MOLLUSCA. 



235 



posterior side its cells have bent over so as to form a narrow tube (in}, the 

 rudiment of the intestine. 



In the next stage (fig. 105 D) the blastopore completely closes, but its 

 position is marked by a shallow pit (in} where the stomodasum is eventually 

 formed. The foot (/) is more prominent, and on its hinder border is 

 formed the operculum. The shell-gland (not shewn in the figure) has 

 flattened out, and its thickened borders commence to extend especially over 

 the dorsal side of the embryo. A delicate shell has become formed. In 

 front of and dorsal to the mouth, a ciliated ring-shaped ridge of cells, which 

 is however incomplete dorsally, gives rise to the velum. On each side of the 

 foot there appears a protuberance of epiblast cells, which forms a provisional 

 renal organ. The hypoblast now forms a complete layer ventrally, bound- 

 ing a cavity which may be conveniently spoken of as the stomach (st\ 

 which is open to the yolk above. Posteriorly however a completely closed 

 intestine is present, which ends blindly behind (in}. 



The shell and with it the mantle grow rapidly, and the primitive 

 symmetry is early interfered with by the shell extending much more 

 towards the left than the right. The anus soon becomes formed and places 

 the intestine in communication with the exterior. 



With the growth of the shell and mantle the foot and the head become 

 sharply separated from the visceral 

 sack (fig. 106). The oesophagus (in} 

 becomes elongated. The eyes and 

 auditory sacks become formed. 



With further growth the asym- 

 metry of the embryo becomes more 

 marked. The intestine takes a trans- 

 verse direction to the right side of 

 the body, and the anus opens on the 

 right side and close to the foot in the 

 mantle cavity which is formed by an 

 epiblastic invagination in this region. 

 The cavity of the stomach (fig. 106, 

 st} increases enormously and passes 

 to the left side of the body, pushing 

 the food-yolk at the same time to 

 the right side, and the point where 

 it communicates with the intestine becomes carried towards the posterior 

 dorsal end of the visceral sack. The walls of the stomach gradually extend 

 so as to narrow the opening to the yolk. The part of it adjoining the 

 oesophagus becomes the true stomach, the remainder the liver ; its interior is 

 filled with coagulable fluid. 



Paludina. Paludina Lankester (No. 263) and Biitschli (No 244) is 

 a viviparous form characterised by the small amount of food-yolk. The 

 hypoblast and epiblast cells are distinguished very early, but soon become of 

 nearly the same size. 



FIG. 106. LONGITUDINAL SECTION 

 THROUGH AN ADVANCED EMBRYO OF 

 NASSA MUTABILIS. (After Bobretzky.) 



f. foot; /;/. mouth; ce.v. cephalic 

 vesicle ; st. stomach. 



