications occur in Teleosts, and to some of them reference is made in subsequent chapters.

In some respects the skull of Dipnoi[205] is remarkably like that of the Holocephali, especially in its typical autostylism; but in possessing both cartilage- and membrane-bones it in some measure approaches the Teleostome skull. The investing dermal bones are not always easy to identify with those of other Fishes. In _Neoceratodus_ an anterior median membrane-bone or dermal mesethmoid covers the ethmo-nasal region, and, on each side of it, forming the anterior boundary of the orbit, there is situated a pre-orbital or dermal lateral ethmoid. Behind the mesethmoid there is a much larger posterior median bone, and on each side a singular backward prolongation of the dermal lateral ethmoid separates it from a squamosal element. The latter bone descends on the outer surface of the quadrate portion of the palato-quadrate cartilage as far as the condyle for the lower jaw. Collectively, these bones form a fairly complete investment to the upper surface of the cranium, but the posterior median bone and the adjacent portions of the dermal lateral ethmoid and the squamosal are widely separated from the underlying chondrocranium by the powerful jaw muscles, and in this respect they differ from the ordinary roofing bones of other Fishes.

In _Protopterus_ (Fig. 133) and _Lepidosiren_ (Fig. 134) the posterior median bone is non-existent, and its place is taken by a large fronto-parietal, which forms the greater part of the cranial roof, internal to the jaw muscles, and is much larger in the latter Dipnoid than in the former. Circum-orbital bones are present only in _Neoceratodus_. A large parasphenoid supports the cranial floor. Vomers are absent, although there are two small vomerine teeth.

{232}

Relatively small opercular and inter-opercular bones are present, and on the inner surface of each may be found vestigial remains of cartilaginous hyoidean rays. The chondrocranium is complete in _Neoceratodus_, but in the remaining genera it has undergone considerable absorption in the inter-orbital region, so that the roof and floor, and, in part, even the side walls of the cranial cavity, are formed by the fronto-parietal and parasphenoid bones. Two exoccipitals are present in all Dipnoi. There are small labial cartilages in relation with the ventrally-placed nostrils, and large lateral outgrowths from the ethmoid cartilage furnish the olfactory organs with conspicuous lattice-like roofs. A pair of strong palato-pterygoid bones fringe the lower margins of the palato-quadrate cartilage, and meeting in front beneath the ethmoid region their symphysial extremities support the large palatal teeth. The Meckelian cartilages are persistent in all Dipnoi. In _Neoceratodus_ each is flanked by a dentary and an angular externally, and internally by a splenial; but in _Protopterus_ and _Lepidosiren_ distinct dentary bones are wanting. The hyoid arch is best {233}developed in _Neoceratodus_,[206] and includes a small hyomandibular cartilage, a partially bony cerato-hyal and cartilaginous hypo-hyal and basi-hyal element. In the other genera (Fig. 133) only a cerato-hyal is retained. The branchial arches are but feebly developed in the Dipnoi. _Neoceratodus_ has five, of which the first four are divided into epi-branchial and cerato-branchial segments, while the fifth is undivided. _Protopterus_ has six, but only the second and third are segmented as in _Neoceratodus_.[207] In _Lepidosiren_ all the arches are simple undivided rods.

In all three genera the skull conforms to the same general type of structure, but it is much more primitive in _Neoceratodus_ than in the other two genera.

With reference to the fossil Dipnoi, it may be stated that, so far as they are known, the cranial roofing bones are more numerous than in the existing genera, and they cannot readily be compared with those of the latter, or with the numerically reduced and more definitely arranged bones of most Teleostomi. There is also evidence that in some fossil Dipnoi (e.g. _Dipterus_) the chondrocranium and the mandibular suspensorium (palato-quadrate) must have been replaced by cartilage bones to an extent which has no parallel in any of the surviving types.[208] Jugular bones were present in _Dipterus_ and _Phaneropleuron_.

{234}MEDIAN FINS AND APPENDICULAR SKELETON

THE MEDIAN FINS.—Whether existing in the form of a continuous fin, or as discontinuous isolated fins, the median fins are provided with skeletal supports, and also with muscles, primitively formed from intrusive clusters of cells derived from a variable number of the neighbouring myotomes, for their varied movements. The skeletal structures of the dorsal and anal fins consist of a series of bony or cartilaginous, rod-like, and typically tri-segmented radial elements or pterygiophores,[209] supporting distally a series of dermal structures in the shape of numerous slender horny fibres or ceratotrichia, as in the Elasmobranchii and Holocephali, or a smaller number of bony dermal fin-rays, which are probably modified scales or lepidotrichia,[210] as in the Teleostomi. The typical tri-segmented character of the radialia is often retained in many existing Elasmobranchs (Fig. 135) and in _Pleuracanthus_, in _Neoceratodus_ amongst the Dipnoi, in the Chondrostei, in existing Holostei (Fig. 136), and to a greater or less extent in several families of Teleosts (_e.g._ Salmonidae, Esocidae, Cyprinidae, and some Acanthopterygii); but in the latter group the radialia are greatly prone to reduction, and hence they are more generally bi-segmented, and sometimes consist of a single proximal segment only (e.g. _Gymnotus_). In all these Fishes the proximal segments are the longest and the most persistent, and when reduction occurs it is at the expense of the middle and distal segments.

