Part 11

All living batrachians, and some of the Stegocephalia, have transverse processes on the vertebrae that succeed the atlas (fig. 4), some of which, in the Caudata, are divided into a dorsal and a ventral portion. Ribs are present in the lower Ecaudata (_Discoglossidae_ and larval Aglossa), but they are never connected with a sternum. It is in fact doubtful whether the so-called sternum of batrachians, in most cases a mere plate of cartilage, has been correctly identified as such. When limbs are present, one vertebra, rarely two (fig. 5) or three, are distinguished as sacral, giving attachment to the ilia. In the Ecaudata, the form of the transverse processes of the sacral vertebra varies very considerably, and has afforded important characters to the systematist. In accordance with the saltatorial habits of the members of this order, the vertebrae, which number from 40 to 60 in the Caudata, to upwards of 200 in the Apoda, have become reduced to 10 as the normal number, viz., eight praecaudal, one sacral and an elongate coccyx or urostyle, formed by coalescence of at least two vertebrae. In some genera this coccyx is fused with the ninth vertebra, and contributes to the sacrum, whilst in a few others the number of segments is still further reduced by the co-ossification of one or two vertebrae preceding that corresponding to the normal sacral and by the fusion of the two first vertebrae, the extreme of reduction being found in the genus _Hymenochirus_, the vertebral column of which is figured here (fig 6.)

[Illustration: FIG. 8.--The skull of _Ichthyophis glutinosus_ A, Dorsal; B, ventral; C, lateral view. The letters have the same signification as below.]

As stated above in the definition of the order, the Stegocephalia have retained most of the cranial bones which are to be found in the Crossopterygian fishes, and it is worthy of note that the bones termed post-temporals may give attachment to a further bone so prolonged backwards as to suggest the probability of the skull being connected with the shoulder-girdle, as in most teleostome fishes. This supposition is supported by a specimen from the Lower Permian of Autun, determined as _Actinodon frossardi_, acquired in 1902 by the British Museum, which shows a bone, similar to the so-called "epiotic cornu" of the microsaurians, _Ceraterpeton_ and _Scincosaurus_, to have the relations of the supra-cleithrum of fishes, thus confirming a suggestion made by C.W. Andrews (28). As in fishes also, the sensory canal system must have been highly developed on the skulls of many labyrinthodonts, and the impressions left by these canals have been utilized by morphologists for homologizing the various elements of the cranial roof with those of Crossopterygians. The pineal foramen, in the parietal bones, is as constantly present as it is absent in the other orders. Although not strictly forming part of the skull, allusion should be made here to the ring of sclerotic plates which has been found in many of the Stegocephalia, and which is only found elsewhere in a few Crossopterygian fishes as well as in many reptiles and birds.

In the orders which are still represented at the present day, the bones of the skull are reduced in number and the "primordial skull," or chondrocranium (fig. 7), remains to a greater or less extent unossified, even in the adult. Huxley's figures of the skull of a caccilian (_Ichthyophis glutinosus_), fig. 8, of a perennibranchiate urodele (_Necturus maculosus = Menobranchus lateralis_), fig. 9, and of a frog (_Rana esculenta_), fig. 10, are here given for comparison.

The skull, in the _Apoda_, is remarkably solid and compact, and it possesses a postorbital or postfrontal bone (marked 1 in the figure) which does not exist in any of the other living batrachians. The squamosal bone is large and either in contact with the frontals and parietals or separated from them by a vacuity; the orbit is sometimes roofed over by bone. The presence, in some genera, of a second row of mandibular teeth seems to indicate the former existence of a splenial element, such as exists in _Siren_ among the Caudata and apparently in the labyrinthodonts.

In the Caudata, the frontals remain likewise distinct from the parietals, whilst in the Ecaudata the two elements are fused into one, and in a few forms (Aglossa, some _Pelobalidae_) the paired condition of these bones has disappeared in the adult. Prefrontal bones are present in the _Salamandridae_ and _Amphiumidae_, but absent (or fused with the nasals) in the other Caudata and in the Ecaudata. In most of the former the palatines fuse with the vomers, whilst they remain distinct, unless entirely lost, in the latter. The vomer is single, or absent, in the Aglossa. In the lower jaw of most of the Ecaudata the symphysial cartilages ossify separately from the dentary bones, forming the so-called mento-meckelian bones; but these symphysial bones, so distinct in the frog, are less so in the _Hylidae_ and _Bufonidae_, almost indistinguishable in the _Pelobatidae_ and _Discoglossidae_, whilst in the Aglossa they do not exist any more than in the other orders of batrachians.

