THE MOLLUSC A 



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 ; Ci/das, 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 



FIG. 10. 



Trochosphere of Yoldia, median sagittal section, a.a, anterior adductor muscle ; ap, apical 

 plate ; W, blastopore ; c.g, cerebral ganglion ; fl, flagellum ; h.a, posterior adductor ; in, intes- 

 tine ; U, liver ; sd, stomodaeum ; t, " test" or reflected velum, with 3 circlets of cilia. (After 

 Drew.) 



(Figs. 13, 1 4, p. a. 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 



