244 



THE LAMELLIBRANCHIA 



also in the Cyrenidae (Cydas, 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, e.g. Ostraea edulis and other herma- 

 phrodite species of Ostraea such as 0. angasi and 0. lurida, and 



nxsi. 



vt 



FIG. 222. 



Lasaea nibra, 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.c, 

 cerebral ganglion ; g.vi, visceral ganglion ; Jiep, liver ; in, intestine ; ot, otoeyst ; p, foot ',*pa, 

 mantle ; pal, labial palp ; st, stomach ; vi, vitellus. 



in Entovalva, 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, etc., and in all these forms artificial 

 fertilisation is possible ; or it may be effected in the pallial cavity, 



