244 THE LAMELLIBRANCHIA 
also in the Cyrenidae (Cyclas, etc.), in which special pouches are 
developed to contain the ova, the maturer ova being the more 
anterior. In the Unionidae of the Old World and of North 
America they are retained in the external interlamellar spaces, and 
in other Unionidae (Quadrula, Schistodesma, Gibbosula, Cuneopsis) and 
in Modiolaria in both the external and internal interlamellar spaces. 
In some other Lamellibranchia, ¢.g. Ostraea edulis and other herma- 
phrodite species of Ostraea such as O. angasi and O. lurida, and 
Ree eS Pe med 
Fic. 222, 
Lasaea rubra, a transverse section through the posterior part of the body, showing embryos 
in the gills. ad’, ad’, anterior and posterior adductor muscles ; br, gill; cae, caecum; g.¢, 
cerebral ganglion ; g.vi, visceral ganglion; hep, liver; in, intestine ; ot, otocyst; p, foot; pa, 
mantle; pal, labial palp ; st, stomach ; vi, vitellus. 
in Hntovalva, the earlier stages of development are passed through 
in the pallial cavity, outside the gills. In all other Lamellibranchia 
the eggs are laid one by one, generally in the spring or summer 
in temperate climates. In Nucula delphinodonta they are collected 
together in a mucous sac fixed to the posterior part of the shell, 
and are there incubated. 
Fertilisation may be effected externally to the maternal parent, 
as, for instance, in Pecten, the dioecious species of Ostraea, Modio- 
laria, Dreissensia, Mactra, Pholas, ete., and in all these forms artificial 
fertilisation is possible ; or it may be effected in the pallial cavity, 
