COMMEXSALISM AND PARASITISM. 171 



terrestrial animals has been satisfactorily worked 

 out, and we know that, in most cases, they must 

 infest two different hosts before they can reach 

 maturity. The first - of these hosts is usually an 

 Invertebrate, and the second a Vertebrate ani- 

 mal. Moreover, it is known that the larvae are 

 extremely particular in their choice of the first 

 host, attacking one species, and one species only, 

 of Snail or Slug, or whatever Invertebrate its 

 first host may be. If the first host dies a natural 

 death or is swallowed by any other animal than 

 the parasite's proper second host, it that is to 

 say, the parasite dies. It seems probable that 

 the Flukes that infest the intestines of marine 

 animals pass through some similar life-history, 

 but owing to the great difficulties that confront 

 the observer their development has not yet been 

 thoroughly investigated. Similarly the life- 

 histories of the Tape-worms, with which a very 

 large number of marine animals are infested, 

 are not yet known to us. It is comforting to 

 know, after looking through the volumes of 

 papers on these internal parasites of marine 

 animals, that none of them have been shown to 

 be, even occasionally, parasitic upon man, and 

 we can continue our Fish diet without any mis- 

 givings on that score. An exception must, how- 

 ever, be made to this statement for the semi- 

 marine Salmon and Sturgeon, which are suspected 

 of being the first hosts of a human Tape-worm. 



It is perhaps unsatisfactory to dismiss the 

 internal parasites of marine animals with so 

 few words, but I feel compelled to do so, not 

 only because I have nearly outrun the limits 



