250 THE LAMELLIBRANCHIA 
municate with one another behind the branchial axis. In the 
European Unionidae the eggs are incubated and pass through the 
earlier stages of their development in the outer gill-plate (see above, 
rl 
\ 
\ 
\ 
Fic. 226. « 
Reconstruction of an embryo of Yoldia limatula at a stage during ‘“ casting,” represented as 
seen from the right side, with the right shell-valve and mantle lobe removed. a.a, anterior 
adductor muscle ; c.g, cerebral ganglion; f, foot; g, rudiment of gill; int, intestine; of, oto- 
cyst; p.a, posterior adductor muscle ; p.g, pedal ganglion ; 7, pouch that leads to the cerebral 
ganglia; r.l, right lobe of the digestive gland; std, stomodaeum; ¢f, adhering test cells of 
velum ; v.g, visceral ganglion. (After Drew.) 
p- 226, for an account of the segmentation and endodermic invagi- 
nation, Fig. 227, II). In Castalia, Arconaia, Pseudodon, etc., the eggs 
are incubated in the internal gill-plate, and in Quadrula, Schisto- 
desma, Gibbosula, and Cuneopsis both gill-plates are used for 
incubation. The shell-gland, as soon as it is formed, produces a 
shell which grows as fast as the mantle, and 
is provided with a large anterior adductor 
muscle (Fig. 227, I). A ciliated disc, corre- 
sponding to the ciliated post-anal surface of 
Dreissensia (Fig. 224, p.a.c), is formed behind 
the blastopore and causes the embryo to 
rotate in the egg-shell (Fig. 227). These 
Fic, 227. first phases of development take about two 
Embyro of Anodonta, left: months for their accomplishment, and in 
pee ee ek ermennaeg European Unionidae the embryos hibernate 
Fyiceit (ater Gora in the interlamellar space without undergoing 
any appreciable structural modification. In 
the following spring they are hatched out, and escape through 
the dorsal or anal pallial aperture in the form of a peculiar larva 
