12 UNIVERSITY OF MISSOURI BULLETIN 



this stage because they are much more transparent than those 

 of Symphynota and their structure can be observed more easily. 

 Harms (9) describes a slight growth of the shell of Unio before 

 the young mussel breaks through the cyst. I did not observe 

 any external changes in any of the glochidia I studied. 



Implantation on Fins 



The proliferation of the epithelium of the fin around an 

 attached glochidium is much less rapid than that of the gill. 

 This is very evident from a comparison of figs. 1 and 9. Figs. 

 9 and 10 show cross-sections through glochidia which have been 

 attached to a fin for one hour and the growth of the epithelial 

 tissue is less advanced than that of the gill, shown in fig. 1, at 

 the end of fifteen minutes. The blood-vessels of the gills are 

 much larger than those of the fins and this probably accounts 

 for the more rapid growth of the epithelium of the gill. 



Sometimes the glochidium in attaching itself to the fin 

 injures the cells around it by mechanical pressure, and they 

 gradually become detached from the other cells and degenerate. 

 The protoplasm appears hyaline and vacuolated and the nuclei 

 contracted (fig. 10). Later, these cells are sloughed off. Im- 

 mediately after attachment a few scattered blood cells which 

 have escaped from torn vessels are usually found around the 

 glochidium (fig. 9). In the intercellular spaces of the tissue 

 forming the cyst and of the fin tissues lying within the mantle 

 cavity are numerous leucocytes and red blood cells. They are 

 easily distinguished from the nuclei of the epithelial cells be- 

 cause they are much smaller and darker. 



As in the case of the gill tissue, when the proliferation of 

 the cells to form the cyst first begins, mitoses are most often 

 seen just below the glochidium (fig. 10). 



The appearance of the tissue of the cyst at the end of 

 three hours is shown in fig. 11. The shell hooks usually be- 

 come turned in and pressed closely against the enclosed tissue 

 of the fish, in this way making the grasp of the glochidium more 



