CHAPTER IX.CLASS II. AMPHIBIA[50].

Fig. 16.A.Skull ofNotidanus× ½ (Brit. Mus.). B.Skull ofCestracion× 1/3 (afterGegenbaur). In neither case are thebranchial arches shown.1. rostrum.6. Meckel's cartilage.2. olfactory capsule.7. teeth.3. ethmo-palatine process.8. labial cartilage.4. palatine portion of9. hyomandibular.palato-pterygo-quadrate bar.10. postorbital process.5. quadrate portion of bar.II. optic foramen.

The simplest type of cranium is that found inElasmobranchs: it consists of a simple cartilaginous box, which is generally immovably fixed to the vertebral column, though in some forms, likeScymnusandGaleus, a joint is indicated, and in others, such as the Rays, one is fairly well developed. The cranium in Elasmobranchs is never bony, though the cartilage is sometimes calcified. It is drawn out laterally into an antorbital process in front of the eye, and a postorbital process behind it. The nasal capsules are always cartilaginous, and the eye, as a general rule, has a cartilaginous sclerotic investment. The cranium is often prolonged in front into a rostrum which is enormously developed inPristisand some Rays. The cartilaginous roof of the cranium is rendered incomplete by the presence of a large hole, the anterior fontanelle.

Two pairs of labial cartilages (fig. 16, B, 8) are often present. They lie imbedded in the cheeks outside the anterior region of the jaws, and are specially large inSquatina.

As regards the visceral arches[44]the simplest and most primitive condition of the jaws is that of the Notidanidae, in which the mandibular and hyoid arches are entirely separate. In these primitive fishes the palato-pterygo-quadrate bar articulates with the postorbital process (fig. 16, 10), while further forwards it is united to the cranium by the ethmo-palatine ligament. The hyoid arch is small and is broadly overlapped by the mandibular arch. The termautostylicis used to describe this condition of the suspensorium. From this condition we pass in the one direction to that ofCestracion(fig. 16, B), in which the whole of the palato-pterygo-quadrate bar has become bound to the cranium, and in the other to that ofScyllium. InScyllium(fig. 6), while the ethmo-palatine ligament is retained, the postorbital articulation of the palato-pterygo-quadrate has been given up, so that the palato-pterygo-quadrate comes to abut on the hyomandibular and is attached to it by ligaments. The pre-spiracular ligament (fig. 16, 20) running from the auditory capsule also assists in supporting the jaws.

Lastly we come to the purelyhyostyliccondition met with in Rays, in which the mandibular arch is entirely supported by the hyomandibular. In some Rays the hyoid is attached to the posterior face of the hyomandibular near its proximal end, and may even come to articulate with the cranium.

Thevisceral arches of Elasmobranchsmay be summarised as follows:—

1. Themandibular arch, consisting of a much reduced dorsal portion, the pre-spiracular ligament, and a greatly developed ventral portion from which both upper and lower jaws are derived. The mandible (Meckel's cartilage) is the original lower member of the mandibular arch, and from it arises an outgrowth which forms the upper jaw or palato-pterygo-quadrate bar. InScymnusthis bears a few branchiostegal rays.

2. Thehyoid arch, which consists of the hyomandibular and the hyoid, and bears branchiostegal rays on its posterior face.

Fig. 17. Dorsal view of the Branchial arches ofHeptanchus. (FromGegenbaur).1. basi-hyal.7. pharyngo-branchial, common2. cerato-hyal.to the sixth and seventh3. second hypo-branchial.arches.4. first cerato-branchial.8. basibranchial of second arch.5. first epi-branchial.9. basibranchial, common to the6. first pharyngo-branchial.sixth and seventh arches.

3. Thebranchial arches, generally five in number, all of which except the last bear branchiostegal rays. In the Notidanidae the number of branchial arches is increasedbeyond the normal series, thus inHexanchusthere are six, and inHeptanchusseven. There are six also inChlamydoselacheandProtopterus.

4. The so-called external branchial arches which are cartilaginous rods attached to all the visceral arches. They are especially large inCestracion.

The skull inHolocephaliis entirely cartilaginous. The palato-pterygo-quadrate bar is fixed to the cranium, and to it the mandible articulates. There is a well-marked joint between the skull and the spinal column.

