Life-history and Anatomy of the Appendiculate Distomes. 383 



with them after tlie inaDuer of au encysted worm until swallowed by 

 some fish. In order to do this, he says, it must be able to swim 

 rapidly like a cercaria, and its organ of locomotion is probably the 

 appendix. And it seems, indeed, exceedingly probable that if the 

 free-swimming worm were predatory, the appendix, with its powerful 

 muscles, would be physiologically, at least, the equivalent of the cer- 

 carian tail. But I have to oppose to this deduction the positive 

 observation that during a period of three or four weeks, when I studied 

 live and free-swimming worms almost daily, I did not see a single 

 one make a true swimming motion, either with the appendix or the 

 body as a whole, or move from place to place, or attack another animal, 

 I am certain that the appendix is not a locomotory organ. 



MoNTiCELLi also argues from the standpoint of ontogeny, but 

 again Willemoes-Suhm is his authority for the facts. The immature 

 worm observed by that author, he says, already possessed an appendix, 

 and it was a Cercaria or but little advanced beyond the cercarian stage ; 

 its appendix, too, was, structurally, the counterpart of the cercarian 

 tail; there is, then, no reason for not believing that the appendix is 

 the homologue of the cercarian tail. 



I have, also, evidence taken from the ontogeny of the animal, 

 but it is proof conclusive against this position. The appendix does 

 not make its appearances as a prolongation of the body, as does the 

 cercarian tail, but as a sac or vesicle within the hinder end 

 of the body, which communicates with the outside by means of a 

 terminal pore. In all the smaller worms I observed it was present 

 in this form alone, and it does not evaginate and become an external 

 body - division until the animal has left its first host and come into 

 the sea-water. I do not believe that the appendix is homologous to 

 the cercarian tail. 



When we inquire: What is its morphological significance, I am 

 unfortunately unable to give a definite answer, but can make certain 

 representations of facts which may point to a solution of the matter. 

 The appendicular vesicle of the smaller worms is lined with a high 

 columnar epithelium; during the free life of the young worm (some- 

 times even before), when evagination of the vesicle has begun, this 

 epithelium is shed and the subepithelial membrane then becomes the 

 external protecting membrane. At the back of the epithelium and its 

 subepithelial membrane are layers of muscle-fibres. The similarity of 

 structure of this vesicle with the typical, Trematode excretory vesicle 

 will be obvious at once ; its function in these young worms is also the 



