PHYLUM VIII VEETEBEATA 115 



In external appearance amphibians more nearly resemble reptiles than 

 fishes. The body is generally elongate, and terminates as a rule in a well- 

 developed tail, although some small forms (Anura) are ecaudate. Among 

 recent amphibians limbs are wanting only in the Coecilians (Gyninophiona). 

 Certain extinct Stegocephalians (Aistopoda) appear also to have been com- 

 pletely apodal ; but elsewhere two pairs of limbs are invariably present, the 

 anterior usually terminating in four digits and the posterior in five. 



Recent amphibians are naked with the exception only of the Coecilians, 

 which are covered with small scales arranged in transverse rings. The fossil 

 Stegocephalians usually have a scaly armature on the ventral surface, and 

 sometimes on the dorsal as well. 



The number of vertebrae in the spinal column is extremely variable 

 (10-150), depending upon the length of the body and especially the tail. 

 Cervical, dorsal, sacral, and caudal regions are distinguished. 



The most primitive form of vertebrae occurs in the Palaeozoic Stego- 

 cephalians, where the notochord is enclosed by thin cylinders of bony tissue, 

 or by separate pleurocentra and hypocentra, as in early Ganoids. Where the 

 column is more completely ossified, three types of vertebrae are exhibited. 

 The first, or amphicoelous, is biconcave ; the procoelous has the anterior vertebral 

 face concave and the posterior convex ; and in the opisthocoelous type the 

 anterior face is convex and the posterior concave. 



The cervical region comprises but a single vertebra, the atlas, although 

 this probably corresponds to both atlas and axis of higher classes. Its con- 

 cave anterior face receives the bony or cartilaginous occipital condyles, and 

 is often provided with a spatulate, forwardly directed basal process. Each of 

 the dorsal vertebrae supports a neural arch (neiirapophysis), which becomes 

 earlier and more completely ossified than the centrum, and may be either 

 suturally united or anchylosed with the latter. The two halves of the neural 

 arch unite above to form a more or less strongly developed spinous process 

 {spina dorsalis) ; and they bear anteriorly and posteriorly a pair of oblique 

 articular processes {processus obliqui, zygapophyses), the forward pair of each 

 vertebra overridden by the hinder pair of the next in frdht. The neural arch 

 also supports, as a rule, a pair of transverse processes {diapophyses) for the 

 attachment of ribs. When the latter are double-headed, as is often the case, 

 another and shorter lateral process {parapophysis) is developed by the body of 

 the vertebra. 



The sacral region is also formed by a single vertebra, which supports the 

 pelvis, the latter being attached either directly by means of exceptionally 

 stout transverse processes, or by sacral ribs, usually of peculiar form. Haemal 

 arches {haemapiophjses, " chevron bones ") are commonly borne by the caudal 

 series, the foremost of which sometimes have ribs attached to the transverse 

 processes of the neural arch. The entire series of caudals in the Anura is 

 fused into a single elongate piece called the coccyx. 



The primordial cranium remains partly cartilaginous throughout life, and 

 is partly replaced by bony pieces, which are either direct ossifications of the 

 cartilaginous capsule (exoccipitals, auditory capsules, quadrate, sphen-ethmoid), 

 or are investing bones (parietals, frontals, nasals, vomer, parasphenoid). The 

 basioccipital and supraoccipital usually remain small cartilaginous tracts ; but 

 save in certain Stegocephcdia and a few other forms, the exoccipitals are completely 

 ossified, and bear the articular condyles. The exoccipitals are of considerable 



