VERTEBRATA 



157 



2. Chelonia, or tortoises and turtles, are of anomalous 

 structure. The skeleton is external, so as to include not 

 only all the viscera, but also the whole muscular system, 

 which is attached internally ; and even the limbs are 



FIG. 136. Hawkbill turtle (Chelone imbricata}. Tropical Atlantic. 



inside, instead of outside, the thorax. The exoskeleton 

 unites with the endoskeleton, forming the carapace, or 

 case, in which the body is inclosed. The exoskeleton 

 consists of horny plates, known as " tortoise shell " (in 



the soft tortoises, 

 Aspidonectes, this 

 is wanting), and 

 of dermal bones, 

 united to the ex- 

 panded spines of 

 the vertebrae and 

 to the ribs, making 

 the walls of the 

 carapace(Fig.3i2). 

 The ventral pieces form the plastron. 2 All are tooth- 

 less. There are always four stout legs ; and the order 

 furnishes the only examples of vertebrates lower than 

 birds that really walk, for lizards and crocodiles wriggle, 



FIG. 137. Box tortoise (Terrapene Carolina}. 

 United States. 