{235}

FIG. 137.—The first four radialia of the dorsal fin of _Mesoprion gembra_, showing the chain-links for the ring-like bases of the fin-rays. _r.e^1_, _r.e^4_, First and fourth proximal radialia.]

The cause of this reduction is often, but not always, to be found in the fact that, whenever the dermal fin-rays take the form of stout spines, as in the anterior dorsal fin in many Acanthopterygian Teleostei, the segmentation of their radialia would obviously detract from their value as skeletal supports, and hence they rarely consist of more than their proximal segments, although the radialia which in the same Fish support soft rays may be bi-segmented or tri-segmented. The radialia are, however, unsegmented, even slightly branched, cartilaginous rods in the Cyclostomata; short simple rods in the Holocephali; and equally simple bony rods in the dorsal fin of _Polypterus_, where they support the strong spines of the numerous finlets; but they are bi-segmented in the soft-rayed anal fin. As previously mentioned, the proportional share taken by the radialia and the horny fibres or the dermal fin-rays in the support of the fins differs greatly in different Fishes. In the Cyclostomata radialia are the sole, and in Elasmobranchs the main supports, and they may extend nearly to the free margin of the fin. In the more specialised Fishes, as in most Teleostomi, the reverse is the case. The radialia sink into the muscles of the body-wall and leave the strongly developed fin-rays as the sole support of the visible portions of the fins. In not a few Fishes there is an obvious segmental correspondence between the radialia and the vertebral neural or haemal spines, to the extent that the former equal the latter in number and articulate with their distal extremities, as, for example, in the caudal region of _Pleuracanthus_ and in existing {236}Dipnoi. In others again, as in most Teleostomi, there is no such segmental relation, and the radialia are more numerous than the vertebrae whenever the two are co-extensive. The exoskeletal fin-supports exhibit similar relations to their radialia, but in inverse order. Much more numerous than the radialia in the Elasmobranchs, Holocephali, and the Dipnoi, the former become gradually reduced in the Teleostomi, until in the Holostei and Teleostei they correspond in number with the supporting radialia. Complete numerical correspondence between the neural and haemal spines and the radialia and fin-rays is very rare, and has only been observed in the caudal region of certain Crossopterygii (_e.g._ the Coelacanthidae).[211]

In not a few Fishes the radialia of the median fins undergo modifications which offer an interesting parallel to an early stage in the evolution of the paired fins from primitively continuous lateral fins. The concentration of radialia which occurs in isolated median fins often results, through growth pressure, in the complete fusion of the proximal segments of more or fewer of the radialia into two or three basal supports, or even into a single basal piece. Examples of such basal fusion are frequent in the dorsal fins of Elasmobranchs, and the same modification may also be seen in the anal fin of _Pleuracanthus_, and especially in the {237}dorsal fin of the Devonian Crossopterygian, _Holoptychius_[212] (Fig. 138), where several radialia, which are free distally, have their bases united into a single basal piece, or basipterygium. In most Teleostomi elevator and depressor muscles arise from the radialia, and are inserted into different points on the bases of the fin-rays, and by their contraction the latter may either be elevated into an erect position, or folded back like a fan along the middle line of the body, where, as in some Teleosts, there is a groove for their reception. When fin-rays are only capable of simple elevation or depression, the connexion between a radial element and its fin-ray is usually by some form of a hinge-joint, the cleft base of the ray clipping the distal segment of the radial (Fig. 139). In some Teleosts the articulation of the two is by means of a kind of chain-link (Fig. 137). In those Fishes in which the median fins are capable of lateral undulatory movements the articulation is of a more mobile character.

In the different types of caudal fin, diphycercal, heterocercal, and homocercal, the supporting elements of the ventral lobe are formed by the haemal spines of the terminal caudal vertebrae which are inclined backwards, and are often greatly expanded for the purpose (Fig. 140). The dorsal lobe may be supported either by the adjacent neural spines, or by radialia, or by both.