In living CartilaginousGanoidsthe primitive cartilaginous cranium is very massive, and is greatly prolonged anteriorly, while posteriorly it merges into the spinal column. Although it is mainly cartilaginous a number of ossifications take place in the skull, and membrane bones are now found definitely developed, especially in connection with the roof of the cranium. InAcipenser(fig. 18) the ossifications in the cartilage include the pro-otic, which is pierced by the foramen for the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid, palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11), cerato-hyal, all the cerato-branchials, and the first two epi-branchials. Most of these structures are, however, partly cartilaginous, though they include an ossified area. The membrane bones too ofAcipenserare very well developed, they include a bone occupying the position of the supra-occipital, and form a complete dorsal cephalic shield. Resting on the ventral surface are a vomer and a very large parasphenoid (fig. 18, 3). There is a bony operculum attached to the hyomandibular, and membrane bones representing respectively the maxilla and dentary are attached to the jaws. The suspensorium is most markedly hyostylic. The palato-pterygo-quadrate bar has a very curious shape and is quite separate from the cranium. It is connected to the hyomandibular by a thick symplectic ligament containing a small bone homologous with the symplectic of Teleosteans.

Polyodondiffers much fromAcipenser, the membrane bones not being so well developed though they cover the great cartilaginous snout.

Fig. 18. Lateral view of the skull of a Sturgeon(Acipenser sturio). Nearly all the membrane bones have been removed (Brit. Mus.).1. nasal cavity.10. palatine.2. orbit.11. hyomandibular.3. parasphenoid.12. pharyngo-branchial.4. vomer.13. epi-branchial.5. pterygoid.14. cerato-branchial.6. maxilla. (The dotted line15. hypo-branchial.running from 6 passes16. coalesced anterior vertebrae.into the mouth cavity.)17. inter-hyal.7. dentary.18. cerato-hyal.8. symplectic.19. rib.

The skull inPolypterus(Crossopterygii) shows a great advance towards the condition met with in Teleostei. The cranium remains to a great extent unossified, and large dorsal and ventral fontanelles pierce its walls. It is covered by a great development of membrane bones, paired nasals, frontals, parietals, supra- and post-temporals, and dermo-supra-occipitals among others being present. The palato-pterygo-quadrate bar is fused to the cranium, and in connection with it the following paired membrane bones appear, palatine, ecto-, meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla and premaxillae. The membrane bones developed in connectionwith each ramus of the mandible are the dentary, angular, and splenial, in addition to the cartilage bone the articular. Several large opercular bones occur. There are also a pair of large jugular or gular plates, and several large opercular bones.

In Bony Ganoids both cartilage bone and membrane bone is well developed. The pro-otics and exoccipitals are well ossified, but the supra-occipital and pterotics are not. Lateral ethmoids are developed, and there are ossifications in the sphenoidal region which vary in different forms. The place of the cartilaginous palato-pterygo-quadrate is taken by a series of bones, the quadrate behind and the palatine, ecto-, meso- and meta-pterygoids in front. InLepidosteus, however, the palatine and pterygoid are membrane bones, as they are inPolypterusand the Frog. Paired maxillae, premaxillae, vomers and a parasphenoid occur forming the upper jaw and roof of the mouth, and a series of membrane bones are found investing the mandible and forming the operculum.

InAmia[45]membrane bones are as freely developed as they are in Teleosteans; they include on each side a squamosal, four opercular bones, a lachrymal, a pre-orbital, one or two suborbitals, two large postorbitals and a supratemporal; while investing the mandible, besides the dentary, splenial, angular, and supra-angular, there is an unpaired jugular. The articular too is double and a mento-meckelian occurs. InAmiateeth are borne on the premaxillae, maxillae, vomers, palatines and pterygoids.

Bony Ganoids are the lowest animals in which squamosal bones are found, and they do not occur in Teleosteans.

The suspensorium in bony Ganoids, as in the Chondrostei, is hyostylic, and there are two ossifications in the hyomandibular cartilage, viz. the hyomandibular, and the symplectic.

The skull ofTeleosteiis very similar to those ofLepidosteusandAmia. Although the bony skull is greatly developed and very complicated, much of the original cartilaginous cranium often persists. Membrane bones are specially developed on the roof of the skull where they include the parietal, frontal, and nasal bones. The same bones are developed in connection with the upper jaw and roof of the mouth as in bony Ganoids, but only two membrane bones occur in the lower jaw, viz. the angular and dentary. A number of large ossifications take place in the cartilage of the auditory capsules. In some forms parts of the last pair of branchial arches are broadened out and form the pharyngeal bones which bear teeth. The opercular bones and those of the upper and lower jaws are quite comparable to those of bony Ganoids.

A full account of the Teleostean skull has been given in the case of the Salmon (pp. 87-96) and the Cod (pp. 96-101).