No batrachian is known to possess an ossified azygous supra-occipital.

[Illustration: FIG. 9.--Lateral, dorsal and ventral views of the cranium of _Necturus maculosus_. In the dorsal view, the bones are removed from the left half of the skull, in the ventral view, the parasphenoid, palato-pterygoid, and vomers are given in outline. The letters have, for the most part, the same signification as before.

VII.p, Posterior division of the seventh nerve. VII. Chorda tympani V^1, V^2, V^3, First, second and third divisions of the trigeminal. s.s.l, Stapedio-suspensorial ligament. h.s.l, Hyo-suspensorial ligament. m.h.l, Mandibulo-hyoid ligament. a, Ascending process of the suspensorium. p, Pterygo-palatine process. q, Quadrate process. o, Otic process. Na, Posterior nares. Mck, Meckel's cartilage. Gl (fig. 10), The position of the glottis. Bb^1, Bb^2, Basilbranchials.]

Although there are four branchial arches in all the larval forms of the three orders, and throughout life in the _Sirenidae_, the perennibranchiate _Proteidae_ have only three (see fig. 11). In the adult Apoda these arches and the hyoid fuse into three transverse, curved or angular bones (see fig. 13), the two posterior disconnected from the hyoid. In the Ecaudata, as shown by F. Gaupp (29) and by W.G. Ridewood (30), the whole hyobranchial apparatus forms a cartilaginous continuum, and during metamorphosis the branchialia disappear without a trace. The hyoid of the adult frog (fig. 12) consists of a plate of cartilage with two slender cornua, three processes on each side, and two long bony rods behind, termed the thyro-hyals, which embrace the larynx. In the Aglossa, which are remarkable for the large size and complexity of the larynx, the thyro-hyal bones are incorporated into the laryngeal apparatus, whilst the recently discovered _Hymenochirus_ is further remarkable for the large size and ossification of the hyoidean cornua (ceratohyals), a feature which, though not uncommon among the salamanders, is unique among the Ecaudata (31).

[Illustration: FIG. 10--Dorsal, ventral, lateral, and posterior views of the skull of _Rana esculenta_. The letters have the same signification throughout.

Pmx, Premaxilla. Mx, Maxilla. Vo, Vomer. Na, Nasal. S.e, Sphen-ethmoid. Fr, Frontal. Pa, Parietal. E.O, Exoccipital. Ep, Epiotic process. Pr.O, Pro-otic. t.t, Tegmentympani. Sq, Squamosal. Q.J, Quadrato-jugal. Pt1, Pterygoid, anterior process. Pt2, Internal process. Pt3, Posterior or external process. Ca, Columella auris. St, Stapes. Hy, Hyoidean cornu. P.S, Parasphenoid. An, Angulate. D, Dentale. V, Foramen of exit of the trigeminal. H, Of the optic. X, Of the pneumogastric and glosso-pharyngeal nerves. V1. Foramen by which the orbito-nasal or first division of the fifth passes to the nasal cavity.]

The pectoral girdle of the Stegocephalia is, of course, only known from the ossified elements, the identification of which has given rise to some diversity of opinion. But C. Gegenbaur's (32) interpretation may be regarded as final. He has shown that, as in the Crossopterygian and Chondrostean ganoid fishes, there are two clavicular elements on each side; the lower corresponds to the clavicle of reptiles and higher vertebrates, whilst the upper corresponds to the clavicle of teleostean fishes, and has been named by him "cleithrum." As stated above, there is strong evidence in favour of the view that some forms at least possessed in addition a "supracleithrum," corresponding to the supra-clavicle of bony fishes. The element often termed "coracoid" in these fossils would be the scapula. The clavicles rest on a large discoidal, rhomboidal, or T-shaped median bone, which clearly corresponds to the interclavicle of reptiles.

