ANATOMY OF A FRESH-WATER MUSSEL. 57 



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

 ments — the bys sal filaments— 2iXQ seen projecting from the larva. 



In this form the embryo is eje^ed 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 ihe tail of a fish. 



the gill-covers, lips, or fins, of a fish, — especially Leuciscus 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 e very-day life of its mother {i)ide, Shell- 

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

 Pisidhwi 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 Garner, it is not a rare thing to find the 

 young of Sphc?,riuni conieuin in the body of the adult animal. 