InDipnoithe skull is mainly cartilaginous, but both cartilage- and membrane-bone occur also. Cartilage-bone is found in the ossified exoccipitals, while of membrane-bonesProtopterushas among unpaired bones a fronto-parietal, a median ethmoid, and a parasphenoid, and among paired bones nasals and large supra-orbitals. The skull ofCeratodus(fig. 19) has an almost complete roof of membrane bones, including some whose homology is doubtful. The ethmo-vomerine region is always cartilaginous, but bears small teeth. The palato-pterygo-quadrate bar is ossified and firmly united to the cranium, and the mandible articulates directly with it (autostylic). Membrane bones are freely developed in connection with the mandible, dentary, splenial, and angular bones being all present. There are two opercular bones.

In the extinct Dipteridae the cranium is very completely covered with plates of dermal bone, and the skeleton in general is more ossified than is the case in recent Dipnoi.

Six pairs of branchial arches occur inProtopterus;CeratodusandLepidosirenhave five, like most other fish. The branchial arches bear gill rakers.

Fig. 19. Dorsal (to the left) and ventral (to the right) views of the cranium ofCeratodus miolepis(afterGünther).1. cartilaginous part of the7. pre-opercular (squamosal).quadrate with which the8. second rib.mandible articulates.9. first rib.2. scleroparietal.10. vomerine tooth.3. frontal.11. palato-pterygoid tooth.4. ethmoid.12. palato-pterygoid.5. nares.13. parasphenoid.6. orbit.14. interopercular.

Ribs.

As has been already mentioned (p. 24), although ribs commonly appear to be the cut-off ends of the transverse processes, they are really elements derived from the ventral or haemal arch.

In Elasmobranchii and other cartilaginous fish they have the form of small cartilaginous structures imperfectly separatedfrom the diverging halves of the ventral arch, and are often absent.

In Teleostei and bony Ganoids they often have different attachments in different parts of the body. In the tail region they are not differentiated from the two halves of the ventral arch, which meet in the middle line, and are prolonged into a haemal spine. In the posterior trunk region they sometimes form distinct processes diverging from the two halves of the ventral arch; while further forward they may shift their attachment so as to arise from the dorsal side of the two halves of the ventral arch and at some distance from their ends, which now diverge as ventri-lateral processes.

Appendicular Skeleton.

Pectoral girdle.

The simplest type of pectoral girdle is found in Elasmobranchs. It is entirely cartilaginous and consists of a curved ventrally-placed rod, ending dorsally in two horn-like scapular processes which are sometimes attached to the cranium or vertebral column. In Rays the shoulder girdle is very large, and has a distinct suprascapular portion forming a broad plate attached to the neural spines of the vertebrae. There is often a cup-like glenoid cavity for the articulation of the limb; this cavity is specially large in Rays and is much pierced by holes. In Dipnoi the cartilaginous girdle still occurs, but on it there is a deposit of membrane bone forming the clavicle, infraclavicle, and supra-clavicle. These bones, which with the exception of the clavicle, are unknown in higher vertebrates, are better developed in Ganoids, and best of all in Teleosteans. They are connected by the supratemporal with the epi-otic and opisthotic regions of the cranium. Owing to this development of dermal bone, the original cartilaginous arch becomes much reduced, but ossifications representing the scapula and coracoid occur in bony Ganoids and Teleosteans.

Pelvic girdle.

In Elasmobranchs the pelvic girdle consists of a short ventral rod of cartilage representing the ischium and pubis, which does not send up dorsal iliac processes. InChimaerathe pelvic girdle has a flattened pointed iliac portion, and ventrally an unpaired movable cartilaginous plate which bears hooks and is supposed to be copulatory in function. Claspers of the usual type are present as well. The Dipnoi have a primitive kind of pelvis in the form of a cartilaginous plate lying in the mid-ventral line and drawn out into three horns anteriorly. In Ganoids the pelvis has almost entirely disappeared, though small cartilaginous vestiges of it remain inPolypterus. In Teleosteans even these vestiges are gone, and in these fish and Ganoids the place of the pelvis is taken by the enlarged basi-pterygia (meta-pterygia) of the fins.

Paired fins[46].

As regards the origin of the limbs or paired fins of fishes there are two principal views. One view, that of Gegenbaur, considers that limbs and their girdles are derived from visceral arches which have migrated backwards. The other view, which probably now has the greater number of supporters, considers that the paired fins of fishes are of essentially the same nature as the median fins.

According to Gegenbaur's view[47]thearchipterygiumofCeratodus(fig. 20) represents the lowest type of fin; it consists of a central cartilaginous axis bearing a large number of radiale. The dorsal or pre-axial radiale are more numerous than the ventral or postaxial, and at the margin of the fin[48]thecartilaginous endoskeletal radiale are replaced by horny exoskeletal fin-rays.

