44 



LAND AND FRESH-WATER SHELLS. 



of the germinal vesicle. Then from the upper surface of these 

 four spheres which are known as macromeres > numerous smaller 

 spheres (micromeres) are budded off. These go on increasing 

 in number to a large extent, macromeres, and micromeres being 

 produced by the division of those previously existing. Mean- 

 while a cavity is formed in the centre of their mass, the segmenta- 

 tion cavity, and the macromeres, becoming invaginated or 

 tucked into the segmentation cavity, form a double-layered 

 embryo, the external wall (ectoderm) being formed of micromeres, 

 the internal wall (endoderm) being made up of macromeres. 

 This stage is known as the gastrula-stage, and the opening 

 produced by the invagination of the macromeres, the blastopore. 

 This blastopore closes as the larva advances in its development, 

 and the place where the future mouth and anus are to be formed 



becomes marked off by invagi- 

 nations of the ectoderm occurring 

 at the anterior and posterior 

 ends of the closing slit as the 

 stomodcBum and proctodaum re- 

 spectively. The embryo now be- 

 comes oval in contour and sur- 

 rounded by a circlet of cilia, 

 the " Trocho sphere- stage " of Lan- 

 kester, which divides the body 

 into two unequal hemispheres, a 

 smaller, which develops into the 

 visceral mass, and a larger, which 



becomes the foot. The ciliated circlet then becomes the velum 

 a structure identical with the disc of a Rotifer the foot grows 

 larger, and becomes bilobed, the mantle and shell become de- 

 veloped, the tentacles begin to appear, and the snail passes 

 from this " veliger stage" into a definite molluscan condition, 

 the whole process of development from the egg to the mature 

 state occupying a period of time varying from twenty to thirty 

 days. 



VeJ 



Fig. 13. Older larva of a Gastropod (after 

 Gegenbaur). S, shell ; P, foot ; l^cl, 

 velum ; 7", tentacles ; Op, operculum for 

 the closure of the shell opening. 



