THE ENDOSKELETON 



183 



tation of the free limb to locomotion in the two difficult ele- 

 ments of water and air (Fig. 50). The chiridial appendage 

 (cheiropterygium) is primarily a walking leg (Fig. 50, a), de- 

 signed to aid in pushing the body along the ground or, when 

 extremely developed, to support the body above the surface 

 and assume the entire function of locomotion. In becoming 



FIG. 50. Modifications of the fore limb. 



(d) Necturus, a primitive salamander. (b) Ichthyosaurus, an extinct marine 

 lizard, (c) Globicephalus, a cetacean (dolphin), (d) Pterodactyl, an extinct flying 

 reptile, (e) Bird, (f) Bat. 



Figure (a) represents an unmodified limb skeleton; (b) and (c), limbs modified 

 for swimming; (d), (e), and (i), those modified for flight. Designations as in the 

 previous figure. 



modified to subserve the needs of an aquatic life it needs to 

 change into a paddle, which it does by elongating the digits, 

 pressing them close together, and surrounding them by a web 

 of integument. The digits also often become hyperphalangeal, 

 that is, they develop an unusual number of phalanges, as is 

 seen both in the marine lizard Ichthyosaurus and in the en- 

 tirely unrelated branch represented by the Cetacea. [Fig. 

 50; compare (b) and (c).] 



In Ichthyosaurus the five regular digits are retained, and 

 there is also a row of small bones along the outer edge, con- 



