RELATIONS BETWEEN ANIMAL ORGANISMS 



497 



of the other, called the host (Chap. XXXII). Parasitism might logically 

 be made also to include the relation of two individuals of the same species 

 when one lives at the expense of the other. Among worms are examples 

 of one sex being carried about and nourished by the other, usually the 

 male by the female. A similar phenomenon is presented in the case of 

 certain fish. If, however, this association of the sexes is considered 

 division of labor, then this is not true parasitism but the relationship of 

 mates. It has been suggested that a young animal living within the body 



jacculina 



B 



Fig. 308. — An extreme case of parasitism. A, semidiagrammatic representation of an 

 individual of Sacculina sp. in the body of a crab and projecting from its ventral surface. 

 B, nauplius larva of the parasite (compare with Fig. 164 A). The nauplius lives free in 

 the water and changes to a form known as a cypris; this attaches itself to a seta on the body 

 of a crab by its antennules and loses its thorax and abdomen with their appendages. The 

 rest of its body undergoes degeneration and becomes a mass of cells. From the antennules 

 rootlike filaments penetrate the body of the host and this mass of cells enters the body 

 ca\aty of the crab and becomes attached to the ventral side of its intestine. The fila- 

 ments of the parasite permeate the tissues of the host, and these tissues are in consequence 

 partly absorbed. Ultimately the parasite develops a sac-like body containing reproductive 

 organs and a ganglion, and this, pressing upon the skin of the ventral surface of the crab's 

 abdomen, finally passes through the skin and shows itself as a tumor-like growth, shown in 

 A. Sacculina belongs to the Cirripedia, or barnacles, and is therefore a distant relative of 

 the crab which it parasitizes. 



of the parent, especially if it receives nourishment directly from the 

 parent, as in the case of mammals, should be considered a parasite. The 

 nourishment of the young, however, seems to be one of the natural func- 

 tions of the parent, and the relationship for that reason ought not to be 

 considered one of parasitism. To extend the term parasitism to all these 

 cases is to limit its significance and impair its usefulness and it seems best 

 to limit its application to two animals of different species. 



Internal parasites, such as intestinal worms and certain protozoans, 

 are called endoparasites, while those which live on the surface of the 

 host, such as fleas, mites, and lice, are called ectoparasites. Parasitism 

 also may be either temporary or permanent. Instances of temporary 



