Chap. XV.] 



THE SNAIL. 



and anus, however, are generally held to be formed by subse- 

 quent stomodseal and proctodseal invaginations. 



At an early stage of development there is formed round the 

 embryo an equatorial circlet of cilia, by means of which the 

 embryo is caused to rotate within the jelly-like walls of the com- 



FIG. 88. POND SNAIL : DEVELOPMENT. 



A. Ovum after segmentation into macromeres (mac.) and micromeres 

 (ro-ic.). 



B. Gastrula Stage. bl. Blastopore. ep. Epiblast liy. Hypoblast. s. c. 

 Segmentation cavity. 



C. Trochosphere stage./. Foot. m. Mouth, sh. gl. Embryonic shell 

 gland, v. Velum. 



D. Veliger stage. coZ. Collar. /.Foot. m. Mouth, t. Tentacle, v. Velum. 



partment in which it lies. Subsequently growth consists in the 

 gradual enlargement of one hemisphere, and the reduction of the 

 other. In the larger hemisphere both mouth and anus will be 

 formed. In Fig. 88, C., the embryo is in what is known as the 

 trochosphere stage. The incomplete circlet of cilia is seen at v. ; 

 the stomodaeum at m. ; and in the larger hemisphere the foot 

 (/.) is becoming marked off from the visceral hump. At the 

 posterior end of this is shown (at sh. gl.) the position of the 

 so-called shell gland. It is found in many molluscan embryos. 

 In Liiimceus it is merely an embryonic organ, the true shell 

 being formed, as in the snail, by the cells in the neighbourhood 

 of the collar. 