The pectoral girdle of the living types of batrachians is distinguishable into a scapular, a coracoidal, and a praecoracoidal region. In most of the Caudata the scapular region alone ossifies, but in the Ecaudata the coracoid is bony and a clavicle is frequently developed over the praecoracoid cartilage. In these batrachians the pectoral arch falls into two distinct types--the _arciferous_, in which the precoracoid (+clavicle) and coracoid are widely separated from each other distally and connected by an arched cartilage (the epicoracoid), the right usually overlapping the left; and the _firmisternal_, in which both precoracoid and coracoid nearly abut on the median line, and are only narrowly separated by the more or less fused epicoracoids. The former type is exemplified by the toads and the lower Ecaudata, whilst the latter is characteristic of the true frogs (_Ranidae_), although when quite young these batrachians present a condition similar to that which persists throughout life in their lower relatives. A cartilage in the median line in front of the precoracoids, sometimes supported by a bony style, is the so-called Omosternum; a large one behind the cora-coids, also sometimes provided with a bony style, has been called the sternum. But these names will probably have to be changed when the homologies of these parts are better understood.

[Illustration: FIG. 11.--Hyoid and branchial apparatus of _Necturus maculosus_.

Hh, Hypo-hyal. Ch, Cerato-hyal. Bb^1, First basibranchial. Bb^2, Ossified second basibranchial. Ep.b^1, Ep.b^2, Ep.b^3, First, second and third epibranchials. Gl, Glottis.]

The pelvic arch of some of the Stegocephalia contained a well ossified pubic element, whilst in all other batrachians only the ilium, or the ilium and the ischium are ossified. In the Ecaudata the ilium is greatly elongated and the pubis and ischium are flattened, discoidal, and closely applied to their fellows by their inner surfaces; the pelvic girdle looks like a pair of tongs.

The long bones of the limbs consist of an axis of cartilage; the extremities of the cartilages frequently undergo calcification and are thus converted into epiphyses. In the Ecaudata the radius and ulna coalesce into one bone. The carpus, which remains cartilaginous in many of the Stegocephalia and Caudata, contains six to eight elements when the manus is fully developed, whilst the number is reduced in those forms which have only two or three digits. Except in some of the Stegocephalia, there are only four functional digits in the manus, but the Ecaudata have a more or less distinct rudiment of pollex; in the Caudata it seems to be the outer digit which has been suppressed, as atavistic reappearance of a fifth digit takes place on the outer side of the manus, as it does on the pes in those forms in which the toes are reduced to four. The usual number of phalanges is 2, 2, 3, 2 in the Stegocephalia and Caudata, 2, 2, 3, 3 in the Ecaudata. In the foot the digits usually number five, and the phalanges 2, 2, 3, 3, 2 in the Caudata, 2, 2, 3, 4, 3 in the Stegocephalia and Ecaudata. There are occasionally intercalary ossifications between the two distal phalanges (33). There are usually nine tarsal elements in the Caudata; this number is reduced in the Ecaudata, in which the two bones of the proximal row (sometimes coalesced) are much elongated and form an additional segment to the greatly lengthened hind-limb, a sort of _crus secundarium_. In the Ecaudata also, the tibia and fibula coalesce into one bone, and two or three small bones on the inner side of the tarsus form what has been regarded as a rudimentary digit or "prehallux."

[Illustration: FIG. 12.--Ventral view of the hyoid of _Rana esculenta_. a, Anterior; b, lateral; c, posterior processes; d, thyro-hyals.]

_Integument._--In all recent batrachians, the skin is naked, or if small scales are present, as in many of the Apoda, they are concealed in the skin. The extinct Stegocephalia, on the other hand, were mostly protected, on the ventral surface at least, by an armour of overlapping round, oval, or rhomboidal scales, often very similar to those of Crossopterygian or ganoid fishes, and likewise disposed in transverse oblique lines converging forwards on the middle line of the belly. Sometimes these scales assumed the importance of scutes and formed a carapace, as in the "batrachian armadillo" discovered by E.D. Cope. A few frogs have the skin of the back studded with stellate bony deposits (_Phyllomedusa, Nototrema_), whilst two genera are remarkable for possessing a bony dorsal shield, free from the vertebrae (_Ceratorphrys_) or ankylosed to them (_Brachycephalus_). None of the Stegocephalia appears to have been provided with claws, but some living batrachians (_Onychodactylus, Xenopus, Hymenochirus_) have the tips of some or all of the digits protected by a claw-like horny sheath.

The integument of tailed and tailless batrachians is remarkable for the great abundance of follicular glands, of which there may be two kinds, each having a special secretion, which is always more or less acrid and irritating, and affords a means of defence against the attacks of many carnivorous animals. A great deal has been published on the poisonous secretion of batrachians (34), which is utilized by the Indians of South America for poisoning their arrows. Some of the poison-secreting glands attain a greater complication of structure and are remarkable for their large size, such as the so-called "parotoid" glands on the back of the head in toads and salamanders.

