260 



ANALYSIS OF THE ENVIRONMENT 



mals meet and fuse in permanent copula- 

 tion. The male of another trematode, the 

 blood fluke, Schistosoma, carries the female 

 in a fold in his ventral body wall. 



Several diverse types of free-living ani- 

 mals that live in sparse populations or have 

 poor powers of sexual "search" have devel- 

 oped sexual parasitism. Perhaps the devel- 

 opment of this habit has enabled these 

 forms to persist in sparse populations. In 

 the typical development of this relationship, 

 a minute male becomes parasitic on or in 

 the body of the large female. This situation 

 exists in the echiuroid worm Bonellia (an 



passive. A common method of transfer is 

 the eating of food contaminated by fecal 

 matter and encysted stages of parasites. De- 

 finitive hosts often prey upon intermediate 

 hosts and so receive parasites they are un- 

 able to digest. There is normally a close cor- 

 relation between the successive stages in 

 the life history of a parasite and the food- 

 chain relationship of the successive hosts. 

 Air and water also are well-known avenues 

 of infection. Other parasites gain transfer by 

 close association, as between dog and man. 

 Insect and other vectors carry many para- 

 sites from one host to another, as anophe- 



Fig. 71. Sexual parasitism in the deep-sea angler fish, Photocorynus spiniceps, in which 

 the diflBculty of one sex finding the other is met by permanent attachment of the much smaller 

 male to the female. The union is so complete that the male has no independent existence 

 at all, being nourished by the blood of the female to which he is attached. (After Norman.) 



aberrant annelid), in several copepods, and 

 in the deep sea species of the angler fishes 

 (Photocorynus spiniceps and others) in 

 which the normal-sized female carries the 

 reduced male attached to her head or some 

 other part of her body. The tiny male fish 

 estabhshes organic connection with the 

 blood vessels of the female. It seems evi- 

 dent that the diflBculties of finding the op- 

 posite sex in the adverse, hghtless deep sea 

 are correlated with this extreme modifi- 

 cation of the sex environment of the 

 individual. 



Dispersal of parasite from host to host 

 presents increasing diflBculties as the para- 

 site becomes less and less capable of active 

 locomotion. In the parasitoid insects, the 

 free-living adult may be mainly a means of 

 dispersal of the parasitic larvae. In many, 

 perhaps most, parasites, even transfer is 



Une mosquitoes carry Plasmodium, the 

 parasite that produces human malaria. 

 Blood parasites are especially hkely to be 

 distributed by blood-sucking insects. Some, 

 like Plasmodium, may be as definitely para- 

 sitic in the insect host as in their sexual 

 stage in the blood of a vertebrate. Para- 

 sites may invade developing ova and so 

 literally grow up with the oncoming gen- 

 eration. Pasteur demonstrated this means of 

 infection in pebrine, a sporozoan disease of 

 silkworms. Some parasites can also pass to 

 the mammalian embryo through the pla- 

 centa. 



Parasites and hosts that have long hved 

 together are often said to develop a mutual 

 toleration such that the two populations are 

 nearly or quite in equilibrium (Chandler, 

 1944; see also p. 707). Ball (1943) ques- 

 tions this interpretation and attributes 



