62 MUSSEL FAUNA OF MAUMEE EIVER. 



disk, which is larger and more complicated, has been aptly compared 

 to the sucking disk of Remora. 



This parasite atfects various species of mussels, but its favorite 

 hosts are species of Anodonta, Symfhynota complanata^ and Lomip- 

 silis alatus^ in which considerable numbers were found. It usually 

 inhabits the pericardial cavity of the host, though when considerable 

 numbers are present they may be found in other parts of the body. 



The finding of a small elongate " hinge pearl " in a mussel affected 

 by these parasites suggested that, under peculiar circumstances, the 

 body of one of these creatures may form the nucleus of such a pearl, 

 since the shape of the two is quite similar. This supposition, however, 

 needs further investigation. Aspidogaster reproduces within the 

 pericardial cavity of the host without any marked metamorphosis, 

 and they are found of all sizes, the minute ones having the same form 

 as the adults." 



3. The marginal-cyst distomid. — A distomid forming spherical 

 cysts was fairly common, especially in Lampsilis ligamentinus and 

 L. ventricosus, along the whole length of the river. A few of these 

 parasites were also found in one Quadnda undulata and one Sym- 

 phynota costata. It is probably the species discovered and briefly 

 described by H. M. Kelly,* who noticed it in four examples of L. 

 ligaonentinK^. 



The cysts are usually found along the edge of the mantle, generally 

 in the muscular portion below the pallial line. They are also fre- 

 quently embedded in other parts of the body, such as the adductor 

 muscles, and especially in the keel of the foot, where they are occa- 

 sionally found in great numbers. There are indications that they 

 prefer muscular tissue. 



Cysts of various ages were frequently present side by side in the 

 same mussels. The youngest cysts are translucent and refractive, 

 faintly yellowish in color, and resemble minute pearls embedded in 

 the edge of the mantle. As they grow older they form conspicuous 

 black dots. One of the smaller cysts measured was 0.3 millimeter 

 in diameter, and a large blackish opaque one was 0.9 millimeter. 



Examined with a microscope the younger cysts appear as brick-red 

 spheres, crossed and recrossed by irregular cracks like those of a dried 

 mud flat. Upon carefully breaking open this crust a minute distomid 

 is released, w^hich slowly crawls about on the slide. One of the dis- 

 tomids thus released was colorless, 0.365 millimeter long and 0.21 

 millimeter wide at its widest portion. Viewed from above, it was 

 pear shaped in outline, the anterior end being acute, the body gradu- 

 ally broadening behind and the posterior end being rounded. It had 



» The development of this species is given by Huxley, Anatomy of Invertebrate Animals. 

 p. 178. 



» Bulletin Illinois State Laboratory of Natural History, vol. v, p. 406. 



