452 COLLEGE ZOOLOGY 



branes) prevent water from entering through the opercular 

 aperture. Expiration (B) results from the contraction of the 

 opercular apparatus; the branchiostegal membrane is opened 

 and water passes out through the gill-slits. The exit of water 

 by way of the mouth is prevented by valves of mucous mem- 

 brane (maxillary and mandibular valves). 



4. General Account of Some of the Principal Group? 



of Fishes 



Subclass I. Teleostomi. — To the Teleostomi belong the 

 majority of fishes. The four orders of living forms are unequa' 

 in number of species, most of which belong to the Teleostei. 



Order 1. Crossopterygii. — Most of the Crossopterygii 

 are extinct, and the order contains only one family and two 



Fig. 380. — Polypterus senegalus. (From the Cambridge Natural History.) 



genera of living forms. One species, Polypterus senegalus (Fig. 

 380), lives in the Nile. It is of special interest to morphologists 

 because it presents many structural features characteristic of 

 ancient crossopterygians. 



Order 2. Chondrostei. — This order contains the sturgeons 

 and paddle-fishes. These have a skeleton largely of cartilage, 

 a heterocercal tail, ganoid scales (Fig. 371, B), and abdominal 

 pelvic fins. 



The family Polyodontid^e contains two species of paddle- 

 fishes, Polyodon spathula (Fig. 381) of the Mississippi Valley, 

 and Psephurus gladius of the Yang-tse-Kiang in China. Poly- 

 odon reaches a length of six feet and a weight of one hundred 

 and sixty pounds, but the specimens usually taken weigh no 

 more than fifty pounds. Its large, paddle-shaped snout is re- 

 garded as a sense-organ, and its use is still unknown. The 



