ASSOCIATIONS 609 



The significance of the terms regression and degeneration is obviously 

 one of definition. The atrophy of sensory, nervous, muscular and locomo- 

 tory systems mentioned above is accompanied by equally great hyper- 

 trophy and specialization of vegetative systems — for example, in the 

 appearance of piercing and sucking mouth parts, haemophilic glands and 

 enlargement of the gut, of ovaries, uteri and brood chambers. Nevertheless, 

 evolutionary advance has come to have certain definite connotations. It 

 implies an increase in the animal's capacity to cope with its environment 

 through an enlargement of its sensory field, an actual increase in the 

 number of sensory modalities perceived and an augmentation of effector 

 mechanisms. Pari passu there is an enlargement of central nervous organ- 

 ization permitting choice, diversity of control and utilization of past 

 experience. It is evident that this whole field of evolutionary progression 

 has been forsaken in the adoption of a parasitic life, and a narrow, fixed 

 and inflexible life-history has resulted. 



Different parasites display much variation in host specificity. In its 

 extreme expression, as seen among gregarines and certain parasitic cope- 

 pods, for example, a given species of parasite is confined to one particular 

 species of host. Other parasites are slightly more adaptable in that they 

 can live on related host species. On the other hand, there are certain ex- 

 ternal parasites such as gnathiids, which attack all kinds of fish indis- 

 criminately and, experimentally, have even been reared on Amphibia. 

 The behaviour of heteroecious parasites is somewhat peculiar in this 

 regard, for strict host specificity may be characteristic of the stages infect- 

 ing either the provisional or definitive host, and not necessarily of both. 

 Thus in trematodes, the gastropod host of the sporocyst may be fixed 

 whereas the sexual stage is found in many different host species (4). 



The influence of the parasite on the host has been brought out in some 

 of the life-histories summarized in previous pages. The parasite may affect 

 the size, form and physiology of the host. In a study of the snail Hydrobia 

 ulvae it has been found that individuals infected with trematode parasites 

 are consistently larger than uninfected specimens, i.e. the parasites in some 

 way favour an increase in size of the host (88). 



The well-known condition of parasitic castration and the tendency 

 towards intersexuality in parasitized crustaceans have received a number 

 of explanations (11). Smith suggested that rhizocephalan parasites impose 

 the same metabolic demands on the male crab as the ovary does in the 

 female. Foodstuff's, especially lipoids, which normally nourish the gonads, 

 were considered in some way to influence gonadal activity and, at the 

 same time, the secondary sexual characteristics. Ovary and parasite each 

 make similar and heavy inroads on trophic balances of the crab and produce 

 similar effects in female and male animals. Other explanations involve 

 hormone production by the parasite and alterations in the amount of 

 hormone secreted in the host. Recent experiments suggest that the para- 

 sites exert their influence on secondary sexual characters through the eye- 

 stalks of the host. The latter structures, it will be recalled, are the site of 



M.A. — 20 