THE APPENDICULAR SKELETON.[213]—It is probable that the skeleton of the paired fins and the pectoral and pelvic girdles have been formed from the supporting radialia of the isolated and enlarged anterior and posterior portions of primitively continuous lateral {238}fins, by a sequence of structural modifications in the same direction as in the median fins. The initial stage was probably marked by the fusion of the proximal portions of the radialia to form a basal support or basipterygium for the free distal portions. Subsequently, it may be, a rudiment of the future limb-girdle became segmented off from the inner extremity of the basipterygium, and by its dorsal and ventral growth in the body-wall the lateral half of a girdle was developed. The subsequent union of the two halves across the mid-ventral line resulted in the evolution of the dorsally incomplete hoop of cartilage which is the primary form of the complete limb-girdle in Craniates. The primitive fin skeleton or "archipterygium" was formed from the residue of the basipterygium in conjunction with the free distal radialia which it carried. The precise structure of the archipterygium is purely hypothetical. Possibly it was a biserial fin of the _Pleuracanthus_ or _Neoceratodus_ type, consisting of a cartilaginous segmented axis, fringed along its anterior and posterior, or pre-axial and post-axial margins, by a series of slender, simple, or jointed radialia (Fig. 147); or it may have been a uniserial structure, somewhat resembling the pelvic fin of _Pleuracanthus_, or the pectoral and pelvic fins of existing Elasmobranchs (Figs. 250, 141), in which an axis formed by the residue of the basipterygium or metapterygium had a fringe of radialia on its anterior or preaxial side only. If the archipterygium was biserial then the uniserial fin was probably derived from it by the subsequent suppression of all the post-axial radialia; or, if uniserial, the biserial fin was evolved by a later extension of radialia on to the post-axial margin. The evidence of comparative anatomy is not conclusive as to the nature of the archipterygium, and palaeontology seems to support either view with puzzling impartiality.[214] It may be admitted that the lateral fin theory offers the best solution of the problem of the origin of the paired fins, but it must be borne in mind that no Fish, living or fossil, is known to possess fins of this nature, unless the singular lateral lobes of some Ostracodermi (_e.g._ the Coelolepidae) are kindred organs[215]; neither do continuous lateral fins ever exist as vestiges, unless, indeed, the bilateral series of spines, which extend between the pectoral and pelvic {239}fins, in some of the Lower Devonian Acanthodei (e.g. _Climatius_), may be regarded in that light.

_The Pectoral and Pelvic Girdles_.—The pectoral girdle is more primitive in _Cladoselache_ and _Pleuracanthus_ than in any other Elasmobranch. In the former (Fig. 145, A) it may be doubted if the girdle has passed beyond the basipterygial stage, and although a definite girdle is present in the latter genus (Fig. 250) its lateral halves retain their primitive distinctness. Existing Elasmobranchs, including the Holocephali, have a pectoral girdle in the form of a dorsally incomplete hoop of cartilage imbedded in the muscles of the body-wall, close behind the last branchial arch (Fig. 141). The upper or dorsal portion of each half is the scapula, and the ventral is the coracoid. Between these two portions of the girdle, and defining their limits, there are articular surfaces for the basal cartilages of the pectoral fin.

_Cladoselache_ (Fig. 145, B) had no pelvic girdle, nor does it appear that this primitive Elasmobranch had acquired even a basipterygium. _Pleuracanthus_, on the contrary, had a pair of pelvic rudiments distinct from well-developed basipterygia. In other Elasmobranchs there is a distinct girdle, formed by the median union of primitively distinct lateral rudiments, consisting of a simple transverse bar of cartilage, imbedded in the ventral abdominal wall, just in front of the cloacal aperture, and having articulated to each of its outer extremities the basal cartilage (metapterygium) of the pelvic fin.

{240}

Sometimes there is a rudiment of a dorsally-directed "iliac" process at each extremity of the girdle, but in no Fish do these processes ever acquire a dorsal connexion with the vertebral column. In the Holocephali the iliac processes are better developed than in any other Fishes, but ventrally the lateral halves of the girdle are united by ligament alone. In the Teleostomi important differences are observable in both girdles. The primary cartilaginous pectoral girdle now consists of distinct lateral halves which have no ventral connexion with each other. In addition, there is developed on the outer surface of each half a series of membrane bones, which form a secondary girdle (Fig. 143). From above downward the series includes a supraclavicle and a cleithrum (clavicle of Teleosts) which are always present, and to these may be added in the {241}Crossopterygii and Chondrostei an infraclavicle or clavicle proper, while one or two "post-clavicles" may be present in relation with the hinder margin of the cleithrum. The infraclavicles, or in their absence the cleithra (_e.g._ Holostei and most Teleostei), usually meet in a median ventral symphysis, so that the secondary girdle tends to acquire the characteristic hoop-like arrangement of its parts which has been lost in the primary girdle. With the development of a bony secondary girdle, the primary girdle (scapula and coracoid) becomes much reduced, and, as a rule, does little more than connect the fins with the cleithra. The secondary girdle acquires a dorsal connexion with the skull on each side by means of the post-temporal bone, which is attached below to the supra-clavicle and above to the periotic capsule. In the Chondrostei and the Dipnoi the primary girdle retains its primitive cartilaginous condition, but in the Crossopterygii, Holostei, and in all Teleosts it is ossified as distinct scapulae and coracoids. To these may be added in some Teleosts a mesocoracoid formed by a separate ossification of the coracoid cartilage (Fig. 143).[216]

