28 _ THE MOLLUSCA 
185), Neomeniomorpha (Fig. 17), Nuculidae (Figs. 16, 225). The 
embryo may in this manner be nearly completely invested by a 
“test” extending posteriorly to an orifice which, however, is only a 
false blastopore. 
In both cases the velum eventually atrophies when the animal 
assumes the definitive habits of the adult. When the whole course 
of the development is undergone within the egg-membranes, there 
is either no veliger stage (Cephalopods, Figs. 119, D; 257, 290, 
291; Cyclas, Entovalva, etc.) or there is no free veliger, but a more 
or less rudimentary velum, postero-dorsally atrophied, may be 
observed, which persists nearly to the time of hatching (viviparous 
Gastropods, Purpura, Fig. 1, Pulmonata). 
A post-anal ciliated area is not uncommonly found, as, for 
example, in Purpura (Fig. 1), among Gastropods, and Dreissensia 
Fic. 16, 
Trochosphere of Yoldia, median sagittal section. a.a, anterior adductor muscle; ap, apical 
plate ; bl, blastopore ; c.g, cerebral ganglion ; jl, flagellum ; h.a, posterior adductor ; in, intes- 
tine; li, liver ; sd, stomodaeum ; ¢, “test” or reflected velum, with 3 circlets of cilia. (After 
Drew.) 
(Figs. 13, 14, p.«.c) among Lamellibranchs. Finally, post-oral ciliated 
rings, secondarily acquired, are found in the larvae of certain Opis- 
thobranchs, Gymnosomata (Fig. 120). 
The foot is nothing more than a projection of the integument 
between the mouth and the anus. Its earliest rudiment is evidently 
paired, since it is formed by the union of the lips of the (commonly) 
elongated blastopore (Fig. 113). It is only at a late stage of 
development that it attains its complete development ; during the 
early stages it is very small and functionless, the velum serving as 
the sole organ of locomotion (Fig. 112). 
An ectodermic invagination, bounded by a ridge, makes its 
appearance at an early period on the dorsal face of the embryo, 
near the formative pole. This invagination, known as the “shell- 
gland” (Ray Lankester) or preconchylian invagination (Fig. 110, 
E, F, sh.gl), is also the precursor of the mantle, since the edge of 
the latter structure is formed by the ridge. The shell-gland 