[Illustration: FIG. 13.--Ventral view of the head and trunk of _Ichthyophis glutinosus_.

Mn, Mandible. Hy, Hyoid. Br^1 Br^2, Br^3, Branchial arches. Gl, Glottis. Tr, Trachea. Ivc, Inferior vena cava. V, Ventricle. Au, Auricles. Rsvc, Lsvc, right and left superior cavae. Ta, Truncus arteriosus. Ao, Left aortic arch. P.A. Right pulmonary artery. The pericardium (lightly shaded) extends as far as the bifurcation of the synangium.]

In all larval forms, in the Caudata, and in a few of the Ecaudata (_Xenopus_, for instance), the epidermis becomes modified in relation with the termination of sensory nerves, and gives rise to organs of the same nature as those of the lateral line of fishes. In addition to diffuse pigment (mostly in the epidermis), the skin contains granular pigment stored up in cells, the chromatophores, restricted to the cutis, which are highly mobile and send out branches which, by contraction and expansion, may rapidly alter the coloration, most batrachians being in this respect quite comparable to the famous chameleons. Besides white (guanine) cells, the pigment includes black, brown, yellow and red. The green and blue, so frequent in frogs and newts, are merely subjective colours, due to interference. On the mechanism of the change of colour, cf. W. Biedermann (35).

One of the interesting recent discoveries is that of the "hairy" frog (_Trichobatrachus_), in which the sides of the body and limbs are covered with long villosities, the function of which is still unknown (36).

The nuptial horny asperities with which the males of many batrachians are provided, for the purpose of clinging to the females, will be noticed below, under the heading _Pairing and Oviposition_.

_Dentition._--In the Microsauria and Branchiosauria among the Stegocephalia, as in the other orders, the hollow, conical or slightly curved teeth exhibit simple or only slightly folded walls. But in the Labyrinthodonta, grooves are more or less marked along the teeth and give rise to folds of the wall which, extending inwards and ramifying, produce the complicated structure, exhibited by transverse sections, whence these batrachians derive their name; a somewhat similar complexity of structure is known in some holoptychian (dendrodont) Crossopterygian fishes. In the remarkable salamander _Autodax_, the teeth in the jaws are compressed, sharp-edged, lancet shaped. The teeth are not implanted in sockets, but become ankylosed with the bones that bear them, and are replaced by others developed at their bases. Teeth are present in the jaws of all known Stegocephalia and Apoda and of nearly all Caudata, _Siren_ alone presenting plates of horn upon the gingival surfaces of the premaxillae and of the dentary elements of the mandible. But they are nearly always absent in the lower jaw of the Ecaudata (exceptions in _Hemiphractus, Amphignathodon, Amphodus, Ceratobatrachus_, the male of _Dimorphognathus_), many of which (toads, for instance) are entirely edentulous.

There is great variety in the distribution of the teeth on the palate. They may occur simultaneously on the vomers, the palatines, the pterygoids and the parasphenoid in some of the Stegocephalia (_Dawsonia, Seeleya, Acanthostoma_), on the vomers, palatines and parasphenoid in many salamandrids (_Plethodontinae_ and _Desmognathinae_), on the vomers, pterygoids and parasphenoid (some _Pelobates_), on the vomers and parasphenoid (_Triprion, Amphodus_), whilst in the majority or other batrachians they are confined to the vomers and palatines or to the vomers alone (37).

As regards the alimentary organs, it will suffice to state, in this very brief sketch, that all batrachians being carnivorous in their perfect condition, the intestine is never very long and its convolutions are few and simple. But the larvae of the Ecaudata are mainly herbivorous and the digestive tract is accordingly extremely elongate and coiled up like the spring of a watch. The gullet is short, except in the Apoda. The tongue is rudimentary in the perennibranchiatea Caudata, well developed, and often protrusile, in the _Salamandridae_ and most of the Ecaudata, totally absent in the Aglossa.

The organs of circulation cannot be dealt with here; the most important addition made to our knowledge in recent years being found in the contributions of F. Hochstetter (38) and of G.B. Howes (39), dealing with the azygous (posterior) cardinal veins in salamanders and some of the Ecaudata. The heart is situated quite forward, in the gular or pectoral region, even in those tailed batrachians which have a serpentiform body, whilst in the Apoda (fig. 13) it is moved back to a distance which is comparable to that it occupies in most of the snakes.