With the possible exception of small paired or median cartilages inserted between the inner extremities of the basipterygia in _Polypterus_ and a few other Teleostomi, the pelvic girdle is absent in all the existing members of this group, having either become completely suppressed, or remaining unseparated from the basipterygia of the pelvic fins.[217] In the Dipnoi (Fig. 144) there is a true pelvic girdle which has some points of resemblance to that of certain of the caudate Amphibia. It is represented by a median, lozenge-shape, cartilaginous {242}plate, produced in front into a long tapering epipubic process, and on each side of this into a forwardly inclined prepubic process. The hinder part of the plate bears two short processes for the basal cartilages of the pelvic fins. There is no trace, however, of iliac processes.

THE PECTORAL FINS.—The skeleton of the pectoral fins exhibits remarkable structural variations in different Elasmobranchs. In the existing members of the group two large basal cartilages, the propterygium and the mesopterygium, are formed by the concentration and fusion of the proximal portions of certain of the preaxial radialia, and they, with the metapterygium, articulate with the pectoral girdle; hence the fin is tribasal as well as uniserial (Figs. 141 and 146, A, B). In striking contrast to all other Elasmobranchs the pectoral fin of _Cladoselache_ (Fig. 145, A) is far more primitive than in any other Fish. Each fin is supported by a distal series of slender, more or less parallel, unjointed, cartilaginous radialia, and basally by a similar series of shorter, stouter, and less numerous cartilages, which apparently were imbedded in the body-wall, the entire fin skeleton presenting a striking resemblance to an isolated median fin in which the supporting radialia have concentrated by growth pressure, and their proximal portions have been reduced in number by partial fusion.[218] _Pleuracanthus_, on the other hand, had a biserial fin, the preaxial and postaxial radialia supporting fan-like clusters of horny fibres at their distal ends (Fig. 250).

{243}

The broadly lobate pectoral fin of the existing Crossopterygii (Fig. 146, G) is uniserial, closely resembling that of the more typical Elasmobranchs.[219] There are three basal elements, a propterygium, a mesopterygium, and a metapterygium, each of which supports a series of partially ossified radialia. Little is known of the endoskeletal elements of the broadly or acutely lobate fins of the fossil Crossopterygii, but it seems probable that their disposition was uniserial and abbreviate in obtusely lobate fins and biserial in acutely lobate fins. In the remaining Teleostomi (Actinopterygii) the endoskeletal elements become {244}gradually reduced in number and importance, their place as fin-supports being usurped by the dermal fin-rays. In addition, more than three, usually several, basal elements articulate directly with the pectoral girdle, and hence the fins become multi-basal. In the Chondrostei and the Holostei a metapterygium is always recognisable, supporting several radialia along its preaxial border, as in _Acipenser_ (Fig. 146, D) and _Amia_ (Fig. 146, E), or only a single one, as in _Lepidosteus_ (Fig. 146, F). The anterior part of the fin is supported by a variable number of cartilaginous or bony radialia, which, with the metapterygium, articulate with the limb-girdle. In Teleosts the process of reduction reaches its maximum. Usually there is but a single row of short, hour-glass-shaped ossicles, of which the postaxial one may represent a vestigial metapterygium, and sometimes there is also a distal row of small cartilages or ossicles, partially hidden in the cleft bases of the dermal fin-rays (Fig. 146, H). In all these Fishes the fin is a much reduced uniserial fin, in which more or fewer of the preaxial radialia have acquired a direct secondary connexion with the pectoral girdle. Of living Dipnoids _Neoceratodus_ has a nearly typical biserial fin, but, as seems to be the case in all fins of this type at present known, there is a marked absence of symmetry in the number and disposition of the radialia on the two sides of the axis. There is also much individual variation. No two fins are precisely alike, and the radialia may sometimes divide. In the very acutely lobate fins of the remaining Dipnoids it is evident that great reduction has taken place. _Protopterus_ has lost all trace of postaxial radialia, and in _Lepidosiren_ even the preaxial have atrophied, leaving only the long jointed axis to represent the originally biserial fin.

