94 Mr. H. A. Bay lis on a 



On the other hand, if the sac is homologous with the usual 

 vesicle, the enormous size of its aperture and its dorsal position 

 have to be accounted for, and there remain also as problems 

 the terminal bladders of the two collecting vessels and the 

 small diverticulum of the sac into which they apparently 

 open. The histology of the walls of these various parts 

 might be expected to throw some light on the question, and 

 the evidence of my material, so far as it goes, seems to be 

 that the thick epithelium of the sac is not continued into the 

 diverticulum and the two " bladders," but that the latter have 

 thin walls consisting of comparatively flattened cells, while 

 the diverticulum has a considerably thinner epithelium than 

 the large sac. Owing, however, to the imperfect preserva- 

 tion for histological details, it is impossible to lay much stress 

 on such a point. 



On the whole, I am inclined to the view that the sac and 

 its diverticulum together probably represent the stem of the 

 Y-shaped excretory vesicle of the Lepodermatidse, the two 

 terminal bladders of the collecting-tubes being the homologues 

 of the divergent anterior branches. Upon this view, the 

 dorsal sac is not a new invagination from the exterior, but a 

 modification of the terminal portion of the vesicle. How it 

 has been developed to this extraordinary extent, and what 

 may be the advantage gained by having the aperture en- 

 larged from a minute pore to a gaping opening nearly as wide 

 as the whole animal, and, instead of being situated, as usual, 

 at the posterior end of the body, moved forward to a position 

 considerably anterior to this on the dorsal side, are at present 

 unanswerable questions. 



I am not aware of any case among Trematodes in which 

 the excretory pore has a similar size and position. In certain 

 forms (e. g., Urogonimus) in which the genital pore is situated 

 at the hinder end of the body, the excretory pore is displaced 

 somewhat to the dorsal side. But in other Digenea, I 

 believe, where the excretory pore or pores are not terminal, 

 they open on the ventral side. In all cases the pores are 

 minute, and closed either by a special sphincter muscle or by 

 the ordinary musculature of the body-wall (Braun, 1893, 

 p. 641). 



It is an interesting and somewhat remarkable fact that the 

 members of the three genera (Ochetosoma, Renifer, and 

 Lechriorchis) which have already been mentioned as, appa- 

 rently, the nearest relatives of the present species are, with 

 very few exceptions, parasitic in snakes. Odhner has pointed 

 out (1911, p. 55) that these genera, together with Pneumato- 

 2>hilus, Odh., and Leptophallus, Liihe, form within the family 