Fig. 20. Lateral view of the skeleton ofCeratodus miolepis(afterGünther).1. ethmoid.9. pectoral girdle.2. scleroparietal.10. proximal cartilage of pectoral fin.3. frontal.11. pectoral fin.4. cartilaginous posterior part of cranium.12. pelvic girdle.5. pre-opercular (squamosal).13. pelvic fin.6. opercular.14. spinal column.7. suborbital.15. caudal fin (diphycercal).8. orbit.

It is impossible here to give a full discussion of the rivalviews, but some of the points which support Gegenbaur's view may be mentioned. The fact that migration of visceral arches has to be assumed is no difficulty, as it is obvious that migration in the opposite direction has taken place in many Teleosteans such as the Cod, whose pelvic fins are attached to the throat in front of the pectorals. If migration did take place, the pelvic fins being older than the pectoral should be the more modified, and this is the case. Again, if the pectoral girdle is a modified branchial arch, it must at some period have carried a gill, and inProtopterusit does bear a vestigial gill.

According to the view more prevalent at the present time, the paired fins have been derived from two continuous folds of skin and their skeletal supports running forward from the anal region along the sides of the body, their character being similar to the fold that gave rise to the median fins. In support of this view it may be argued that the paired and unpaired fins are often identical in structure, and that some Elasmobranch embryos do show a ridge running between the pectoral and pelvic fins. Then from this continuous fold two pairs of smaller folds may have been specialised off, and in each a number of cartilaginous radiale may have been developed. The fin ofCladoselachefrom the Carboniferous of Ohio apparently illustrates this condition. It consists of certain basal pieces which do not project beyond the body wall and bear a number of unsegmented cartilaginous radiale, which show crowding together and are sometimes bifurcated distally; they extend throughout the whole fin from the body wall to the margin. From this fin the archipterygium might be easily derived by the enlargement of one of the middle radiale and the segmentation and partial fusion of them all.

Whether the archipterygium be a primitive or secondarytype of fin, when it is once reached it is easy to derive all the other types from it. The fins of the other living Dipnoi,—ProtopterusandLepidosiren—are simply archipterygia from which the radiale have almost or completely disappeared, leaving only the segmented axes. Archipterygia too are found in the pectoral fins of the Ichthyotomi, but the postaxial radiale are much reduced.

Theichthyopterygium, or type of fin, characteristic of many modern Elasmobranchs such asScyllium, may have been derived from the archipterygium by the gradual reduction of the rays on the postaxial side of the axis and their condensation on the pre-axial side. The Ichthyotomi such asXenacanthusshow one stage in the reduction of the postaxial rays, and a further stage is seen in the Notidanidae and some other sharks likeScymnusandAcanthias, in which a few postaxial rays still remain. The condensation of the pre-axial rays when further continued leads to one of the rays getting an attachment to the girdle. Thus the fin comes to articulate with the girdle by two basalia or basal pieces; a third attachment is formed in the same way and the three basalia are called respectively pro-, meso-, and meta-pterygia. By some authors the meta-pterygium and by others the meso-pterygium is regarded as homologous with the axis of the archipterygium.

The pectoral fins of Elasmobranchs vary very much in their mode of attachment. In some of the sharks, including the Notidanidae andScyllium, all three basalia articulate with the pectoral girdle, while in others such asCestracionthe meta-pterygium is excluded. In Rays the propterygium and the meta-pterygium are long and narrow and diverge much from one another; other basalia work their way in between the meso-pterygium and meta-pterygium, and come to articulate with the pectoral girdle. Sometimes they fuse and form a second meso-pterygium. The radiale are greatly elongated and are segmented.

InChimaeraall three basalia are present, but the meso-pterygium is shifted and does not articulate with the pectoral girdle[49].

InAcipenserandPolyodonthe pectoral fin is built on the same type as in Elasmobranchs, but becomes modified from the fact that the propterygium is replaced by dermal bone which forms a largemarginal ray. Extra meso-pterygia are formed in the same way as in Rays.

InPolypterusthe pro-and meta-pterygia have ossified while the meso-pterygium remains chiefly cartilaginous; the fin-rays are also chiefly ossified.

InAmia,Lepidosteus, and certain Teleosteans likeSalmo, not only the propterygium but the meso-pterygium is almost suppressed by the marginal ray.

In the great majority of Teleosteans a still further stage is reached, the endoskeletal elements, the basalia and radiale are almost entirely suppressed and the fin comes to consist entirely of ossified fin-rays of dermal origin.

In some Teleosteans—Exocaetus, a herring, andDactylopterus, a gurnard—the pectoral fins are so enormously developed that by means of them the fish is able to fly through the air for considerable distances. The skeleton of these great fins is almost entirely composed of dermal bone.

