Anatomy aid Physiology of Anodonta Fluviatilis. 191 



the generative opening during the latter part of summer, passing into 

 the cavity formed by the two lamellae of the outer gills, and there re- 

 main until the following spring, .when they are sufficiently developed 

 to leave the protection of the parent. 



The yolk divides into two portions, nearly, but not quite, separating. 

 The valves are connected by a hinge ; near the hinge line is an 

 adductor muscle, which by its frequent contractions gives a napping 

 motion to the valves ; in the angle formed by the junction of the two 

 valves is a short hollow tube, from which is produced a long filament 

 known as a byssus ; the valves are slightly convex-trigonal in outline, 

 presenting very much the appearance of a shield (PL 4, fig. 2), in no 

 respect resembling the adult shell. The apex is prolonged into a ser- 

 rated spine at right angles to the shell, the teeth or serrations are on the 

 upper portion of the spine and are in two rows. After the valves have 

 become definitely closed by the action of the adductor muscle, the 

 halves are blended together, the gills are developed, the foot grows, the 

 form of the shell changes, and the young Anodonta becomes like the 

 parent in every respect except size. 



