Trematode from Protopterus. 93 



with the peculiar dorsal sac and wide aperture. In these 

 respects, however, it appears to differ from any other known 

 group of Trematodes as much as from the Lepodermatid&e. 

 Taking into consideration, therefore, its general agreement 

 with the members of this family, I am inclined to the view 

 that it should be included here and regarded as a specialized 

 form, whose peculiarities seem to require the creation of a 

 new genus. The genera to which it appears to be most 

 closely allied are Ochetosoma, Braun (1901, 1902), Renifer, 

 Pratt (1903), and Lechriorchis, Stafford (1905; redefined by 

 Nicoll, 1911). To all of these genera it bears a very close 

 resemblance in most of its morphological features, among 

 which the displacement of the genital pore towards the left 

 side is especially important ; but it differs from all of them 

 alike in the shape and unequal size of the testes, in the 

 presence of a well-developed receptaculum seminis, and in 

 the unique condition of the excretory apparatus. In all the 

 three genera mentioned the testes are equal in size, more or 

 less lobate, and not elongate, while the excretory vesicle has 

 the usual shape of a Y, already mentioned as characteristic of 

 the Lepodermatidse. In Renifer eUipticus and R. elongatus, 

 Pratt (1903), the stem of the vesicle widens considerably in 

 the middle, and the same seems to be the case in Lechriorchis 

 validus, Nicoll (lyll) ; but in all these cases the external 

 pore is small and situated at the posterior end of the body. 



To what extent the specialization of this parasite may 

 have been brought about by the fact of its inhabiting a host 

 so specialized in structure and mode of life as Protopterus is 

 a question of great interest, but one to which it is scarcely 

 possible to attempt an answer. One is tempted to speculate 

 on the possible function of the dorsal sac with its enormous 

 aperture, and to question whether it is in any way an adapta- 

 tion to the circumstances which arise owing to the aestivation 

 of the host during the dry season — whether, for example, it 

 is a reservoir for the excretory products during this period, 

 or whether it may have an additional function, e. g., in sub- 

 serving respiration. 



Quite apart from this question of function, the homologies 

 of the various parts of the excretory system are by no means 

 clear. It would be of great interest to decide whether the 

 dorsal sac really represents the usual excretory vesicle of the 

 digeuetic Trematodes, or whether it is a new structure 

 specially developed, possibly as an invagination of the 

 external skin. The latter view has a serious drawback in the 

 existence of a special lining epithelium in the sac, which 

 differs totally in character from the outer covering of the body. 



