188 ANIMAL FORMS 



nally of any trace of limbs. However, in some of the 

 pythons and boas hind limbs are present in the form of 

 small groups of bones embedded beneath the skin and ter- 

 minating in a claw. There thus appears to be no doubt 

 that the ancestors of the modern snakes were four-footed, 

 lizard-like creatures, which have assumed the present form 

 in response to the necessity of adaptation to new conditions. 



More than any other order of vertebrates do the snakes 

 deserve the name of creeping things, and yet their method 

 of locomotion enables them to crawl and swim with a ra- 

 pidity equal to that of many of the more highly developed 

 animals. This depends chiefly upon certain peculiarities 

 of the skeleton, which consists merely of a skull, vertebral 

 column, and ribs. The vertebrae, usually two or three hun- 

 dred in number, are united together by ball-and-socket 

 joints, and each attaches by similar joints a pair of slender 

 ribs. These in turn are attached to the broad outer plates 

 upon which the body rests, and the whole system is operated 

 by a powerful set of muscles. Upon the contraction of the 

 muscles the ventral plates are made to strike backward 

 upon the ground or other rough surface, which drives the 

 body forward. Also, the ribs may be made to move back- 

 ward and forward, and the snake thus progresses like a 

 centiped or " thousand-legs." 



178. The turtles (Chelonia). In many respects the tur- 

 tles are the most highly modified of all the reptiles. The 

 body (Fig. 113) is short and wide and enclosed in a shell or 

 heavy armor, consisting of an upper portion, the carapace, 

 and a flat ventral plate, the plastron. The shape of the 

 carapace varies greatly from a low, flat shield to a highly 

 vaulted dome, remaining cartilaginous throughout life, as 

 in the soft-shelled turtles, or becoming bony and of great 

 strength. The two portions of the shell form a box-like 

 armor through whose openings may be extended the head, 

 tail, and limbs. As a means of protection the turtle may 

 retract these organs within the shell. The head is generally 



