284 



THE VERTEBRATE ANIMALS 



Sand shark, an elasmobranch. Note the slits leading from the gills. From photo- 

 graph loaned by the American Museum of Natural History. 



V 



Sturgeon (Acipenser sturio), a ganoid fish. 



In some members of this group the young are born alive. Sharks, rays, 

 and skates are elasmobranchs. 



2. Ganoids. The bodies of these are ganoids protected by a series of 

 platelike scales of considerable strength. These fishes are the only rem- 

 nant of what once was the most powerful group of animals on the earth, the 



great armored fishes of the Devonian 

 age. The gar pike is an example. 



3. The Teleosts, or Bony Fishes. 

 They compose 95 per cent of all living 

 fishes. In this group the skeleton is 

 bony, the gills are protected by an 

 operculum, and the eggs are numerous. 

 A bony fish. Most of our common food fishes belong 



to this class. 



4. The Dipnoi, or Lung Fishes. This is a very small group, in many 

 respects more like amphibians than fishes, the swim bladder being used 

 as a lung. They live in tropical Africa, South America, and Australia, 

 inhabiting the rivers and lakes there. They withstand drying up in the 

 mud during the dry season, lying dormant for long periods of time in a 

 ball of mud and waking to active life again when the mud coat is removed 

 by immersion in water. 



