366 ANIMAL BIOLOGY 



sitic habits may lead, as well as the importance of 

 a knowledge of development in order to determine an 

 animal's true affinities. 



In the tape-worms degeneration has not proceeded so 

 far, but higher sense organs are lacking and there is no 

 trace of a digestive system. Digestive organs are quite 

 unnecessary for the tape-worms as these animals absorb 

 the digested intestinal contents of their hosts. 



Frequently parasitic animals are compelled to live in 

 the bodies of two kinds of host before completing their 

 life history. The common liver fluke of the sheep, 

 Fasciola hepatica, passes a part of its life history in the 

 body of a snail before it is taken into the alimentary canal 

 of a sheep. Most tape-worms, as we have seen, live in two 

 different animals, usually an herbivore and a carnivore. 

 The same is true of the trichina; and the life history of the 

 malarial parasite is spent partly in the mosquito and partly 

 in man. This change of host makes the perpetuation of 

 the life of a parasite more than usually precarious. A 

 failure to meet with either of the hosts would naturally 

 be fatal to the parasite's career, but the increased dangers 

 of such a method of propagation are offset by an extra- 

 ordinary degree of fecundity. 



Some organisms are found more or less constantly asso- 

 ciated, although neither subsists in any way upon the 

 other. Such forms are called commensals, or messmates. 

 An example of the commensal relation is afforded by 

 the small oyster crab, Pinnotheres, which lives between 

 the valves of the shells of oysters and other bivalves. 

 A third kind of association is called symbiosis; in this case 

 each organism confers some benefit upon the other, so that 

 the partnership is mutually advantageous. Many primi- 

 tive organisms such as the green Hydra, the flat worm 

 Convoluta, and many species of anemones and corals 



