OF MOLLUSCA. 



295 



of the cup become less conspicuous at the periphery, by an in- 

 crease in transparency, so that the embryo appears to be en- 

 closed in a hyaline shell. In the course of three days its longi- 

 tudinal axis becomes more decidedly marked by an elongation 

 of the body in that direction, (fig. 195,) and its bilaterality evinces 

 itself by a symmetrical projection of the 

 right and left sides of the region about 

 the mouth, so as to form a sort of broad 

 tongue (/, t ] ). As the latter is the incip- 

 ient creeping disc, we are enabled by its 

 position, even at this early period, to de- 

 termine the upper and lower sides of the 

 body; and as we know the point at which 



,/ 



the mouth opens, we can at the same time Fi S- 195 



decide upon the relative situation of the head (a) and tail, and 

 right (c 1 ) and left (c). So far, then, the embryo, in all its parts, 

 is perfectly symmetrical right and left of the axial plane. 



In the next stage, which is about twenty-four hours older 

 than the last, the various regions of the body are quite promi- 

 nent, and new features of an important character have become 

 conspicuous. Premising that the embryo still retains its sym- 

 metrical proportions right and left, I will illustrate the relations 

 of its organs by a profile view (fig. 196). The creeping disc 

 (/) is quite marked as a distinct swell- 

 ing which projects below and behind the 

 mouth (m). The mantle, which forms a 

 complete envelope to the body in the adult 

 state, has just begun to form, and covers 

 the end of the body, which is exactly op- 

 posite to the mouth, with a sort of low 

 cap (s/t). The digestive cavity appears much 



an 



19g 



Fig. 195. The same as fig. l&l, but three days older. 200 diam. A view from 

 the back, a, the head; t, t l , the right and left corners of a ; c, c 1 , the right and 

 left sides of the anterior region of the back ; </, the enclosed yolk mass ; e, the 

 transparent space about d ; f, the envelope of the body. From Lercboullet. 



Fig. 196. The same as figs. 194, 195, but one day older than the last. 100 



