70 
FISHES 
those in the hip are termed the pelvic (pel'vik) girdle. In 
the snakes, only traces of legs are found (Figures 83, 88, 
and 89). 
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 de¬ 
veloped 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 
Figure 61. — Brook Trout. 
This is the most highly prized fish among fishermen. It is rapidly becom¬ 
ing extinct except where protected by law. 
many vertebrates the skin is an active agent in the inter¬ 
change of oxygen and carbon dioxide and particularly in 
those animals which have a thin, moist skin like frogs. 
55. Fishes. — The fishes are vertebrates, that is, they 
have a notochord which, as they develop, gives place to a 
vertebral column. There are four large divisions of fishes: 
(1) the lampreys (lam'priz) and relatives, (2) the sharks 
and relatives, (3) the bony fishes, and (4) the small group 
of fishes with lungs. The most important group in numbers 
and economic importance is the bony fishes. This group 
