REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 69 



As may be seen in the table, with hookless glochidia (aside from the extreme cases 

 mentioned) the variation in the period has been from 12 to 36 days at average tempera- 

 tures ranging from 24.4° to 17.8°; but even at practically the same temperature the 

 difference may be quite marked, as in experiments no. 8 and no. 9. Experiment no. 6 

 should be noticed as being a case in which, contrary to expectation, quite a long period 

 (32 to 36 days) was recorded at 19.1°, whereas in other experiments (no. 5 for example) 

 the time was only 14 to 21 days at the lower temperature of 17.8°. 



It would seem clear that, although within certain wide limits the duration of the 

 parasitism is dependent upon the temperature of the water, nevertheless other factors 

 may enter into the case to either accelerate the metamorphosis or prolong it over a period 

 which is much longer than the usual duration of the parasitism. These factors would 

 seem to be associated with individual physiological differences in the interaction between 

 the fish and the parasite and are probably nutritive in nature, for on one and the same fish 

 some glochidia may remain several days longer than others. 



As may be seen from an examination of the table, in which the period of liberation 

 is given in each experiment, not all of the young mussels leave the fish at the same time, 

 but, on the contrary, the liberation may occupy a week or more. Harms found that it 

 required from 5 to 6 days, the greater number leaving the fish during the middle of the 

 period. Our experience has usually been in accord with these obser\-ations, but we have 

 found the period to be somewhat more variable, from 2 to 11 days, or even much longer. 



IMPLANTATION AND CYST FORMATION. 



As has been described, the glochidium attaches itself to the fish by closing its shell 

 firmly over some projecting region which can be grasped between the valves, like the 

 free border of a fin or a gill filament. In so doing, a portion of the epithelium and 

 underlying tissue, including blood vessels and lymphatics and varying in amount with 

 the extent of the "bite," becomes inclosed within the mantle space of the glochidium. 

 This tissue early disintegrates into its cellular constituents, which are taken up by the 

 pseudopodial processes of the lar^^al mantle cells, and, as Faussek (1895) has described, 

 are utilized as food during the early stages of metamorphosis. In figure 60, plate xv, 

 drawn from a glochidium six hours after attachment to a fin, the disintegrated tissue, 

 consisting of loose epithelial cells, blood corpuscles, and fibers which lie scattered in the 

 mantle cavity, is seen in the process of being ingested by the mantle cells. Figure 61, 

 plate XV, shows a later stage, 24 hours after attachment, in which the detritus has been 

 entirely taken up, and the mantle cells are now heavih- charged with food material. 



Almost immediately after attachment proliferation of the epithelium begins as the 

 initial step in the formation of the cyst which eventually incloses the entire glochidium. 

 The overgrowth of the larva has been described by Faussek (1895) and Harms (1907-1009) 

 as a healing process on the part of the fish's tissues, resulting from the irritation caused by 

 the wound. The proliferation starts around the line of constriction produced by the 

 pressure of the edges of the valves on the epithelium, and, since the glochidium lies 

 between and prevents the immediate closure of the lips of the wound, the extending 



