MOLLUSC A. 235 



The various modifications of the above type of development of the 

 alimentary tract are to be regarded as due to the disturbing influence 

 of food yolk. Where primitively the hypoblast cells are very bulky, 

 though invaginated in a normal way, the wall of the hepatic region becomes 

 immensely swollen with food yolk, e.g. Natica. In other cases amongst 

 certain Pteropods (Fol, No. 249) where the hypoblas-t is still more bulky, 

 part of the archenteric walls becomes converted into a bilobed sack 

 opening 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 

 investigated 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 per- 

 manently 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 pecxiliar mass of albuminous 

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

 become ciliated, and form the stomach. The} 7 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 blastopoi'e closed, is found iu Nassa. and 

 has already been described (vide p. 193). 



The stomodceiim. The stomodseum in most cases is formed as 

 a simple epiblastic imagination which meets and opens into the 

 mesenteron. When the blastopore remains permanently open at the 

 oral region the stomodseum is formed as an epiblastic wall round its 

 opening. In all cases the stomodseum gives rise to the mouth 

 and oesophagus. At a subsequent period there are developed in 

 the oral region of the stomodceum 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 subse- 

 quently to the mouth. Its formation is usually preluded by the 

 appearance of two projecting epiblast cells, but it is always developed 