Pelvic fin.

The pelvic fin is almost always further removed from the archipterygial condition, and is in general more modified than is the pectoral. Thus in the Ichthyotomi, while the pectoral fins are archipterygia similar to those ofCeratodus, the pelvic fins consist of an axis bearing rays on the postaxial side only, and prolonged distally into a clasper. In Dipnoi however the pelvic fins are very similar to the pectoral. In Elasmobranchs the meso-pterygium is missing, the propterygium is small or absent, and the fin is mainly composed of the meta-pterygium(generally called basi-pterygium) and its radiale. The males in Elasmobranchii and Holocephali have the distal end of the meta-pterygium prolonged into a clasper.

In Ganoids and in Teleosteans the loss of the pelvic girdle causes the pelvic fin to be still further removed from the primitive state. There is always a large basi-pterygium which lies imbedded in the muscles and meets its fellow at its proximal end. In Cartilaginous Ganoids it has a secondary segmentation. Its relation to its fellow is subject to much variation in Teleosteans, sometimes as in the Perch the two are in contact throughout, sometimes as in the Salmon they meet distally as well as proximally, but are elsewhere separated by a space, sometimes as in the Pike and Bony Ganoids they diverge widely. The radiale are articulated to the basi-pterygium. In Cartilaginous Ganoids andPolypterusthey are well developed, in other Ganoids and in Teleosteans they are in the main replaced by dermal fin-rays.

In some Teleosteans such as the Cod the pelvic fins have migrated from their usual position and come to be attached to the throat in front of the pectoral fins. Fish with this arrangement are grouped together asjugulares.

Amphibiadiffer markedly from Pisces in the fact that in the more abundant and familiar forms the skin is naked, and that when the integument is prolonged into median fins they are devoid of fin-rays. The notochord may persist, but bony vertebral centra are always developed. These are sometimes biconcave, sometimes procoelous, sometimes opisthocoelous. There is only one sacral vertebra, except in rare cases. The cartilaginous cranium persists to a considerable extent but is more or less replaced by cartilage bone, and overlain by membrane bone. The basi-occipital is not completely ossified, and the skull articulates with the vertebral column by means of two occipital condyles formed by the exoccipitals.

There is a large parasphenoid, but there are no ossifications in the basisphenoidal, presphenoidal, and alisphenoidal regions. In most cases the epi-otics and opisthotics are ossified continuously with the exoccipitals.

The palato-pterygo-quadrate bar is firmly united to the cranium, so the skull is autostylic. The palatines and pterygoids are membrane bones. Teeth are nearly always borne on the vomers and commonly on the maxillae andpremaxillae. There are no sternal ribs, and the sternum is very intimately related to the pectoral girdle. There are no obturator foramina. The limbs are as in the higher vertebrata, divisible into upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin, and pes (ankle and foot) respectively. The posterior limb is, as a rule, pentedactylate, but in nearly every case the pollex is vestigial or absent.

Order 1.Urodela[51].

The Urodela are elongated animals with a naked skin, a persistent tail, and generally four short limbs.

The vertebral centra are opisthocoelous or biconcave, and there are numerous precaudal vertebrae. Portions of the notochord commonly persist in the intervertebral spaces. In the skull there is no sphenethmoid forming a ring encircling the anterior end of the brain, its place being in many cases partly taken by a pair of orbitosphenoids. There is no quadratojugal, and the quadrate is more or less ossified. The mandible has a distinct splenial, and the articular is ossified.

There is no definite tympanic cavity. The hyoid apparatus is throughout life connected to the quadrate by ligament, and a large basilingual plate does not occur. The ribs are short structures with bifurcated proximal ends. In the pelvis the pubis remains cartilaginous, and there is a bifid cartilaginous epipubis. The bones of the fore-arm and shin remain distinct, and the manus never has more than four digits.

Suborder(1).Ichthyoidea.

The vertebrae are amphicoelous, but the notochord remains but little constricted throughout the whole length of the vertebral column. Three or four branchial arches nearly always persist in the adult. The cartilages of the carpus and tarsus remain unossified.

The Ichthyoidea may be subdivided again into two groups:—

A.Perennibranchiata, whose chief distinguishing skeletal characters are that the skull is elongated, the premaxillae are not ankylosed, the maxillae are vestigial or absent; there are sometimes no nasals, and the palatines bear teeth;

e.g.Siren,Proteus,Menobranchus.

B.Derotremata, whose chief distinguishing skeletal characters are that there are large maxillae and nasals; teeth are borne by both maxillae and premaxillae; there are no palatines; and both pectoral and pelvic limbs are always present;

e.g.Amphiuma,Megalobatrachus,Cryptobranchus.

