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, 



Reconstruction of an embryo of Yoldi Umatula 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 /, foot ; g, rudiment of gill ; int, intestine ; ot, oto- 

 cyst ; p.a, posterior adductor muscle ; p.g, pedal ganglion ; r, pouch that leads to the cerebral 

 ganglia ; r.l, right lobe of the digestive gland ; stil, stomodaeum ; t, 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 

 first phases of development take about two 

 months for their accomplishment, and in 

 European Unionidae the embryos hibernate 

 ln ^ e 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 



Fio. 227. 

 Embyro of Anodonta, left- 



