EXTERNAL FEATURES AND RELATED STRUCTURES 39 



vertebrates. Head, body, and tail regions are recognizable, with 

 few exceptions. In all but the lowest subdivision of the Verte- 

 brata, the Cyclostomata, there are two pairs of appendages, 

 the fins of fishes and the limbs of amphibians, reptiles, birds, and 

 mammals (Fig. 1, p. 10). A neck, distinguishable externally as a 

 constriction between head and body, is found in certain reptiles and 

 in birds and mammals. Modifications occur (Figs. 1, 2, 3, p. 10), 

 but the same general organization persists 

 throughout. Compare the head, body, 

 and tail in a fish, salamander, hzard, 

 turtle, snake, ostrich, sparrow, seal, 

 elephant, horse, and man. In the snakes 

 and in a small subdivision of the amphi- 

 bians (Apoda, Fig. 6, p. 14), limbs are 

 absent in the adult, although present 

 as rudiments in the embryo. In a few ^_^ 



vertebrates the tail is rudimentary, as in ^^ | ] JjA 

 frogs and toads, the higher apes and l/irft(^<x^ 



man; or it may be greatly developed as '"^/^(f ^ ^^T^^ 

 . the kangaroo and alligator. yi^ jj [^ ^^ 



Modifications of the appendages are of P/^ ^ fl ^^ 



particular interest, since these organs are ^ o 



so closely related to the habits of the § ^ 



animal. Thus, the paired fins of fishes are ,„ ci w f + r,; 



,., 1 J. • -1 Fig. 18. — Skeleton of typi- 



usually paddle-hke organs but, m special ^^^ vertebrate appendage, 



cases they are modified in whole or in part ^ ^^^^^. ^ ^^^^^. ^,_ 



for other purposes, such as crawling on the metatarsals; p, phalanges; t, 

 , ,. / T-\t( r j;^™ " I'r. +Vir. oir- tibia; <a, tarsals. (Redrawn from 



bottom (sea-robm), ghdmg m the air ^.^^^^^^^-^^^ -comparative 



(flying fish), and as COpulatory organs Anatomy of vertebrates," copy- 



(sharks and skates). More famiUar exam- -J;,^; ll^^Ji;^^^^^^ 

 pies of modification in appendages appear 



in the limbs of terrestrial vertebrates. The five-toed plan of struc- 

 ture (Fig. 18), from which all the varied types seem to have 

 evolved bv the loss or specialization of parts, is modified to fit 

 particular" functions in appendages so diverse in their external 

 appearance as the fore limbs of man and the elephant, the flipper 

 of a seal, the wings of bats and birds, and the fore limbs of horses 

 and such familiar animals. Underlying an endless variety m 

 detail, there appears a unity in essential organization. Such 

 modifications of an animal's body are of interest as examples of 



