AFFINITY OF TREMATODES TO HTRUDINEA. 107 



generations which it undergoes after transference to its definitive host, 

 of multiplying the number of its descendants by the number of the 

 sexual animals which it produces. This fact also proves that the 

 tape-worms are Helminths, which have adapted themselves much more 

 completely to the conditions of parasitism than the Trematodes. 



If tape-worms are in reality to be regarded as creatures which have 

 sprung through a further adaptation to the conditions of parasitic 

 existence, from Trematodes or Trematode-like ancestors, then the ques- 

 tion regarding the origin of these two groups resolves itself into one 

 that is to say, the inquiry concerns itself only with the relations which 

 the Trematodes bear to free-living worms. 



In the discussions of this question only two groups of known 

 animals can be considered here ; these are the leeches and the Plan- 

 arians, both of which show in their external appearance and internal 

 structure a manifold resemblance to the Trematodes. The leeches, 

 by their mode of life, show an analogy with the Trematodes, for it is 

 well known that the greater number of them live as parasites, although 

 they are to some extent predatory (Aulastomum vorax, for instance, 

 feeds chiefly on earth-worms and snails). The smaller and weaker 

 forms of leeches are almost as persistent in their parasitism as the 

 ectoparasitic Trematodes, some of which they also resemble in size 

 and appearance (e.g., Astacobdella, which is parasitic upon the cray-fish, 

 and Udonella parasitic upon Caligus). One might indeed be easily 

 tempted to imagine a direct connection between these two groups. 



But upon closer comparison there are considerable difficulties 

 opposed to this hypothesis. Not only do the leeches possess a dis- 

 tinctly segmented body the segmentation being evident also in their 

 internal structure, especially in the formation of their nervous system 

 and excretory organs but also the mode of their development and the 

 organization of their embryos manifest many and vital differences from 

 the Trematodes, which at present forbid any attempt to connect them. 

 What similarity there is between the two forms is either more ap- 

 parent than real (structure of the intestine and sexual organs), or is 

 only found in points of inferior importance (possession of suctorial 

 discs, absence of body-cavity). It is evidently more in accordance 

 with our present knowledge of the morphological relations of the 

 Hirudinea to regard them as parasitic forms allied to the earth-worms, 

 than to connect them with the Trematodes. 



But if the Hirudinea do not furnish a link to the Trematodes, 

 there remain only the Planarians which can be regarded as their 

 ancestors. These prove in reality to be very closely related to 

 the Trematodes in their general structure, and the formation of their 

 individual organs, In both cases, the short unsegmented parenchyma- 



