392 Relations between Species 



spawned in the same shore area have grown to the feeding stage and 

 eat the copepods harboring the larvae of the tapeworm. A close cor- 

 relation is thus requiied between the time for the copepod popula- 

 tion to develop, for the larvae of the parasite to hatch, and for the 

 young fish to appear. The larvae taken in with the copepods can 

 resist being digested, as can the other stages of the parasite, and make 

 their way into the body cavity, liver, or other organs of the small 

 fish which is a tertiary host. When the infected young fishes are 

 eaten by the voracious larger bass (5), the tapeworm larvae are 

 passed on, and this transfer from smaller fish to larger fish may take 

 place several times. The larvae do not develop into adults while in 

 the body cavity or internal glands of the small fish. Only if the in- 

 fected fish is eaten by a large bass at the moment when the larvae 

 are ready to metamorphose, do they develop into adult tapeworms. 

 This final stage establishes itself in the intestine of the primary host 

 and thus completes the life cycle. In another species of fish tape- 

 worm the adult stage is found in man and in other fish-eating mam- 

 mals. The life cycles of flukes that parasitize birds, snails, and fish 

 successively are described by Hunter ( 1942 ) . These multiple rela- 

 tions involve many special adaptations of the parasites and of the 

 species in the environment on which they depend. 



Predation. The typical predator is free living and catches, kills, 

 and devours individuals of another species for food, in contrast to the 

 typical parasite that lives on or in its host and derives nourishment 

 from it without killing it. Thus the predator is said to live on "capi- 

 tal" whereas the parasite lives on "interest." Although this concept 

 is generally true as far as the individual aggressors and victims are 

 concerned, it does not necessarily apply to the populations as a whole. 

 Parasites may kill a sufficient number of their hosts so that their "capi- 

 tal stock" is very greatly reduced. For example, in the grouse dis- 

 ease, which strikes this game bird periodically, the host population is 

 so depleted by the ravages of the infectious parasite that the parasite 

 population in turn sinks to a low ebb. Similarly, in an area in which 

 the predator and prey populations have struck more or less of a bal- 

 ance we may find that the predators are limiting themselves to "in- 

 terest" in the sense that they are devouring only the increment to the 

 prey population each year. In such a situation the predator popula- 

 tion may continue indefinitely to take a limited number of the prey 

 without endangering the breeding stock of the species on which it 

 depends. 



Predatory organisms are almost entirely animals but a few kinds 



