274 PROCEEDINGS OF SECTION D. 



Two explanations of the fact that four such similar groups of 

 trematodes now inhabit the frogs of Europe, North America, Australia, 

 and Asia ma 3^ be put forward : — 



(1) That these similar groups have come into existence by con- 

 vergence. This explanation has been suggested, but can scarcely be 

 taken seriously. The probability is too remote. This hypothesis would 

 ask us to believe that, under somewhat similar conditions, in regard to 

 Pneumonoeces, for instance, four groups of species have come into 

 existence in which a certain arrangement of the coils of the uterus and 

 many other peculiar features have been separately evolved four distinct 

 times, and that similar happenings occurred in the case of all the other 

 genera. The mathematical probability makes the proposition absurd. 



(2) That the members of each of the subfamilies of the worms are 

 derived from common ancestors, which were parasitic in the ancestors 

 of the frogs long ago, v/hen the distribution of that group was not so 

 widespread as it is to-day. 



It will help us to understand how these very similar groups of 

 trematodes have come to inhabit the frogs of the various regions if we 

 consider briefly how the parasitic mode of life came to be adopted by 

 the flatworms. 



There is no doubt that parasites have been derived from ancestors 

 that once upon a time were free-living. Of the three classes of flat 

 worms, the Turbellaria, the Trematoda, and the Cestoda, the last two 

 are almost exclusively parasitic, whilst the members of the first-named 

 class are almost entirely free-living. Now the Turbellaria are looked 

 upon as the most primitive of the three, as representing most nearly- 

 in their structure the ancestral flat worm. The trematodes and the 

 cestodes are more specialized, and the differences they show compared 

 with the Turbellaria represent more recent developments. 



We can imagine a species of free-living worms, especially 

 one with some form of clinging apparatus like the more or 

 less feeble suckers possessed by some turbellarians, that 

 adopted the habit of attaching themselves to the surface of 

 the body of some larger and slow-moving animal. Finding an 

 abundance of nourishment in the small organisms that cling 

 to the host or stick in the mucus covering its body, the worm 

 gradually forms the habit of living the greater part of its life on its host. 

 Some of these worms would, no doubt, sometimes temporarily take 

 shelter in one of the cavities opening on the surface of the host, such as 

 the gill cavity. This position would be found so congenial through 

 the ready food supply in the shape of blood to be sucked, that, in the 

 course of generations, it would be adopted as the usual place of abode. 

 Later on, some tough-skinned individuals would venture further into 



