CIRCULATION 97 



94. Respiration. — The clam, like other aquatic animals, 

 gains its oxygen from the water and gives off carbon 

 dioxide. A close inspection of the mantle shows the pres- 

 ence of blood vessels which are more numerous than in 

 the gills. For this reason, the mantle is regarded as the 

 main organ of respiration, although the gills also assist. 



95. Life History. — In clams the sexes are distinct. Kggs 

 formed in the ovaries of the female fuse with sperm cells 

 from the males taken in with the water through the 

 siphon. These sperm cells have reached 



the water through the upper siphon. 

 Thousands of embryos form in the body 

 of the female and develop into larvse 

 in the outer gills which, thus become 

 greatly distended. Later the larvse 

 pass into the water through the upper FlGUR o F 96 ^ MBRYO 



1 * That attaches itself 



The larvae of many fresh water clams to a fish. 



have hooks on their shells and by means 



of these they are able to cling to the gills or body of a 



fish, where they live as parasites for several weeks. They 



absorb food from their host and are carried from one place 



to another and are thus scattered. After a few weeks they 



leave the host and settle down to lead an independent 



life. 



96. Excretion. — The wastes of the body are absorbed 

 by the kidneys and passed out into the water through the 

 upper siphon. 



97. Circulation is well developed. 1 From the heart the 

 colorless blood is carried through arteries into smaller 

 tubes, and returns, through veins, back to the heart. 



1 The three chambered heart lies in the dorsal region, near the hinge, in a 

 little soft-walled chamber, the pericardium (pe"r-i-c&r'di-um : Greek peri, 



around; cardia, heart). 



