79 



CLASS IV. PISCES. 



Remarks. In this class the process answering 

 to respiration is performed by presenting the blood 

 to the air contained in water by means of the gills. 

 In effect the gills are as lungs exposed inside out. 

 Fish cannot live in water, from which air has been 

 expelled by boiling, until it has been exposed a 

 considerable time to the atmosphere, or been agi- 

 tated in it. They take water into the mouth and 

 compress it upon the gills by the gill-covers ;- 

 whereby they bring the air contained in it into 

 close contact with the gills. 



Most species of fish have an air-vessel which 

 they can compress and dilate at pleasure ; by 

 which operation they ascend and descend in water 

 without much exertion. They move in water by 

 their fins, which they use like oars, or wings. 

 Those fins which answer to fore feet are called 

 yectoral fins to hind feet, caudal fins those on 

 the back, dorsal fins those near the vent, anal 

 fins those of the abdomen, ventral fins. The ribs 

 in the fins and gills are called rays. These rays 

 are thorny, soft* articulated, or branched. 



Some fish are migratory like birds ; as the shad. 

 They move in vast numbers from the ocean into 

 fresh water rivers in the spring season, and depos- 

 ite their eggs. Some reside permanently in fresh 

 water, some in salt water, and others seem to live 

 in either indifferently. Those which pass into 

 fresh water rivers in the spring season, encrease 

 in number in proportion to the extent of agricul- 

 tural improvement in the countries through which 

 the rivers run. The shad of the Hudson have en - 



