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 vertebra, 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. These two portions of the shell form a box-like 
armor, through whose openings the head, tail, and limbs 
may be extended. The latter organs are superficially unlike 
those of any other order of animals. The head is generally 