{245}THE PELVIC FINS.—In the simplicity of their endoskeletal supports the pelvic fins of _Cladoselache_ are the most primitive type of paired fins at present known (Fig. 145, B). In general structure they resemble the pectorals, but the radialia are fewer in number, less modified by concentration, and exhibit little, if any, trace of basal fusion. Add to such features as these the apparent absence of any trace of pelvic rudiments, or of basipterygia, and it will be obvious that the pelvic fins differ but little from the median fins of the same Fish except that they are paired. In _Pleuracanthus_ the pelvic fins differ from the corresponding pectorals in being uniserial instead of biserial (Fig. 250). All other Elasmobranchs, including the Holocephali, have uniserial fins, which consist of a large metapterygium, supporting a preaxial fringe of segmented radialia. A propterygium is sometimes present, notably in some of the Skates and Rays, and, like the metapterygium, it is directly connected with the pelvic girdle.

The skeleton of the pelvic fins of the Teleostomi is often extremely degenerate. It is perhaps best developed in the Chondrostei,[220] where each fin is supported by numerous segmented radialia, more or fewer of which fuse towards the base of the fin, and those form a large and slightly ossified basipterygium (Fig. 148). In the living Crossopterygii, Holostei, and Teleostei, the pelvic fins are similar in essential structure, but are very degenerate. The basipterygium is usually well developed and is always bony (Fig. 149), and in many Teleosts it acquires so extensive a sutural connexion with its fellow that, physiologically, it supplies the place of a true pelvic girdle. At its distal end there may be a single row of small cartilaginous or bony nodules, representing vestigial radialia, as in the Crossopterygii, Holostei, and {246}Teleostei, but even these may be absent, and the dermal fin-rays then articulate directly with the basipterygium. Little is known of the skeleton of the pelvic fins in the fossil Crossopterygii, but there is evidence of the existence of a higher grade of structure than in their surviving allies. In _Eusthenopteron_,[221] for example, the fin is supported by an axis of at least three bony segments, with at least three ossified preaxial radialia; hence, it has obviously undergone less degeneration than in _Polypterus_, where the fin-skeleton is essentially Teleostean. In the Dipnoi the pelvic fins are similar to the corresponding pectoral fins, but individual variation is more marked and even the central axis may divide.[222] In the males of all existing Elasmobranchs, including the Holocephali, certain of the more distally situated metapterygial radialia become modified to form a supporting skeleton for the copulatory organs, the claspers, or mixipterygia. In the latter group the anterior claspers are also provided with cartilaginous supports articulating with the pelvic girdle directly in front of the pelvic fins.

{247}CHAPTER IX

THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS

The alimentary canal is a muscular tube with an epithelial lining, formed for the reception and the digestion of the food. It begins with a mouth, and from thence it extends backwards through the coelom, finally communicating with the exterior either by a cloacal or by an anal orifice. The oral or buccal cavity into which the mouth leads is a stomodaeum, and is lined by inpushed epidermis, while the hinder portion of the cloaca and the anus are lined by a somewhat similar inpushing of the epidermis which forms the proctodaeum. The rest of the alimentary canal, consisting in succession of a pharynx, an oesophagus, a stomach, and an intestine, constitutes the mesenteron, and is lined by endoderm. Teeth are developed from the walls of the stomodaeum, and glands for the secretion of digestive fluids from the endoderm of the mesenteron.

DENTITION.

In the Lampreys among the Cyclostomata teeth are developed in the form of yellow conical structures on the inner surface of the buccal funnel, and on the extremity of the rasping "tongue" (Fig. 91, A). Each tooth consists of an axial papilla of the dermis, sometimes enclosing a pulp-cavity, and invested by the epidermis, and also by a stratified horny cone which forms the projecting hard part of the tooth. The dermal papilla with its ectodermal investment bears a superficial resemblance to the germ of a true calcified tooth, but no odontoblasts are formed, nor any calcic deposit, the laminated horny teeth being formed by the gradual conversion of the successive strata of the {248}epidermic cells into horny layers.[223] The old teeth are vertically replaced by new teeth developed beneath the functional teeth. With the exception of a median tooth above the oral aperture, _Myxine_ and its allies have only lingual teeth. These are comb-like, and they are formed by the basal fusion of primitively distinct tooth-germs. The structure and development of the teeth of the Cyclostomes lend no support to the view that the teeth are degenerate calcified structures. With greater probability they represent a stage in the evolution of teeth and dermal spines, which has been succeeded by a later stage in which calcification superseded cornification as a method of hardening.

True calcified teeth first make their appearance in Fishes, where they assume the form of modifications of exoskeletal structures.[224] The teeth of Elasmobranchs are identical in essential structure, as well as in the manner of their development, with the ordinary dermal spines of the skin, and in the embryo the {249}dermal spines form a continuous series with those which invest the jaws and eventually become teeth (Fig. 151). It is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted, and the two structures assume their distinctive characters.