Suborder(2).Salamandrina.

The vertebrae are opisthocoelous. The skull is broad, and teeth are borne by both premaxillae and dentaries. Nasal bones are present. The remains of only two branchial arches are found in the adult. The carpus and tarsus are more or less ossified.

This suborder includes the Newts (Molge), Salamanders (Salamandra), andAmblystoma.

Order2.Labyrinthodontia[52].

These are extinct Amphibia with a greatly developed dermal exoskeleton, which is generally limited to the ventral surface. The body and tail are long and in some cases limbs are absent. The teeth are pointed and often have the dentine remarkably folded. The vertebrae are amphicoelous, and are generally well ossified. The skull is very solid, and has a greatly-developed secondary roof which hides the true craniumand is very little broken up by fossae. Paired dermal supra-occipitals are found, and there is an interparietal foramen. The epi-otics and opisthotics form a pair of bones distinct from the exoccipitals. Four simple limbs of moderate length are generally present, and in some cases all four limbs are pentedactylate. Among the better known genera of Labyrinthodonts areMastodonsaurus,Nyrania, andArchegosaurus.

Order 3.Gymnophiona[53].

These animals form a group of abnormal worm-like Amphibia having an exoskeleton in the form of subcutaneous scales arranged in rings. The vertebrae are biconcave and are very numerous; very few however belong to the tail. The skull has a complete secondary bony roof, the mandible bears teeth and has an enormous backward projection of the angular. The hyoid arch has very slender cornua and no distinct body, it is attached neither to the cranium nor to the suspensorium. The ribs are very long and there are no limbs or limb girdles.

Order 4.Anura.

These are tailless Amphibia, which except in a few instances, are devoid of an exoskeleton. The vertebrae are as a rule procoelous, and are very few in number. The post-sacral part of the spinal column ossifies continuously, forming an unsegmented cylindrical rod, the urostyle. Remains of the notochord persist, lyingvertebrally, i.e. enclosed within the centra of the several vertebrae, and not as in Urodela lying between one vertebra and the next. The skull is very short and wide. The mandible is almost always, if not invariably, toothless.

The frontals and parietals on each side are united so as to form a pair of fronto-parietals, and a girdle-like sphenethmoid is present.

The quadrate is not generally ossified. A predentary or mento-meckelian bone is commonly present in the mandible, and a single bone represents the angular and splenial. The branchial arches are much reduced in the adult, and the distal ends of the cornua unite to form a flat basilingual plate of a comparatively large size.

Ribs are very little developed. Clavicles are present. The ilia are very greatly elongated. The anterior limb has four well-developed digits and a vestigial pollex, and is of moderate length; the radius and ulna have fused. The posterior limb is greatly elongated and is pentedactylate; the tibia and fibula are fused, while the calcaneum and astragalus are greatly elongated, and it is largely owing to them that the length of the limb is so great. The group includes the Frogs and Toads, the predominant Amphibia of the present time.

I. EXOSKELETON.

The skin of the Newt is quite devoid of any exoskeletal structures. The only exoskeletal structures that the animal possesses are the teeth, and these are most conveniently described with the endoskeleton.

II. ENDOSKELETON.

The endoskeleton of the Newt, though ossified to a considerable extent, is more cartilaginous than is that of the frog. It is divisible into anaxial portionincluding the vertebral column, skull, ribs, and sternum, and anappendicular portionincluding the skeleton of the limbs and their girdles.

1.The Axial Skeleton.

A.The Vertebral column.

This consists of about fifty vertebrae arranged in a regular continuous series. The first vertebra differs a good deal from any of the others; the seventeenth or sacral vertebra and the eighteenth or first caudal also present peculiarities of their own. The remaining vertebrae are divided by the sacrum into an anterior series oftrunkvertebrae which bear fairlylarge ribs, and a posterior series ofcaudalvertebrae, all of which except the first few are ribless.

The trunk vertebrae.

Any vertebra from the second to the sixteenth may be taken as a type of the trunk vertebrae.

The general form is elongated and somewhat hour-glass shaped, and thecentraare convex in front and concave behind; an opisthocoelous condition such as this is quite exceptional in Anura. Thenotochordmay persist intervertebrally[54], but in the centre of each vertebra it becomes greatly constricted or altogether obliterated, and replaced by marrow. The superficial portion of the centrum is ossified, while the articular surfaces are cartilaginous. Theneural archesare low and articulate together by means ofzygapophysesborne on short diverging processes. The anterior zygapophyses look upwards, the posterior downwards. Each neural arch is drawn out dorsally into a very slight cartilaginousneural spine.