_The Respiratory Organs._--The larynx, which is rudimentary in most of the Caudata and in the Apoda, is highly developed in the Ecaudata, and becomes the instrument of the powerful voice with which many of the frogs and toads are provided. The lungs are long simple tubes in some of the perennibranchiate Caudata; they generally shorten or become cellular in the salamandrids, and attain their highest development in the Ecaudata, especially in such forms as the burrowing _Pelobates_. Although the lungs are present in such forms as preserve the gills throughout life, it is highly remarkable that quite a number of abranchiate salamanders, belonging mostly to the subfamilies _Desmognathinae_ and _Plethodontinae_, are devoid of lungs and breathe entirely by the skin and by the bucco-pharyngeal mucose membrane (20). Some of the _Salamandrinae_ show the intermediate conditions which have led to the suppression of the trachea and lungs. In the Apoda, as in many serpentiform reptiles, one of the lungs, either the right or the left, is much less developed than the other, often very short.

_Urino-genital Organs._--The genital glands, ovaries and testes, are attached to the dorsal wall of the body-cavity, in the immediate vicinity of the kidneys, with which the male glands are intimately connected. The oviducts are long, usually more or less convoluted tubes which open posteriorly into the cloaca, while their anterior aperture is situated far forward, sometimes close to the root of the lung; their walls secrete a gelatinous substance which invests the ova as they descend. In most male batrachians the testes are drained by transverse canals which open into a longitudinal duct, which also receives the canals of the kidneys, so that this common duct conveys both sperma and urine. In some of the discogloesid frogs, however, the seminal duct is quite independent of the kidney, which has its own canal, or true ureter. Many of the Ecaudata have remnants of oviducts, or Mullerian ducts, most developed in _Bufo_, which genus is also remarkable as possessing a problematic organ, Bidder's organ, situated between the testis and the adipose or fat-bodies that surmount it. This has been regarded by some anatomists as a rudimentary ovary. Female salamandrids are provided with a _receptaculum seminis_. Copulatory organs are absent, except in the Apoda, in which a portion of the cloaca can be everted and acts as a penis. The urinary bladder is always large.

The spermatozoa have received a great share of attention, on the part not only of anatomists and physiologists, but even of systematic workers (40). This is due to the great amount of difference in structure and size between these elements in the various genera, and also to the fact that otherwise closely allied species may differ very considerably in this respect. The failure to obtain hybrids between certain species of _Rana_ has been attributed principally to these differences. The spermatozoa of _Discoglossus_ are remarkable for their great size, measuring three millimetres in length.

_Pairing and Oviposition_--Batrachians may be divided into four categories under this head:--(1) no amplexation; (2) amplexation without internal fecundation; (3) amplexation with internal fecundation; (4) copulation proper. The first category embraces many aquatic newts, the second nearly all the Ecaudata, the third the rest of the Caudata, and the fourth the Apoda.

In the typical newts (_Molge_) of Europe, the males are adorned during the breeding season with bright colours and crests or other ornamental dermal appendages, and, resorting to the water, they engage in a lengthy courtship accompanied by lively evolutions around the females, near which they deposit their spermatozoa in bundles on a gelatinous mass, the spermatophore, probably secreted by the cloacal gland. This arrangement facilitates the internal fecundation of the female without copulation, the female absorbs the spermatozoa by squeezing them out of the spermatophore between the cloacal lips. Other newts, and many salamanders, whether terrestrial or aquatic, pair, the male embracing the female about the fore limbs or in the pelvic region, and the males of such forms are invariably devoid of ornamental secondary sexual characters; but in spite of this amplexation the same mode of fecundation by means of a spermatophore is resorted to, although it may happen that the contents of the spermatophore are absorbed direct from the cloaca of the male. The spermatozoa thus reach the eggs in the oviducts, where they may develop entirely, some of the salamanders being viviparous.

In all the tailless batrachians (with the exception of a single known viviparous toad), the male clings to the female round the breast, at the arm-pits, or round the waist, and awaits, often for hours or days, the deposition of the ova, which are immediately fecundated by several seminal emissions.

The fourth category is represented by the Apoda or Caecilians in which, as we have stated above, the male is provided with an intromittent organ. Some of these batrachians are viviparous.