PARASITES AND PARASITISMUS. 203 



mals. At one stage they inhabit the tissues of mollus- 

 cous animals. Their ova are numerous^ the developmental 

 forms multiply by budding, and numbers simultaneously 

 gain entry to the system. The contingencies of the lives 

 of these parasites are less numerous than those of Taeniae. 

 Flukes, therefore, are often most prejudicial. The best 

 means to counteract them are to endeavour to remove ani- 

 mals fitted to act as bearers from the grounds in which 

 fluke larvae abound, and to ensure a pure water supply. 

 Another means is to check the passage of mollusca across 

 pastures, either by a well-limed border against every por- 

 tion of marshy land and every stagnant ditch, or by salt- 

 ing the pastures. Our evidences as to the action of anti- 

 parasitics is not very good. We know of some Taeniafuges 

 and some Tseniacides, but flukes and certain round worms 

 are very inaccessible, and we can only back up the system 

 against their attacks by tonics. Parasitism degrades ; 

 animals not entirely dependent on their own resources 

 lose many organs essential to independent life, therefore 

 much difficulty has been found in placing parasites in their 

 proper position in the animal kingdom. The Nematodes 

 and Acanthocephala puzzle even Huxley. Thus, some of 

 these comparatively high organisms respire simply by 

 means of the general surface of the body, like the Amoeba 

 and other Protozoa. In some also, the general surface takes 

 in nutritive matter. There is evidence to show that in 

 the present day some forms are becoming parasitic which 

 have hitherto been living in independence, and perhaps the 

 reverse also holds. The phenomenon of partial parasitism 

 is shown when an animal only lives as a parasite through 

 one or more of its developmental phases, in the others being 

 free and independent. This is a lazy method of life adopted 

 by gad-flies and the like, which feed remarkably when in 

 the larval stage, at the expense of their animal hosts, remain 

 for a time in the chrysalis stage, and then spend a brief but 

 exciting existence in the acts of reproduction and provision 

 for the preservation of their own race, to the discomfort of 

 that of far higher beings. We may now draw up a short 

 classificatory notice in a tabular form of bovine parasites. 



