In the next stage (fig. 105 D) the blastopore completely closes, but its position is marked by a shallow pit (m) where the stomodæum is eventually formed. The foot (_f_) 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, bounding 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 (_m_) becomes elongated. The eyes and auditory sacks become formed. [FIG. 106. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF NASSA MUTABILIS. (After Bobretzky.) _f._ foot; _m._ mouth; _ce.v._ cephalic vesicle; _st._ stomach.] With further growth the asymmetry of the embryo becomes more marked. The intestine takes a transverse 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 Bütschli (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.