104 LIFE IN THE SEA [CH. 



the larval host is eaten by another animal which acts 

 as the adult host. When this occurs the parasite under- 

 goes full development and attains sexual maturity, 

 and its eggs pass out of the body of the host and 

 infect further larval hosts. In other cases still the 

 parasite never lives in the open except as an egg 

 which is eaten by the larval host, and this in turn is 

 eaten by the adult host. 



The parasitic habit is advantageous to the parasite 

 but is detrimental to the host, since substances may 

 be excreted by the parasite which may be to some 

 extent poisonous. The association is also harmful to 

 the host since the parasite uses up the food-stuffs of 

 the latter. A typical parasite lives in the alimentary 

 canal of another animal and it is bathed in the food 

 matter which is undergoing the process of digestion. 

 It may or may not possess an alimentary canal and 

 mouth: no tapeworm does but most trematodes do. 

 If an alimentary canal is absent the parasite simply 

 absorbs the soluble food matter of the host through 

 its skin; if it is present the food matter may be 

 absorbed through the skin or taken into the mouth. 

 In whatever way the food-stuffs are absorbed they 

 always consist of soluble peptones or amino-acids, of 

 soluble fatty acids and glycerine, and of soluble 

 sugars ; and all these substances have been prepared 

 by the digestive enzymes of the host. This is the 

 complete elimination of the process of digestion and 



