220 METAZOAN PHYLA 



freely about in search of food, have thin shells with the hinge teeth 

 greatly reduced or absent, and the larvae develop on the fins, opercula, 

 or margins of the mouths of fishes, as well as on the gills. 



An anodonta living in quiet water and not subject to rough treat- 

 ment needs neither a thick shell nor well-developed hinge teeth to safe- 

 guard itself against injury. Not being able to count upon its food being 

 brought to it, it wanders freely, especially at night, in search of the food 

 it needs, usually remaining quite stationary during the daytime. When 

 the glochidia from these forms are released from the parent, they fall to 

 the bottom and lie there until stimulated by the contact of some part 

 of a fish coming to rest on the bottom or until carried into the mouth of 

 a fish in its breathing. They then seize the surface of the fish thus pre- 

 sented and become attached by the teeth in the margin of the shell and 

 by the larval thread in the same manner as do the glochidia of the 

 unios. Here the closure of the valves seems to result from a contact 

 stimulus. In anodontas the eggs are usually fertilized between the middle 

 of July and the middle of August, and the glochidia are discharged the 

 next spring or early summer. When these glochidia attach themselves 

 to the surface of a fish, the skin of the fish grows around them and they 

 become known as blackheads. 



Not all unios occur in rapidly running streams, and when one lives 

 under conditions described as proper for the anodontas it exhibits to a 

 considerable degree both the structure and the habits of an anodonta, 

 as well as a similar manner of reproduction. In a Minnesota lake, 

 surrounded by sandy beaches, unios of the species Lmnysilis luteola 

 (Lamarck) have been observed to come in from the deeper water during 

 quiet nights and to migrate freely about in the shallow water alongshore, 

 returning to the deeper water with the coming of the dawn and leaving a 

 tortuous furrow as evidence of their nocturnal wandering. 



One result of the parasitic period in the life of the mussel is a far more 

 rapid dispersal of the species than if that depended entirely upon the 

 locomotor ability of the mussel itself or even upon the current, since 

 transportation by fish permits the spreading of mussels from the lower 

 portion of a stream toward its source. 



