10 UNIVERSITY OF MISSOURI BULLETIN 



cellular spaces appear cell-division may take place and occasion- 

 al mitoses are seen in the sections (fig. 6). 



After this change takes place, the wall of the cyst is much 

 weakened and pieces of the tissue composing it are often broken 

 off, even before the glochidium has completed its development. 



Fig. 7 represents a section through a glochidium in which 

 this has occurred, leaving the larva only partly enclosed. Some- 

 times the glochidium is in this way prematurely liberated. 

 When the cyst wall is ruptured, the cells become swollen and the 

 nuclei enlarged, while the intercellular spaces entirely disap- 

 pear. 



Fig. 8 is a section through a young mussel which is partly 

 freed from the cyst. It is still attached to the tip of a gill- 

 filament, but the cyst on one side has broken off and on the 

 other the cells are degenerating. 



The time required for the complete embedding of the 

 parasite varies somewhat with the different types of glochidia. 

 The smaller hookless ones usually become enclosed in two 

 hours, but the tissue forming the cyst may grow more slowly 

 and not form a complete covering for the glochidium until 

 as much as six hours after attachment. In the case of the 

 larger hooked glochidia of Symphynota the cyst on the fins 

 is usually completed in from three to eight hours, but as has 

 been mentioned before it seems to be somewhat difficult for 

 these glochidia to become embedded on the slender gill-filaments 

 and occasionally they may be found five days after attachment 

 not more than half covered. When this occurs, the glochidium 

 still clings to the gill and the tissue around it appears entirely 

 normal. 



A glochidium often attaches itself to two filaments lying 

 close together and the epithelium of both filaments grows up 

 around the parasite to form the cyst. This is especially apt to 

 occur in the case of the large hooked glochidia of Symphynota. 



The glochidium continues to grasp the gill tightly until it 

 is completely embedded. During the first few days of the para- 

 sitism, after the cyst is fully formed, but while it is still loose, 



