ANATOMY OF A FRESH-WATER MUSSEL. 57 



the foot is small and slightly developed, and in its place two fila- 

 ments the by ssal filaments are seen projecting from the larva. 



In this form the embryo is ejected from the gill of its brood- 

 mother into the water. Then, sinking down to the bottom, the 

 shell gapes widely, for the single adductor muscle is not strong 

 enough to keep the valves together. Swimming by the flapping 

 of its valves, when it becomes a little more developed, the young 

 Anodon attaches itself by means of its byssal filaments to either 



Fig. 25. Young Mussels on the tail of a f sh. 



the gill-covers, lips, or fins, of a fish, especially Leudscus and 

 Gobio, and fixes its sharp teeth into its body. Remaining for a 

 time in this parasitic condition, the single adductor muscle and 

 byssal threads atrophy, and in their place the anterior and pos- 

 terior adductors become developed, and the foot more developed. 

 Changes go on until the larva has become like the parent from 

 which it originated, and then the young Anodon loses its hold, 

 drops down into the bottom of the water in which it exists, 

 and commences the every-day life of its mother (vide, Shell- 

 Collector's Handbook for the Field, p. 29). In Spharium and 

 Pisidium development takes place in a special brood-pouch 

 developed near the inner gill-plate. The young bivalve is 

 nourished by a secretion from the walls of this pouch, just as the 

 young Anodon is nourished by a secretion from the gills. As 

 was first recorded by Gamer, it is not a rare thing to find the 

 young of Spharium corneum in the body of the adult animal. 



