390 Relations between Species 



defenses of antibodies and phagocytes. The parasite is nsually pro- 

 vided with a protective covering and with special ways for gaining 

 entrance, maintaining position, and avoiding digestion by the host. 

 On the other hand, certain of the parasite's unused organs have de- 

 generated. The efficient parasite gains all the advantage possible 

 without seriously curtailing the life of the host. If the host is killed, 

 or when it eventually dies, the parasite must have the means of reach- 

 ing a new host. Both host and parasite may become so profoundly 

 modified as to be almost unrecognizable. In its young free-swimming 

 stage the barnacle Sacculina has the appearance of a typical crustacean 

 larva, but, as an adult parasitizing the shore crab, it becomes little 

 more than a sac suitable for the absorption of food and for reproduc- 

 tion. The crab host is in turn reciprocally affected by the presence of 

 the parasite for its reproductive glands are caused to atrophy and cer- 

 tain metabolic changes are brought about. 



In many instances the transmission of the parasite to new hosts is 

 left to chance aided by a very high natality; in others the parasite 

 is specifically oriented to the host (Thorpe and Jones, 1937) or the 

 host is attracted to the parasite. A remarkable instance of the latter 

 procedure is displayed by the fresh-water mussel Lampsilis ventricosa 

 in which the mantle edge is modified to appear like a small fish. 

 When a real fish, attracted by this mimic, swims over the mussel, cast- 

 ing a shadow, the mussel discharges its glochidial larvae. Some of 

 these larvae reach the gills or fins of the fish to which they attach and 

 live as parasites until they are ready to metamorphose into adults. 

 Certain fishes thus parasitized wander upstream where the young 

 mussels drop off and begin a new life as independent bottom ani- 

 mals. In this way these sessile forms are distributed against the 

 current to the upper reaches of the stream. 



Many parasites require more than one host for the completion of 

 their life cycles; a large number of instances could be cited for both 

 plants and animals. One familiar example is the white pine rust that 

 is dependent upon the barberry (Ribes) as a secondary host. The 

 bass tapeworm that causes stunting and sterility in the small-mouthed 

 bass also illustrates the complexities of multiple parasitism. In the 

 spring when the bass swims into the shoal water of a lake for spawn- 

 ing, segments of the tapeworm (1 in Fig. 10.12) living in the fish's 

 gut are discharged into the water where they produce eggs ( 2 ) . The 

 eggs are eaten by copepods (3), a secondary host, and hatch out 

 within the alimentary tract of these primitive crustaceans. The re- 

 sulting larvae pass through the wall of the intestines into the body 

 cavity of this host. Meanwhile young bass and other fish (4) 



