280 PAKA.SITISM IN PLANTS AND ANIMALS. 



Lecture No. 4. — Parasitism in Plants and Animals. By Prof. 

 Geoege Stewaedson Beady, M.D., LL.D., F.R.S., 4th March, 

 1893. 



The subject of parasitism is so extensive that we might well 

 occupy not only one, but a great many evenings in the consid- 

 eration of it. Possibly it may not have presented itself to many 

 of you in this light, but a moment's reflection will suffice to 

 show that I have not overstated the matter. In the first place, 

 all animals are pestered with parasites, both internal and ex- 

 ternal; — most animals having one or more species peculiar to 

 themselves, besides others which are less restricted in their 

 range. Cobbold enumerates 120 species as being found in the 

 interior of the human body, but a rigorous investigation of this 

 list would lead us to exclude probably about one-half as being 

 merely accidental interlopers. Still a tale of sixty is sufficiently 

 disquieting. Among parasites of the frog we may reckon about 

 twenty species, and of the cockroach a like number. And then 

 these creatures are not restricted to pairs like the unclean beasts 

 of the ark, but are found sometimes in countless swarms and 

 very frequently in numbers which, if not countless, are yet 

 considerable. Thus in a single stork there were found, in the 

 respiratory tract, forty worms of different kinds, besides one 

 hundred similar creatures in the stomach, while from other parts 

 of the animal were got one hundred flukes and many hundred 

 Holostoma, besides external parasites. So that it is evident 

 that the number of species of parasitic animals must be greatly 

 in excess of the non-parasitic, while if we take into account the 

 number of individuals the discrepancy will be vastly larger. 

 And this without considering vegetable parasites at all. 



There are many kinds and degrees of parasitism, but in its 

 most pronounced form we may say that a parasite is a plant or 

 animal which derives its entire nutriment from the organism in 

 or on which it lives. But between tliis extreme condition of 

 dependence, and that not infrequent condition in whicli certain 

 animals are associated together in a state of mutual interde- 

 pendence, without being actually parasitic, we find numerous 



