24 PEARLS AND PEARLING 



found in most of the other mussels. One example of 

 Lampsilis rectus contained sixty-seven of these parasites. 

 They do not seem to harm the mussels. 



2. Aspidogaster conchicola, Von Baer is similar to 

 the one just mentioned, but much larger and has a larger 

 adhering disk. This parasite affects various species of 

 mussels, but prefers Anadonta, Symphynota Complanata 

 and Lampsilis Alatus, in which many are found. It usu- 

 ally lives in the pericardial cavity of the host. It is pos- 

 sible these may occasionally form hinge pearls, as ti'.e 

 shape of the two are quite similar. 



3. The Marginal Cyst Distomid. This distomid forms 

 spherical cysts and is fairly common, especially in Lamp- 

 silis ligamentinus and L. ventricosus. Some were found 

 in one Indrula undulate and one Symphynota costata. It 

 is probably the species discovered by H. M. Kelly, who 

 noticed it in four examples of L. ligamentinus. The 

 cysts were usually found along the edge of the mantle, 

 generally in the muscular portion below the pallial line. 

 They are also frequently found in the adductor muscles, 

 and especially in the lower part of the foot where occa- 

 sionally many are found. They seem to prefer muscular 

 tissue. Sometimes cysts of various ages are found in the 

 same mussels. The scientists carefully broke the crust 

 surrounding one of the younger cysts, and the released 

 distomid crawled about slowly. It was necessary to use 

 the miscroscope to study the minute distomid. This para- 

 site often causes irregular blisters and stains the shell a 

 steel blue color, where the cysts are near the outer sur- 

 face of the mantle. Then, too, as the cysts increase in 



