478 JOSEPH STAFFORD, 



regards it as phylogenetically equivalent to a Redia and like Ärchi- 

 getes having become sexually mature. 



Material and occurrence. 



Much of my material was procured from Plön , for which my 

 best thanks are due to Dr. Zacharias. About the last of October 1894 

 I received the first mussels from there, and almost every Anodon 

 contained one to many Aspidogasters. I have been using material 

 pretty constantly since that time and I find that the worms are 

 present, in difierent developmental stages together in the same Anodon, 

 in every mouth of the year. The mussels I used from the regions 

 round Leipzig contained none, or only one, or a very few Aspido- 

 gasters ; but those from Plön had a dozen or more , and in one I 

 found forty-five, in anothor ninety-eight. 



On opening the Anodon the parasites are often visible in the 

 transparent pericardium. It was in this organ that the great mass 

 of my Aspidogasters was obtained; and generally they were found 

 closely packed into the anterior corners, at the entrance into the 

 kidney and pericardial gland. In these latter organs I have found a 

 good number too, but in no other organ have I succeeded in finding 

 any, although I have taken considerable trouble to find evidence of 

 migration of the young animals. In one Anodon, only, I found in 

 the pericardial gland a couple of cysts. These were very non-trans- 

 parent, and the one yielded upon comi)ression a mass of globules 

 somewhat resembling fat, while from the other came an Aspidogaster 

 just in the stage when the yolk-glands begin to develop. This animal 

 was very languid, almost non-transi)arent, filled with protoplasmic 

 granules similar to those in an embryo, rapidly became more and 

 more opaque, and altogether gave me the impression of approaching 

 death and decomposition. I mounted it in my usual way in order to 

 see if, under high magnification, it presented a normal structure, but 

 could only make out that the skin and excretory vessels were ap- 

 parently as in others. 



Methods. 



On account of the small size of the animal it permits easy pre- 

 paration as a living object and is yet not too small to give serial 

 sections. Accordingly, one can employ whichever method best suits his 

 particular purpose - for the fine anatomy and for a study of the 

 organs in continuothe former ; for the undistorted relative positions 



