FRESH-WATER MUSSELS. 121 



enhances the resemblance. The enlarged marsupia distended with glochidia lie close 

 to these flaps, one on each side. It has been suggested that a fish darting at this 

 tempting bait may cause the extrusion of the glochidia and then become infected. 

 (See Wilson and Clark, 1912, pp. 13, 14.) The unwelcome members in the associations 

 to which mussels belong are discussed in the following section on "Parasites and 

 Enemies." 



PARASITES AND ENEMIES. 



PARASITES. 



Long green algae are occasionally found attached to the exposed tips of the shells 

 of mussels, and these may cause some erosion of the shells. Marly concretions, com- 

 posed of intermingled low algae and lime often form knoblike lumps on shells in lakes. 



Among the most common of mussel parasites are water mites which dwell in the 

 gill cavity and lay their eggs within the flesh of the mussel, in the inner surface of the 

 mantle, in the foot, or in the gills. These water mites, which belong to the genus Atax, 

 vary in size and color and to some extent in shape (Wolcott, 1899). One is black with 

 a white Y-like marking on its back; others may be reddish. The largest and most 

 degenerate is of a honey color with white treelike markings, but because of its incon- 

 spicuous coloration it is often overlooked. The different species of Atax are hard to 

 distinguish without special preparation and study. Under magnification these water 

 mites look somewhat hke spiders. Small pearls are sometimes formed about Atax eggs. 



Leeches are occasionally seen on the inner surface of the mantle of some mussels, 

 especially in Anodontas (floaters) in ponds. They probably feed on the mucus of the 

 mussel. 



A small organism closely resembling a minute leech in general shape and appear- 

 ance is occasional in the axils of the gills of mussels in some lakes. This is Cotylaspis 

 insignis, one of the trematodes or flukes (Leidy, 1904, p. no). One mussel may 

 harbor a dozen or more of these parasites. Rather similar to Cotylaspis insignis but 

 considerably larger and pink instead of yellowish, is the trematode Aspidogaster con- 

 chicola. It is more complex than Cotylaspis insignis and is usually found in the peri- 

 cardial cavitv of the host mussels, although in severe infection it may overflow into 

 other organs. 



Distomids, both free and encysted, are found in mussels. The distomid occurs 

 in almost any muscular part of the body but most frequently in the foot or along the 

 edges of the mantle. Sometimes pearls are formed around distomid cysts. The free 

 distomids are usually found on the mantle surface next to the shell; they are chiefly 

 confined to the flesh along the hinge line but may extend lower down. They are often 

 associated with small irregular pearls. Sporocysts of distomids are common, especially 

 in some Quadrulas. Many distomid parasites of mussels appear to be harmless, but 

 one, Biicephalus polymorphus, destroys their reproductive organs (Kelly, 1899, p. 407; 

 Wilson and Clark, 1912", pp. 69, 70; Lefevre and Curtis, 1912, p. 121). An ascarid 

 worm is occasionally found in the intestine of mussels. 



A worm ^vith peculiar hooks on its head was found encysted in the margin of the 

 mantle of some mussels in a pond near Fairport, Iowa. It was probably a trematode 

 but has been found only once and never identified. 



