ANATOMY AND PHYSIOLOGY OF ANODONTA FLUVIATILIS. 191 
the generative opening during the latter part of summer, passing into 
the cavity formed by the two lamellze 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 flapping 
motion to the valves; in the angle formed by the junction of the two 
valves is ashort hollow tube, from which is produced a long filament 
known as a byssus; the valves are slightly convex-trigoual 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 aser- 
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. 
