Sub-Class I. Archaeobatrachi s. Stegocephali.
Sub-ClassII. Neobatrachi.
Order 1. URODELA.
Order 1. a. Salamandroidea. [The Urodela.]
Order 1. b. Coeciloidea = Amphiumidae + Coeciliidae.
Order 2. ANURA.
The classification adopted in this volume is as follows:–
Class Amphibia.
Sub-Class I. Phractamphibia.
Order III. Stegocephali Lepospondyli.
Sub-order 1. Branchiosauri.
Sub-order 2. Aistopodes.
Order III. Stegocephali Temnospondyli.
Order III. Stegocephali Stereospondyli.
Sub-Class II. Lissamphibia.
Order III. Apoda.
Order III. Urodela.
Order III. Anura.
Sub-order 1. Aglossa.
Sub-order 2. Phaneroglossa.
CHAPTER II
SKELETON OF URODELA AND ANURA–SKIN–COLOUR-CHANGING MECHANISM–POISON-GLANDS–SPINAL NERVES–RESPIRATORY ORGANS–SUPPRESSION OF LUNGS–URINO-GENITAL ORGANS–FECUNDATION–NURSING HABITS–DEVELOPMENT AND METAMORPHOSIS
Skeleton of the Urodela
The vertebral column.–The number of vertebrae is smallest in the terrestrial, greatest in the entirely aquatic forms, and is exceptionally large in the eel-shaped Amphiuma. In the following table the sacral vertebra is included in those of the trunk.
| Trunk. | Tail. | |
| Siren lacertina | 22 | 35 + |
| Necturus maculatus | 19 | 29 |
| Proteus anguinus | 30 | 28 + |
| Cryptobranchus alleghaniensis | 20 or 21 | 24 + |
| C. scheuchzeri | 21 | |
| C. japonicus | 22 | 22 to 26 |
| Amphiuma means | 63 | 35 + |
| Amblystoma tigrinum | 17 or 16 | 32 + |
| Salamandra maculosa | 17 | 27 |
| Triton cristatus | 17 | 36 |
| Triton taeniatus | 14 or 15 | 36 + |
| Triton palmatus | 14 | 23 to 25 |
| Salamandrina perspicillata | 15 | 32 to 42 |
| Spelerpes fuscus | 16 | 23 |
The vertebrae of the Urodela and those of the Apoda differ from those of all the other Tetrapoda[[11]] by possessing no special centra or bodies. That part which should correspond with the centrum is formed either by the meeting and subsequent complete co-ossification of the two chief dorsal and ventral pairs of arcualia (tail-vertebrae), or entirely by the pair of chief dorsal arcualia. There is consequently no neuro-central suture. Moreover, the central region of each vertebra is strongly pinched in laterally, widening towards the ends. Another feature of the vertebral column of the Urodela is the possession of a considerable amount of intervertebral cartilage, by which the successive vertebrae are held together. This cartilage does not ossify, and it either remains continuous, serving in its entirety and owing to its flexibility as a joint, or it becomes more or less imperfectly separated into a cup and ball portion, the cup belonging to the posterior end of the vertebra. Such joints are called opisthocoelous, and occur in the Desmognathinae and Salamandrinae. In the adult the cup and ball frequently calcify, and the chorda dorsalis or notochord is completely destroyed. Those vertebrae between which the intervertebral cartilage remains unbroken, are called amphicoelous, since in them, most obviously in macerated or dried skeletons, the vertebrae appear hollowed out at either end. In such amphicoelous vertebrae a considerable amount of the chorda always remains, running in an unbroken string through the whole length of the vertebral column. Towards adult life the chorda becomes constricted, and is ultimately squeezed out or destroyed, in the middle of the vertebra, by the invasion of cartilage from the chief arcualia. This intravertebrally situated cartilage has been described erroneously as chordal cartilage.
The development of the vertebrae proceeds as follows. First appear a pair of basidorsalia and a pair of basiventralia (Fig. 1, 1, B.D, B.V), blocks of cartilage, imbedded in and resting upon the thin sheath of the chorda dorsalis. Next appears a pair of interdorsal blocks, immediately behind the basidorsals; and somewhat later appears a pair of interventral blocks. These four pairs of cartilages or "arcualia" each meet, above or below the chorda, and form semi-rings, which again by extending upwards or downwards fuse into complete rings, in such a way that the interdorsal and interventral elements form the intervertebral mass spoken of above. The basidorsals fuse with the basiventrals, and form the body of the vertebra, the fusion being effected chiefly by the calcification and ossification of the lateral connecting portion of the skeletogenous layer. The basidorsalia form the neural arches with their unpaired short spinous or neural, and the paired anterior and posterior zygapophysial processes. Concerning the basiventralia we have to distinguish between the trunk and the tail. In the latter they produce a pair of ventral outgrowths or haemapophyses, which ultimately enclose the caudal blood-vessels. In the trunk the basiventral blocks of cartilage are suppressed; they appear in the early larvae, but disappear during or even before metamorphosis.