The tissues of which the teeth of Fishes are composed are (1) _dentine_, which is a non-vascular, calcified tissue, traversed by numerous radiating, branched, dentinal tubuli, into which extend protoplasmic prolongations from the cells (scleroblasts) by which the dentine is secreted. Dentine forms the greater part of the body of a tooth. (2) _vasodentine_ and (3) _osteodentine_ are modifications of ordinary dentine, the former containing blood-vessels ramifying in its substance but no dentinal tubules, and the latter more closely resembling bone. (4) _enamel_, an exceptionally dense, non-vascular, non-tubular tissue, which may or may not exhibit traces of the prismatic structure so characteristic of this tissue in the higher Vertebrates, forms the outer investment of the teeth.

As regards their fixation, the more primitive forms of teeth, such as those of Elasmobranchs, are simply embedded in the gums, and are only connected with the jaws by fibrous tissue; but in some of the older fossil Sharks the fixation of the teeth is effected by the mutual articulation of the basal plates of the teeth with one another. The Chondrostean _Polyodon_, so {250}shark-like in many other respects, also has teeth implanted basally in the gums, and quite free from any special connexion with the jaw-bones. In some Teleosts with movable teeth, the latter are merely attached to the jaws by fibrous, and often elastic, ligaments, as in the Pike (_Esox_) and the Angler-Fish (_Lophius_). As a rule, however, the teeth are directly ankylosed to the bones developed in relation with the jaws. Very rarely, as, for example, in some Characinidae, are the teeth implanted in sockets.

Nearly all Fishes are polyphyodont, that is, the old teeth are constantly replaced by new teeth as fast as they become worn down or fall out. In the Sharks and Dog-Fishes, for example, where the teeth are arranged in rows parallel to the axis of each jaw, the functional teeth along the upper edge of the jaw are usually erect, while those in the rows more internally situated point inwards towards the oral cavity; and behind these again there are rows of developing teeth in different stages of growth, and partially hidden beneath a projecting fold of the oral mucous membrane (Fig. 152). As the teeth in use become lost they are successively replaced by the inner rows, which, with the mucous membrane in which they are embedded, move forwards to the edge of the jaw, where they become erect and functional. The teeth of the Holocephali and of the Dipnoi are not shed, but the loss which they sustain through wear and tear is made good by persistent growth at their bases. In the Teleostomi the succession is less regular, new teeth being formed between or at the bases of the old teeth. In the case of socketed teeth the succession is usually vertical, the new teeth being formed at the sides of the old ones; and by the absorption of the bases of the latter, the former come to lie directly below them, and eventually they occupy the same sockets.

As might be expected from the remarkable diversity in the {251}habits and in the food of different Fishes, the teeth exhibit an equally striking diversity in form, size, and structure. The most primitive type of tooth resembles an ordinary dermal spine, and is little more than a simple pointed cone. A few Elasmobranchs and many Teleostomi possess teeth of this kind. By the flattening of the cone parallel to the axis of the jaw, the tooth becomes triangular, and then the margins may either remain smooth and trenchant, or they may become complicated by the formation of marginal serrations or of accessory basal cusps, and by such modifications the characteristic teeth of most Elasmobranchs are formed. The simple cone may also be modified to form crushing teeth—short, blunt, more or less hemispherical teeth—or even transformed into a mosaic of hexagonal plates, as in the Myliobatidae amongst Elasmobranchs. Massive, flattened, scroll-like crushing teeth are also formed by the fusion of adjacent teeth, or of several successional teeth, and of such composite teeth we have examples in the Heterodontidae and in the Palaeozoic Cochliodontidae. By a somewhat similar process of concrescence the anomalous composite teeth of such Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish (_Scarus_), have been evolved. The singular dental structures of the Holocephali are probably composite teeth, and it is certain that the highly characteristic teeth of the Dipnoi have resulted from the basal fusion of primitively distinct simple conical denticles. The dentition is often heterodont. In _Heterodontus_ (_Cestracion_), for example, the anterior teeth in each jaw are pointed and prehensile, while the hinder ones are scroll-like and crushing. Prehensile and crushing molar-like teeth are also present in such Teleosts as many of the Sparidae, and in the Wolf-Fish (_Anarrhichas_). The existence of sexual differences in the dentition is illustrated in the Skates and Rays (_Raia_), where teeth which are simple and pointed in the male become flattened and plate-like in the female. A few Teleosts, like the Syngnathidae, Cyprinidae, and some Siluridae, are entirely devoid of jaw-teeth.

