310 OSBORN. 



to belong to the genus Distomum. It seems to be a case of 

 arrested development of the final form. The inactivity of the 

 germinal tissue is indicated by the fact that no advance is 

 noticed during the six weeks in which they have been observed, 

 and by the fact that mitotic figures are not seen in the pre- 

 sumably germinal tissue noted above. The form of the body 

 is clearly that of the final stage, and not a larval form. There 

 is no evidence that I know of to show what the earlier stages 

 are, but they are apparently passed in some other host ; and 

 the same is true of the final mature form, the Anodonta seem- 

 ingly serving as a sort of halting place in which the flukes pass 

 a certain period. It is also peculiar to find a distomid in this 

 stage of its life history in an invertebrate host. The long list 

 of hosts for distomids given in Bronn, '93, does not mention a 

 single instance in which the mature form of the fluke is found 

 in an invertebrate, though Aspidogaster is so found ; but in 

 that case there is only one host. It is also unique, as far as I 

 have been able to learn, to find a distomid that is not located 

 parasitically on some portion of the alimentary tube or on some 

 hollow organ connected with it. Here, however, the fluke has 

 no such relation. I have inferred from the restriction of the 

 fluke to Anodonta, when there are so many allied animals at 

 hand, that there is a close relation between the two, doubtless 

 for the benefit of the fluke, and that when the case is better 

 understood we shall find that the residence in Anodonta in this 

 situation is an essential middle part of a life history, both ends 

 of which are at present unknown or unrecognized in connection 

 with this part. 



I shall hope during the coming summer to make further 

 observations upon these parasites and to plan for Anodontas 

 being sent to me at regular intervals during a year, so that 

 careful examinations can be made extending over a longer 

 period. Also it is hoped to make examinations of the mussels 

 at a number of different points on the lake. In the mean- 

 time it seems to be worth while to publish the results thus 

 far obtained. 



HAMLINE UNIVERSITY BIOLOGICAL LABORATORY, 

 January 28, 1898. 



