238 MARTIN F. WOODWARD. 



foi'Di in Scutum and Fissurella. Traces of this organ are 

 also found in that pi-imitive tsenioglossan, Nassopsis. 

 The almost universal occurrence of this organ in the Dioto- 

 cardia suggests that it is a structure of great antiquity and 

 fuuctioual importance, although we are unable to ascribe any 

 special physiological function to it. 



This caecum is in most cases connected with the postero- 

 dorsal wall of the stomach (postero-ventral in Phasianella), 

 and its lips are invariably related to the opening of the bile- 

 duct. Regarding the stomach as a U-shaped structure 

 composed of an oesophageal and an intestinal chamber, the 

 C£ecum invariably arises close to the junction of the two, but 

 essentially belonging to the intestinal chamber, and is closely 

 associated with the bile-ducts. 



This structure has no homology with the crystalline style sac 

 of other Gastropoda or of the Lamellibrauchia; the two struc- 

 tures are undoubtedly co-existent in Nassopsis (Moore "^), 

 and possibly in some Diotocardia. It is, however, extremely 

 suggestive of the spiral ctecum present in the Cephalopoda, 

 which, like the cEecum described above, is a postero-dorsal 

 outgrowth from the stomach, closely related to the bile-ducts 

 and to the point of origin of the intestine. 



An attempt to homologise the spiral cascum found in two 

 such distinct orders of Mollusca as the Gastropoda and the 

 Cephalopoda may at first sight seem unjustifiable, but the rela- 

 tions of the two organs to the alimentary canal are so precisely 

 alike that one cannot help being struck with their similitude. 



It is, moreover, generally accepted that the Cephalopoda 

 and Gastropoda are descended from a common ancestor, so 

 that presence in the two groups of a spiral stomach-cascum 

 is not so surprising, and would only suggest that this struc- 

 ture was present in that ancestral form. Unfortunately we 

 know nothing of the connecting type, which is not astonishing 

 when we remember that both the Cephalopoda and the Dioto- 

 cardia extend back to the Cambrian epoch. The only group 



» " The Molluscs of the Great African Lakes. IV. Nassopsis and Bytho- 

 ceras," 'Quart. Journ. Micro. Sci.,' vol. xlii, 1899. 