In addition to jaw-teeth, many Teleosts possess pharyngeal or gill-teeth, developed in connexion with the inner margins of the branchial arches, to which they are usually firmly ankylosed (Figs. 352, 412 and 413). As a rule "the pharyngeal dentition is inversely proportional to the extent of tooth development {252}on the jaws."[225] Pharyngeal teeth differ greatly in size and structure in different Teleosts, and, like the jaw-teeth, they are capable of replacement by vertical succession. The teeth are sometimes restricted to the inferior pharyngeal bones (cerato-branchials of the last branchial arch), and then, as in the Carp (_Cyprinus_), they may bite against a callous pad on the under surface of the basioccipital bone; or, as in some of the Wrasses (_Labrus_), the inferior teeth are opposed to superior teeth on the upper pharyngeal bones (pharyngo-branchials of more or fewer of the branchial arches). When pharyngeal teeth are present it is probable that they are the principal masticatory organs, the jaw-teeth being used for seizing or holding the prey.

ALIMENTARY CANAL.

A protrusible tongue is never developed in Fishes. A rudiment of that organ is present in the Elasmobranchs (Fig. 153) and Dipnoi, and also in the Crossopterygii, and usually consists of an elevated area of mucous membrane provided with free lateral edges and a forwardly projecting apex; it is supported by the basi-hyal element of the hyoid arch. In the Crossopterygii (e.g. _Polypterus_) the tongue contains muscle fibres, and in the Dipnoi, where the organ is better developed than in any other Fishes, special lingual muscles are present.

The pharynx succeeds the oral cavity, and is perforated on each side by the branchial clefts (Figs. 153, 154). The rest of the alimentary canal differs considerably in various Fishes in the degree of distinctness of its several regions, and in the extent to which it is convoluted. As a rule the pharynx is followed in succession by an oesophagus, a stomach, and an intestine (Fig. 153), the latter terminating in a portion usually termed the "rectum." The boundaries of these regions are not always very obvious, but are indicated by variations in calibre, by changes in the character of the lining epithelium, by special valves or sphincter muscles, or by the entrance of the ducts of certain glands like the pancreas and liver.

{253}

The oesophagus is occasionally separated from the stomach by a slight constriction, but more frequently the replacement of the squamous epithelium of the oesophagus by the columnar epithelium of the stomach and the appearance of gastric glands in the wall of the latter cavity afford the only distinction between the two regions. The {254}commencement of the intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in the form of a ring-like, inwardly projecting thickening of the circularly-disposed muscle fibres of the terminal extremity of the stomach, and usually also by the entrance of the distinct or united ducts of the liver and pancreas; sometimes, as in certain Elasmobranchs and in the Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The rectum, or terminal portion of the intestine, is distinguished from the rest of the gut by its straight course to the cloacal aperture or the anus, and sometimes by an increase in calibre. In _Box vulgaris_ and a few other Teleosts[226] a caecal diverticulum indicates the commencement of the rectum, while in a few cases the pre-rectal portion of the intestine communicates with the enlarged rectal segment by a much constricted valvular orifice which is suggestive of the ileo-colic valve of the higher Vertebrates,[227] as in the Teleosts _Amiurus catus_,[228] _Trigla gurnardus_, and _Cyclopterus lumpus_.

The relation of the regional divisions of the intestine in Fishes to those of other Vertebrates are somewhat difficult to determine. If we may regard the "rectal" gland of Elasmobranchs and the intestinal caecum of certain Teleosts as homologous with each other, and with the caecum coli of the higher Vertebrates, then it would seem that by far the greater part of the intestine of Fishes, including that portion in which a spiral valve may be developed, is homologous with the pre-caecal segment of the gut or small intestine in other Vertebrates, and that the post-caecal section, or large intestine, of the latter is represented in Fishes only by that relatively short portion of the gut which lies posterior to the rectal gland or its homologue in Teleosts, the equivalent of the colon of Mammalia being, as in Amphibia, Reptiles, and Birds, practically undifferentiated.[229]

In the Cyclostomata the alimentary canal retains much of its primitive simplicity. It pursues a straight course from mouth to anus, and the usual regions are very obscurely indicated. The same remarks apply also to the Holocephali and a few Teleosts, although in these Fishes the limits of the different regions are somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a contracted sigmoid curve between the somewhat dilated stomach and the spacious intestine is the only departure from the straight course of the preceding groups.