On each centrum, at a little behind the middle line, there arise a pair of short backwardly-directedtransverse processes; each of which becomes divided into two slightly divergent portions, a dorsal portion which meets the tubercular process of the rib and is derived from the neural arch, and a ventral portion which meets the capitular process of the rib and is derived from the ventral or haemal arch. The division between these two parts of the transverse processes can be traced back as far as the sacrum.

Thefirst vertebraas already mentioned differs much from all the others. It has no ribs, and presents anteriorly two slightly divergent concave surfaces which articulate with the occipital condyles of the skull. Between these surfaces the dorsal portion of the anterior face of the centrum is drawn out into a prominentodontoid process, the occurrence of which renders it probable that the first vertebra ofthe newt is really the axis, and that the atlas with the exception of the odontoid process has become fused with the skull. The sacral vertebra orsacrumdiffers from the vertebrae immediately in front of it only in the fact that its transverse processes are stouter and more obviously divided into dorsal and ventral portions.

The caudal vertebrae.

Thecaudal vertebraeare about twenty-four in number. The anterior ones have hour-glass shaped centra, and short backwardly-directed transverse processes. The middle and posterior ones have rather shorter centra, and are without transverse processes. The neural arches resemble those of the trunk vertebrae, but each is drawn out into a rather high cartilaginous neural spine abruptly truncated anteriorly. All the caudal vertebrae except the first have also a haemal arch, which is very similar to the neural arch, and is drawn out into a haemal spine quite similar to the neural spine. Both neural and haemal arches are ossified continuously with the centra.

B.The Skull.

The skull of the newt is divisible into three principal parts:—

(1) an axial part, thecranium proper, which encloses the brain and to which

(2) thecapsulesof theauditory and olfactory sense organsare fused;

(3) the skeleton of thejaws and hyoid apparatus. The skull is much flattened and expanded, though not so much as in the frog.

(1)The cranium proper.

Thecranium properorbrain caseis an unsegmented tube which remains partly cartilaginous, and is partly converted into cartilage bone, partly sheathed by membrane bone. The roof and floor of the cartilaginous cranium are, as isthe case also in the frog, pierced by holes or fontanelles, and these are so large that the main part of the roof and floor comes to be formed by membrane bone.

Two pairs of large ossifications take place in the cranial walls. Of these the more posterior on each side represents theexoccipitaland all threeperioticbones. It bears a small convex patch of cartilage for articulation with the atlas, and with its fellow forms the boundary of the foramen magnum.

Two foramina pierce the exoccipital just in front of the occipital condyle and transmit respectively the glossopharyngeal and pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve openings is seen a patch of cartilage, thestapes, which is homologous with the stapes or proximal element of the columellar chain in the frog. Further forward in front of the stapes is the small opening for the exit of the facial nerve, and seen in a lateral view close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).

In front of these large bones the lateral parts of the cranial walls remain cartilaginous for a short distance, and then there follow two elongated bones, theorbitosphenoids(fig. 21, B and C, 11), pierced by the foramina for the exit of the optic nerves. These bones partly correspond to the sphenethmoid of the frog.

Themembrane bonesconnected with the cranium are theparietals,frontalsandprefronto-lachrymalson the dorsal surface, and theparasphenoidon the ventral surface.

Theparietals(fig. 21, A and C, 6) roof over the posterior part of the great dorsal fontanelle and overlap the exoccipito-periotics. They meet one another along a sinuous suture in the middle line, as do also thefrontalswhich overlap them in front. Thefrontalsandparietalsboth extend for a short distance down the sides of the cranium and meet the orbitosphenoids. Theprefronto-lachrymals(fig. 21, A and C, 7) connect the frontals with the maxillae.

Fig. 21. A dorsal, B ventral, and C lateral views of the skullof a Newt(Molge cristata) × 2½ (afterParker).The cartilage is dotted, the cartilage bones are marked with dots anddashes, the membrane bones are left white.1. premaxillae.14. pro-otic region of2. anterior nares.exoccipito-periotic.3. posterior nares.15. quadrate.4. nasal.16. quadrate cartilage.5. frontal.17. exoccipital region of6. parietal.exoccipito-periotic.7. prefronto-lachrymal.18. articular.8. maxillae.19. articular cartilage.9. vomero-palatine.20. dentary.10. parasphenoid.21. splenial.11. orbitosphenoid.22. middle narial passage.12. pterygoid.II. V. VII. IX. X. foramina for13. squamosal.the exit of cranial nerves.

On the ventral surface is the largeparasphenoid(fig. 21, B, 10), which is widest behind and overlapped anteriorlyby the vomero-palatines.

(2)The sense capsules.

