|()}{ Comparative Morphology of Chordates 



lacking. Among fishes are eels without paired fins, among amphibians 

 are legless caecilians, and among reptiles are legless lizards and snakes. 

 Jn modern reptiles the connection of the skeleton of the pelvic legs 

 with the vertebral column involves two sacral vertebrae, in contrast 

 to the one of amphibians. This strengthening of the sacral connection 

 is consistent with the fact that greater strain is incurred by skeletal 

 parts in land animals than in aquatic animals, because in the latter 

 the body is buoyed up by the water. 



In addition to these terrestrial adaptations of the skin, respiratory 

 organs, and locomotor appendages, there are many internal differences 

 between aquatic anamniotes and reptiles. Most of these differences 

 more or less directly reflect the change of environment. 



The reptilian skeleton is, in general, more strongly built than 

 that of fishes and amphibians. In many fishes the bones are so deficient 

 in calcium that they are soft and flexible. Reptilian bones are heavily 

 calcified. 



The reptilian skull is, with some exceptions (the tuatara and some 

 lizards), strongly ossified and contains a much greater number of bones 

 than the skull of any modern amphibian, resembling rather the skulls 

 of early labyrinthodont amphibians (Stegocephalia) and crossop- 

 terygian fishes (Fig. 361). In the absence of gills, it might be expected 

 that such skeletal arches as support them in anamniotes would not 

 occur in reptiles. The fact is, however, that in the ventral region of the 

 mouth and pharynx are found certain skeletal parts, the hyoid appa- 

 ratus, which, in position and relation to other structures, clearly corre- 

 spond to portions of several of the more anterior visceral arches of 

 lishes (Fig. 362). This hyoid apparatus supports the base of the tongue 

 and the larynx, which corresponds to the "voice-box" (larynx) of 

 mammals. It serves also for attachment of small muscles related to 

 neighboring structures. 



The embryonic notochord, except in some lizards (geckos) and in 

 the New Zealand "tuatara" (Sphenodon: Fig. 374), is completely 

 obliterated by development of the thoroughly ossified vertebral column 

 (Fig. 363). The vertebral centra are of various forms, but only excep- 

 tionally of the amphicoelous type characteristic offish vertebrae. With 

 the differentiation of a neck, the vertebral column becomes defi- 

 nitely marked out into four regions: (1) cervical, the first cervical 

 vertebra (atlas) being a bony ring articulating with the skull and 

 separated from its own centrum, which has become secondarily joined 

 to the anterior face of the second vertebra (axis), forming the conical 

 odontoid process on which the atlas is pivoted (Fig. 364) ; (2) trunk, 

 occupying the trunk region of the body and terminated by (3) the 



