7 6 NATURE STUDIES. 



OUR UNBIDDEN GUESTS. 



BY DR. ANDREW WILSON, F.R.S.E. 



'THE fact that in most animals there may reside, as 

 " guests," within unconscious or unwilling " hosts/' 

 certain other animal forms, is, of course, widely known. 

 These animal " guests " form the " parasites " of the 

 natural historian. But, although the fact of their 

 existence is known, the general "history of even the 

 commonest parasites is a matter concerning which the 

 general public are, as a rule, lamentably ignorant. 

 I say ' ' lamentably," and I mean what I say. A vast 

 .amount of disease, and that of a preventible nature, 

 is caused by the carelessness of man in the preparation 

 of his food. This carelessness is, in its turn, founded 

 upon gross ignorance, for there are not a few persons 

 who believe that parasites come, like Dogberry's 

 reading and writing, by nature, and that they are 

 part and parcel of an animal's constitution. That 

 this opinion is very far removed from the true state of 

 matters can easily be shown. It is perfectly provable 

 that animals were not created with the parasites 

 infesting them as we find them to-day. Common 

 sense forbids such a supposition, and the organised 

 common sense we call " science " shows us that the 

 reverse is the case. All parasites are acquired, and 

 not original " guests." This alone is provable by the 

 facts of parasite-development. There is a bag-like 

 parasite called Sacculina, for instance, which attaches 



