1890.] Studies of Pelecypoda. 1133 
The development of the oyster is followed from the egg up to 
the adult. It is a highly modified Pelecypod, and has a very 
accelerated development. The completed prodissoconch stage of 
Ostrea (Fig. 1) is an important period in development, and yields 
facts of much phylogenetic significance. Two adductor muscles 
exist instead of one, as in earlier and later stages. No foot, but 
filamentous gills, a velum, and a plain mantle border exist. The 
anus overlies the posterior adductor muscle, and the velum and 
mouth lie on the ventral border of the 
anterior adductor, the normal condi- 
tion in dimyarian Pelecypods. The 
larval umbos are directed posteriorly, 
—a fact of some significance. In all ` 
Pelecypods apparently, as in Ostrea, 
the anterior adductor is developed first, 
both muscles always being present, 
Fic. 1.—Ostrea virginiana; 
a ad, p ad, anterior and posterior 
adductor muscles; g, gills; % as far as known, at the completed 
velum; 27, ps; a, anus; m, 
mantle border. X go diam. prodissoconch stage. 
In the nepionic stages, which succeed the prodissoconch, 
changes take place rapidly. Fig.2 is the same specimen as Fig. 
1, but three days older. The anterior adductor muscle has dis- 
appeared, and the gill filaments are connected ventrally by cross- 
connecting bars. Later the antero-posterior axis becomes re- 
volved, so that the mouth parts are directed toward the hinge 
line. An intricate system of cross- 
connecting bars is built up between 
the gill filaments, until we find the 
condition characteristic of the adult. 
Whereas but a single pair of gills 
exists in early stages, later two new 
gills originate, one on the outer side 
of each previously formed gill. The 
new gills do not originate simulta- 
A A Fic. 2.—Ostrea virginiana 
neously, but there is a considerable 2, palps; ad, adductor muscle; 
r, rec $ gills; mantle 
interval, which forms a three-gilled Suthers ioeo. 
period. 
