MUSSEL FAUNA OF MAUMEE RIVER. 



63 



two suckers, an anterior one at the small anterior end and a ventral 

 one, about the same size, placed far back near the swollen posterior 

 end. The posterior part of the distomid contained a mass of black 

 material, probably food. 



As the cyst increases in age it becomes black and forms a conspic- 

 uous dot about the size of a pinheacl in the tissue. It is found on 

 examination that the cyst itself, which is surrounded by the black 

 material, has not increased in size^ and the wall retains its brick-red 

 color. The appearances indicate that the black material, which is 

 probably a product of excretion of the parasite, has been passed 

 through the cyst wall. 



In our alcoholic material the cysts, which appear as small black 

 spots to the naked eye, show under the simple lens as minute orange 

 globes, surrounded by a semitranslucent halo, the alcohol having 

 apparently dissolved out the black material around the cyst wall. 



The marginal-cyst distomid reminds one in its general appearance 

 of Diplostomum^ which is frequently found encysted in the skin of 

 fishes, especially minnows, forming small black spots. In the fishes, 

 however, the black color surrounding the cyst appears to be due to a 

 deposit of pigment by the fish. 



The effect of this parasite (the marginal-cyst distomid) upon the 

 mussel appears to be quite various, according to the location of 

 the cyst. In many cases, probably where the cysts are near the 

 inner surface of the mantle, no effect whatever appears to be pro- 

 duced upon the shell. Under other cysts the shell is stained brown, 

 and where the cysts are near the outer surface of the mantle the whole 

 edge of the shell is blistered irregularly and stained a steel-blue color. 

 Moreover, as the cysts increase in size they form firm nodules in the 

 mantle and cause the shell to be built out around them, so that they 

 leave pits in the nacre similar to those left by pearls. 



In studying the collection of shells of L. ligamentinus at the 

 Bureau of Fisheries, which contains perhaps 2,000 specimens of this 

 species and is represented by shells from widel}^ separate parts of the 

 country, peculiar malformations of the shell were observed, which 

 were probably caused by the parasite under discussion. Among these 

 markings are pits near the margin of the shelly such as we have 

 frequently observed in the living mussel underneath the cyst, and 

 sometimes these contain small attached pearls. One of the most 

 marked features is a tendency of the antero-ventral portion of the 

 shell to cease growing outwardly and either become thickened or 

 gape away from the middle line so that the mussel can not tightly 

 close its shell in front. It appears that the presence of the cysts 

 keeps the affected portion of the mantle in a constant state of con- 

 traction, thus preventing it from building the shell outward in the 

 22367°— 12 5 



