THE DEVELOPMENT OF LAMELLIBRANCHS. 325 



the organ of locomotion at a much later stage of develop- 

 ment. It is shown at the right side of Fig. 169. 



The two shells grow rapidly, and soon become quite 

 regular in outline, as shown at s, in Figs. 169 and 172, 

 but for some time they are much smaller than the body, 

 which projects from between their edges around their 

 whole circumference, except along a short area, the area 

 of the hinge, upon the dorsal surface, where the two 

 valves are in contact. 



The two shells continue to grow at their edges, and 

 soon become large enough to cover up and project a little 

 beyond the surface of the body, as shown in Fig. 172, and 

 at the same time muscular fibres make their appearance, 

 and are so arranged that they can draw the edge of the 

 body and the velum in between the edges of the shell. In 

 this way that surface of the body which lines the shell 

 becomes converted into the two lobes of the mantle, and 

 between them a mantle cavity is formed, into which the 

 velum can be drawn when the animal is at rest. While 

 these changes have been going on over the outer surface 

 of the body, other important in- 

 ternal modifications have taken 

 place. We left the digestive 

 tract at the stage shown in Fig. 

 1 68 , without any communication 

 with the exterior. 



FIG. 170. A still older embryo. 

 an. Anus. m. Mouth, s. Shell. 



Soon the outer wall of the body becomes pushed in- 

 wards, to form the true mouth, at a point (Fig. 169, m), 

 which is upon the ventral surface, and almost directly 

 opposite the point where the orifice of invagination was 



