No. 6.] PARASITISM OF ANODONTA PLANA LEA. 309 



The excretory system consists of a terminal pore and a 

 vesicle, into which in the living specimen two lateral passages 

 are seen to lead. These lateral vessels (Fig. 3) in living speci- 

 mens were traced forward where they were lost. They were 

 ciliated so as to produce a current toward the terminal pore, 

 and small side branches were seen which were lost in the 

 parenchyma. Indications of a second similar and probably 

 excretory vessel were seen at the hinder end of the body in 

 compressed living specimens, but I was not able to see its con- 

 nection (if it has one) with the more distinct lateral vessel 

 already mentioned. In the preserved specimens (Fig. 4) the 

 only portion of the excretory system that was visible is the 

 terminal vesicle and the pore. The lateral vessels do not 

 show ; doubtless they are too delicate and similar to the paren- 

 chymatous cells to be distinguishable from them. I have not 

 been able to recognize any nerves, and there are no noticeable 

 sense organs on the surface of the body. 



The reproductive system has not developed ; there is, how- 

 ever, in the posterior end of the body in front of the vesicle 

 of the excretory system a mass of undifferentiated cells. They 

 are clearly seen in total preparations where they are deeply 

 stained, and in contrast with the looser surrounding tissues. 

 The cells of this mass are seen in sections as spherical objects 

 with a large central nucleus, and the mass is clearly a mass of 

 undifferentiated cells. Their future destination is not deter- 

 mined, but it seems very probable that it is the "Anlagen" of 

 the reproductive organs. The gonads in some of the distomids 

 are posterior in position, though the passages may run forward 

 and open anteriorly. 



V. THEORETICAL POINTS. 



Since the reproductive organs are not as yet developed in 

 these flukes, it is impossible to identify them by means of 

 anatomical data ; and since the case has not been reported 

 heretofore, no light is thrown on the matter from the work of 

 other observers. There seems, however, no doubt that the 

 animal is of the family Distomidae, and it seems not unlikely 



