VERTEBRATES 105 



each side of the neck that in the fishes allow the water to 

 pass over the gills. Such structures are of use only to 

 aquatic animals, and yet all vertebrates have them at some 

 time in their development. 



In most vertebrates the skeleton is composed of bone. 

 There are usually two pairs of appendages (legs, wings, 

 or fins) attached to the body at the shoulder and hip. 

 Here special bones join the limb to the body. The bones 

 in the shoulder are known as the pectoral (pek't6-ral) 



Figure 107. — Catfish, Bullhead, or Horned Pout. 



girdle ; while those in the hip are termed the pelvic 

 (pel'vik) girdle. In the snakes, only traces of legs are 

 found (Figures 104, 139, and 158). 



A further distinguishing feature of all vertebrates is 

 the well-developed nervous system, with its large brain. 

 The sense organs, eyes, ears, and the like, are also better 

 developed than in any of the invertebrates. 



Oxygen is obtained by external or internal gills in most 

 aquatic animals and by lungs in all other vertebrates. In 

 many vertebrates the skin is an active agent in the inter- 



