IMPLANTATION OF THE GLOCHIDIUM ON THE FISH 11 



the glochidium usually lies with the shell valves slightly gaping. 

 During this time it often moves slightly by opening and closing 

 the valves. It may even bite off the piece of gill-filament which 

 it has seized and become completely turned around in the cyst. 

 Even when this takes place the glochidia seem to develop 

 normally, or they are often found in this position during the 

 late stages of the parasitism. 



Schierholz (14), Faussek (4), and Harms (7), all describe 

 the cyst as being very thin at first, and gradually becoming 

 thicker, but in the cases which I observed it reached its maxi- 

 mum thickness at once, and as the cells continued to divide 

 became thinner and more dense during the first few days of 

 the parasitism until it assumed the appearance shown in fig. 5. 



The time at which the tissue of the cyst becomes vacuol- 

 ated varies according to the length of the parasitic period. In 

 those infections which last fourteen days the intercellular spaces 

 begin to appear about the fourth day, while in those extending 

 over a period of thirty days the tissue assumes this appearance 

 on the eighth or ninth day. As stated before, after the formation 

 of these numerous lymph-spaces the cyst is much looser and 

 weaker, and many glochidia which have not yet completed the 

 metamorphosis fall off during this time. Parts of the cyst may 

 become broken off as shown in fig. 7, but more often the tissue 

 breaks below the point at which the parasite is attached and 

 the glochidium falls to the bottom still surrounded by the wall 

 of the cyst. This happens most often in the case of the larger 

 hooked glochidia of Symphynota, but the lighter glochidia of 

 Lampsilis may become separated from the fish in the same way. 

 Although the glochidia are usually alive at the time they leave 

 the fish and may often be seen to move slightly, the development 

 does not continue and they soon die. Occasionally a fully 

 metamorphosed glochidium will fall off still covered by the 

 tissue of the cyst. When this occurs, the foot may be seen to 

 move around within the mantle cavity, but the glochidium 

 seems never to be able to free itself from the surrounding tissue. 

 The glochidia of Lampsilis are the most favorable for study at 



