164 ANIMAL BIOLOGY 



of the body, advantage is taken of this fact in the arti- 

 ficial propagation of many of the more valuable species. 

 The United States government and various states support 

 fish hatcheries where young fishes are reared and then let 

 loose, to replenish, so far as possible, the numbers taken 

 by fishermen. To obtain the eggs a ripe female is taken 

 in the hands and "stripped" by slowly compressing the fish 

 from before backward, thus forcing the eggs out of the 

 body. The sperm, or milt, of the male, which is obtained 

 in a similar manner, is mixed with the water containing the 

 eggs and causes them to be fertilized. The eggs undergo 



FIG. 132. A shark, Squalus acanthias. (After Dean.) 



their development in specially prepared receptacles and 

 the young are set adrift. Through the fish commis- 

 sions, various lakes and streams are stocked with desired 

 species, sometimes by introducing the eggs, in other cases 

 by transferring the mature fish. 



Fishes may be divided into three sub-classes: the 

 Elasmobranchs, or cartilaginous fishes; the Teleostomi, 

 or bony fishes; and the Dipnoi, or lung fishes. In the 

 elasmobranchs which are represented by sharks, skates, 

 rays, etc., the skeleton is composed of cartilage, the gill- 

 slits open directly to the outside instead of being covered 

 by an operculum, and the tail fin is typically asymmetrical, 

 or heterocercal. The mouth and usually the nostrils are 

 situated on the ventral surface of the head, and the body 



