284 ALIMENTARY TRACT. 



into the pyloric region, in the walls of which a large deposit of food material 

 is stored, which gradually passes into the remainder of the alimentary tract 

 and is there digested. The bilobed nutritive sack, as it is called by Fol, is 

 eventually completely absorbed, though the liver in some, if not all cases, 

 grows out as a fresh sack from its duct. 



The formation of the permanent alimentary tract, when the hypoblast is 

 so bulky that there is no true archenteric cavity, has been especially investi- 

 gated by Bobretzky (No. 242). 



In the case of a species of Fusus the hypoblast, when enclosed by the 

 epiblast, is composed of four cells only. The blastopore remains perma- 

 nently open at the oral region, and around it the oesophagus grows in a 

 wall-like fashion. The protoplasmic portions of the four hypoblast cells are 

 turned towards the cesophageal opening, and from them are budded off 

 small cells which are continuous at the blastopore with the epiblast of the 

 oesophagus. These cells give rise posteriorly to the intestine and anteriorly 

 to the sack, which becomes the stomach and liver. This sack always 

 remains open towards the four primitive yolk cells. The cells of the 

 posterior part of it become larger and larger and form the hepatic sack, 

 which fills up the left and posterior part of the visceral sack, pushing the 

 yolk cells to the right. The cells lining the hepatic sack become pyramidal 

 in shape, and each of them is filled with a peculiar mass of albuminous 

 material. The cells adjoining the opening of the oesophagus remain small, 

 become ciliated, and form the stomach. They are not sharply separated off 

 from the cells of the hepatic sack. The yolk cells remain distinct on the 

 right side of the body during larval life, and their food material is gradually 

 absorbed for the nutrition of the embryo. 



A modification of the above mode of development, where the food 

 material is still more bulky and the blastopore closed, is found in Nassa, 

 and has already been described (vide p. 233). 



Tlie stonwdcEuui. The stomodseum in most cases is formed 

 as a simple epiblastic invagination which meets and opens into 

 the mesenteron. When the blastopore remains permanently 

 open at the oral region the stomodaeum is formed as an epiblastic 

 wall round its opening. In all cases the stomodaeum gives rise 

 to the mouth and oesophagus. At a subsequent period there are 

 developed in the oral region of the stomodasum the radula in a 

 special ventral pit, and the salivary glands the latter as simple 

 outgrowths. 



The oesophagus is usually ciliated. 



The proctodceum. Except where the blastopore remains as 

 the permanent anus (Paludina) the proctodaeum is always formed 

 subsequently to the mouth. Its formation is usually preluded 

 by the appearance of two projecting epiblast cells, but it is 



