276 PROCEEDINGS OF SECTION D, 



and redia. The cercaria derived from the redia becomes encysted on 

 grass and the like, and must be eaten by the sheep or some other animal 

 that will serve as its final host, before it can become the sexually mature 

 fluke. 



The life history of Diplodiscus or Opisthioglyphe is somewhat 

 similar to that of Fasciola, the miracidium or ciliated larva finding its 

 way into a snail of some kind, the cercaria of Diplodiscus encysting in 

 insect larvae, while that of Opisthioglyphe makes its way into the 

 crustacean Gramraarus or into another snail. 



In their long-passed history such forms as these must have been 

 able to develop their sexual maturity in their invertebrate host, just 

 as the primitive Aspidogaster does to this day. When vertebrates 

 became evolved and fed upon invertebrate animals harboring parasites, 

 the latter would, of course, be swallowed along with their host, and would 

 at first be protected from the more active digestive juices by the body 

 of their invertebrate host. Becoming freer lower down in the intestine 

 of the vertebrate through the breaking up of the body of their inverte- 

 brate host, the position is not found to be untenable ; instead of active 

 and corroding juices, they find themselves in the midst of abundance of 

 digested or partly digested food. Finding their circumstances here 

 more affluent they waxed fat and strong, this advance finding its 

 expression in increased fertility ; and thus gradually they took on the 

 custom of only developing their sexual maturity in the larger 

 vertebrate host. For the most part each kind would only become 

 established in one vertebrate host, the staple food of which was the 

 invertebrate host. 



Those forms that eventually found their way into vertebrates and 

 established themselves there outstripped their fellows in the struggle 

 for existence, so that this mode of life, more or less accidentally intro- 

 duced, came to be the normal mode. 



When the amphibian ancestors of the frogs appeared in the world — 

 long before the frogs themselves — they became, in this way, infected 

 with a number of forms of trematodes. These trematodes probably 

 much more closely resembled the present day trematode parasites of 

 frogs than did those amphibian ancestors the frogs of to-day, for the 

 worms by that time were an old group of animals, and less likely than tha 

 newly-evolved amphibian to be very plastic. And, not only so, but 

 their mode of life rendered them less likely to be rapidly affected by 

 environmental changes than free-living animals. As the descendants of 

 those early amphibians dispersed to the four corners of the earth, they 

 took their parasites with them, and while the old amphibians have 



