MOLLUSCA. 283 



connected with the aortic heart (fig. 127, kd], but soon become 

 completely separated from it. 



Alimentary tract. The formation of the archenteron, and 

 the relation of its opening to the permanent mouth and anus, has 

 already been described and needs no further elucidation. It will 

 be convenient to treat the subject of this section under three 

 headings for each group viz. (i) the mesenteron, (2) the sto- 

 modaeum, and (3) the proctodaeum. 



The mesenteron. In the Gasteropoda and Pteropoda the 

 mesenteron, as has already been mentioned, forms a simple sack, 

 which may however, owing to the presence of food-yolk, be at 

 first without a lumen. Of this sack an anterior portion gives 

 rise to the stomach and liver, and a posterior to the intestine. 

 This latter portion is the first to be distinctly differentiated as 

 such, and forms a narrowish tube connecting the anterior dila- 

 tation with the anus. In the meantime the cells of a great 

 part of the anterior portion of the mesenteron undergo peculiar 

 changes. They enlarge, and in each of them a deposit of food 

 material appears, which is often at any rate derived from the 

 absorption of the albumen in which the embryo floats. The cells 

 on the dorsal side, adjoining the cesophageal invagination, and 

 the whole of the cells on the ventral side do not however undergo 

 these changes. There thus arises an anterior and ventral region 

 adjoining the oesophagus, which becomes completely enclosed by 

 small cells and forms the true stomach. The part behind and 

 dorsal to the stomach is lined by the large nutritive cells and 

 forms the liver. It opens into the stomach at the junction of the 

 latter with the intestine, which in the later stages becomes bent 

 somewhat forwards and to the right. Still later the hepatic 

 region becomes branched, the albuminous contents of its cells 

 are replaced by a coloured secretion, and it becomes bodily 

 converted into the liver. The stomach is usually richly ciliated. 



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 hypoblast is still more bulky, 

 part of the archenteric walls becomes converted into a bilobed sack opening 