Theauditory capsulesbecome almost completely ossified continuously with the exoccipitals; they have been already described.

Thenasal capsulesare large and quite unossified though they are overlain by membrane bone. They appear on the dorsal surface between the anterior nares and the nasal process of the premaxillae. They enclose the nasal organs, bound the inner side of the anterior narial opening, and are connected with one another posteriorly by a cartilaginous area.

Developed in connection with the nasal capsules are a pair of rather largenasals(fig. 21, A and C, 4), which lie on the dorsal surface immediately in front of the frontals. Each forms part of the posterior boundary of one of the anterior nares, and the two are separated from one another in the middle line by the nasal process of the premaxillae (fig. 21, A, 1), and the opening of themiddle narial passage(fig. 21, A and B, 22), which passes right through the skull.

On the ventral surface of the skull and forming the greater part of the boundary of the posterior nares are two large bones, thevomero-palatines(fig. 21, B and C, 9). Each consists of a wide anterior portion, partly separated from its fellow in the middle line by the ventral opening of the middle narial passage, and of a long pointed posterior portion which is separated from its fellow by theparasphenoid, and bears a row of small pointed teeth formed of dentine capped with enamel.

(3)The jaws.

Theupper jawof the newt is a discontinuous structure divided into two parts, an anterior part which consists of membrane bones, themaxillaeandpremaxillae, and a posterior part which remains mainly cartilaginous.

Thepremaxillaeare united, forming a single bone, which ina ventral view is seen to meet the maxillae and vomero-palatines, and in a dorsal view to send back a nasal process (fig. 21, A, 1) between the nasals.

Themaxillaeare large bones, each terminating in a point posteriorly. A single row of teeth similar to those on the vomero-palatines runs along the outer margin of the maxillae and premaxillae.

The posterior part of the upper jaw forms a mass of cartilage which extends forwards towards the maxillae as a long pointed process whose ventral surface and sides are overlapped by a membrane bone, thepterygoid(fig. 21, 12).

The suspensorial bones include thequadrateandsquamosal. Thequadrate(fig. 21, 15) which forms the truesuspensoriumis directed forwards and outwards, and is terminated by a patch of cartilage with which the mandible articulates.

The lower jaw or mandible remains partly cartilaginous, while its ossifications include two membrane bones and one cartilage bone. The cartilage bone is thearticular(fig. 21, C, 18), it forms the posterior part of the ramus, extends forwards for some distance along its inner side, and is terminated posteriorly by a patch of cartilage which articulates with the quadrate. Thedentary(fig. 21, C, 20) is a large bone which forms the anterior part and nearly all the outer half of each ramus, and bears teeth similar to those of the upper jaw. Attached to its inner face is a long slendersplenial(fig. 21, C, 21).

The Hyoid apparatus.

This consists of the hyoid arch and part of the first two branchial arches.

Thehyoid arch(fig. 29, A, 2) consists of a pair ofcornua, each of which is divided into two halves. The dorsal half forming thecerato-hyalis mainly ossified though tipped with cartilage, and is connected by ligament with the suspensorium. The ventral half (hypo-hyal) is cartilaginous, and is connected with the basibranchial.

Thebranchial archesconsist of a median piece, thebasibranchial, which is ossified in the centre and cartilaginous at either end, and of two pairs ofcerato-branchialswhich are attached to the cartilaginous part (fig. 29, A, 8) of the basibranchial. The first cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a good deal smaller and is cartilaginous. Both are united dorsally to a singleepi-branchial, which is terminated by a small cartilaginous area at the free end but is elsewhere well ossified.

C.The Ribs.

The ribs are short imperfectly ossified structures, bifid at their proximal end where they articulate with the transverse processes, and tipped both proximally and distally with cartilage. The dorsal portion of the proximal end corresponds to thetuberculumof the ribs of higher animals, and the ventral portion to thecapitulum. Some of the anterior ribs have a step-like notch on their dorsal surfaces.

The second to twelfth ribs are fairly equal in size, but further back they decrease slightly. The ribs which connect the sacral vertebrae with the ilia are however large. The short ribs borne on the anterior caudal vertebrae are cartilaginous.

D.The Sternum.

The sternum (fig. 22, A, 6) is a rather broad plate of cartilage, drawn out posteriorly into a median process marked by a prominent ridge. On its antero-lateral margins it bears surfaces for articulation with the pectoral girdle.

2.The Appendicular Skeleton.

A.The Pectoral girdle.

This is of a very simple character, and remains throughout life in an imperfectly ossified condition. It consists of a dorsalscapular portion, and a ventralcoracoid portionpartially divided into an anterior part, the precoracoid, and a posterior part, thecoracoid.

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