PROCEEDINGS OF SECTION D. 275 



the alimentary caual, and, by natural selection, as the generations went 

 on, the cuticle would become thicker and thicker, and the cilia become 

 lost ; our tyro-parasite is developing the kind of integument necessary 

 for its mode of life. 



The earliest parasites would pass their whole life, or most of it, in 

 a single host. Their larvee, still possessed of the ancestral cilia, would 

 have a free existence for a time, but never far away from the host, 

 "which becomes adopted by the worm as it matures, just as it has been 

 adopted by the worm's parents for generations. This is what we know 

 to happen in the case of Polystomum, for instance. 



The life history of all the more specialized parasites is much more 

 complicated than this ; but if this represents the way in which para- 

 sitism developed, we would expect to find something like it in the more 

 primitive forms living at the present day. And this is what we do find. 



Aspidogaster, with its simple saclike intestine and primitive 

 reproductive organs, is obviously a primitive form, in spite of its complex 

 clmging organ. It passes its whole life in one animal. It lives in the 

 kidney of the freshwater mussel, though the young are found only in 

 the intestine, so that we may infer that its more immediate ancestors 

 formerly lived in the intestine, and only later contracted the habit of 

 emigrating to the kidney. 



Polystomum, again, is also more primitive than the digenetic 

 trematodes, and exhibits a simpler life-history. Its free-living larva 

 darts into the gDl cavity of the young frog that it selects for its host ; 

 and these larvae may be found in various stages of immaturity in 

 different parts of the intestine, where, doubtless, its ancestors remained 

 during their sexual maturity. But nowadays the sexually mature 

 forms are found as a rule in the urinary bladder,to which they find an 

 entrance after having traversed almost the whole length of the host's 

 alimentary canal. 



Temnocephala, which, though it lives upon the surface or gills of 

 crayfishes mostly, is not a parasite at all, occupies a position, in regard 

 to its habits of life, somewhere between the non-parasitic TurbeUaria 

 and the parasitic Trematoda. 



The remaining group of trematodes, in the number of forms 

 immensely greater than all the others put together (for archaic forms 

 amongst living animals are never represented by numerous species), 

 consists of members more specialized than the others, and all undergoing 

 a more complex life history. 



The complicated life history of the common liver-fluke of the sheep, 

 Fasciola hepatica, may be cited as an example. The ciliated larva 

 derived from the hatched egg is free-swimming for a short time, and 

 has to find some kind of pond snail, the body of which it enters, there 

 to pass through two stages of its life-history, those of the sporocyst 



