SUGGESTIONS FOE RELIABLE HYPOTHESES 183 



would have been also from the beginning those shape- 

 less forms which we see often enough in Parasites. 

 On this assumption, however, how could it be explained 

 that there are transitions between shapeless Parasites 

 and normal ones i.e. free-living representatives of a 

 defined type in which otherwise the separation of type 

 appears to be so clear ? 



The Platodes, for instance, have their tropical repre- 

 sentatives in the free-living Planaria (Turbellaria) ; they 

 possess an intestine and numerous cilia as organs of 

 locomotion which cover the whole body like a garment. 

 There is also known a large group of similar animals 

 which may also be recognized as Platodes and are called 

 Trematodes. They are thoroughly parasitical and 

 provided with an intestine, but without cilia, bearing 

 suckers and hooks which serve to attach them in or 

 outside their hosts. 1 In many forms the really digesting 

 portion of the intestine disappears entirely ; there 

 remain only the oesophagus and the anus. Again, 

 other similar animals are the Tapeworms, all parasites, 

 without intestine or cilia ; they are also the most 

 extreme parasites of the whole Worm group. One 

 is inclined to regard the three classes cited as represen- 

 tatives of one and the same animal form which, in the 

 free-living Turbellaria (there are also parasitical ones), 

 exhibit the unchanged original type, since there are 

 also among the Turbellaria the one or the other extreme 

 parasitical kind, which shows similar retrogression to 

 that of the worms ranked with the Tapeworms. That 



1 Graff : Wissenschajt und Bildung, p. 25. 