{255}

{256}In the remaining Fishes the degree of convolution varies within rather wide limits. The oesophagus is usually straight and wide, but in _Lutodeira_, among Teleosts, it is long and even convoluted, and in the Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"), which extends anteriorly as far as the head, and posteriorly to the beginning of the tail, and communicates with the oesophagus by two apertures. The stomach may be U-shaped with the concavity directed forwards, and consisting of a right limb passing backwards from the oesophagus, and a left limb curving forwards to its junction with the intestine (Fig. 153). In such instances as these the stomach and the adjacent section of the intestine describe a characteristic siphonal curve. In certain other Fishes (Fig. 160), the oesophageal portion of the stomach terminates behind in a tubular or sac-like dilatation at some distance posterior to the laterally situated pylorus, which indicates the origin of the intestine. The intestine is straight, or nearly so, in Elasmobranchs, Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and very generally in Teleosts, it is more or less convoluted, notably in some of the Mugilidae, and in the Loricariidae, where, as in _Plecostomus_, it is disposed in numerous spiral coils like a watch-spring. The terminal portion of the intestine or rectum either opens into a cloaca, which also receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of the separate or united urinogenital ducts, as is the case with all the remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably situated near the junction of the caudal and trunk regions, and as a rule is median in position, rarely, as in the Dipnoi, displaced to the right or left of the middle line; but the anus differs greatly in position, sometimes retaining its primitive position at the hinder end of the trunk, as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many Teleosts, or occupying almost any position between that point and, as in the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig. 351.)

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{258}

The whole length of the alimentary canal from the oesophagus to the rectum is invested externally by the visceral layer of the peritoneum (Fig. 156), which histologically consists of a stratum of connective tissue, supporting on its free surface an epithelial stratum (coelomic epithelium). Primarily, the investing peritoneum is continued both dorsally and ventrally into bilaminar suspensory folds, the dorsal and ventral mesenteries (_d.ms_, _v.ms_), which extend to the mid-dorsal or mid-ventral line of the abdominal cavity. The two layers then separate and become continuous with the parietal layer of the peritoneum lining the whole of the inner surface of the body-wall. Embryologically, the two mesenteries owe their formation to the fusion above and below the mesenteron of the contiguous walls of two laterally situated and primitively distinct coelomic cavities. The dorsal mesentery in the adult is occasionally complete, as in the Myxinoid Cyclostomata and in the Elasmobranch _Hypnos subnigrum_,[230] and also in some Dipnoi and in a few Teleosts, but much more frequently it is reduced by absorption to anterior and posterior remnants, or to a series of isolated bands, or even, as in the Lamprey (_Petromyzon_), to a few filaments accompanying the intestinal blood-vessels. The ventral mesentery, on the contrary, is rarely present, and if present is never complete. In _Lepidosteus_[231] a ventral mesentery {259}is said to be present in connexion with that part of the intestine which contains the spiral valve. In _Protopterus_,[232] and also in _Neoceratodus_,[233] there is a well-developed ventral mesentery in relation with the greater part of the length of the intestine, although in the former Dipnoid its continuity is interrupted by one or two vacuities, and in the latter the mesentery is incomplete posteriorly. A ventral mesentery is also present in the intestinal region of some of the Muraenidae among Teleosts.[234]

Internal to its peritoneal investment the wall of the alimentary canal consists in succession from without inwards of (1), a {260}muscular coat, (2) the submucosa, and (3) an epithelial stratum or mucous membrane, the first two of these layers, with the addition of the peritoneum, being derivatives of the inner or splanchnic portion of the embryonic mesoblast.[235]

Excluding the oesophagus, where the muscular coat is mainly composed of striated fibres, the musculature of the alimentary canal usually consists solely of non-striated, spindle-shaped fibres disposed in two layers, an external stratum of longitudinally arranged fibres, and an inner stratum of circularly disposed fibres (Fig. 157), with the addition, in the stomach, of an oblique layer between the two. In the oesophagus the reverse arrangement may exist, the circular layer being external and the longitudinal internal. The muscular coat varies considerably in thickness in different regions and in different Fishes, and in the Cyclostomata, the Holocephali, some Teleosts, and the Dipnoi may be very feebly developed, or even entirely absent, as in the intestine of the Hag-Fish (_Myxine_). In the Gillaroo Trout (_Salmo stomachicus_),[236] on the contrary, the distal section of the siphonal stomach has its musculature unusually thickened, so as to form an incipient gizzard for the crushing of the shells of the freshwater Molluscs on which the Fish feeds. In some of the Mullets (Mugilidae),[237] a true gizzard is developed by the enormous thickening of the muscular coat of the caecal stomach, the cavity of which, in consequence, is reduced to a mere vertical fissure, and is lined by an exceptionally thick, horny epithelium.

There are a few exceptions to the rule that the muscular fibres are of the non-striated variety. Thus in some Teleosts, as in the Tench (_Tinca vulgaris_), striated fibres are continued from the oesophagus into the walls of the stomach and intestine, and there form an outer longitudinal and an inner circular layer, situated externally to the corresponding layers of the non-striated stratum. {261}In the Siluroid, _Amiurus_, the striated fibres of the outer circular layer of the oesophagus are continued, although but sparsely, into the inner circular layer of the stomach.