THE TANGANYIKA PROBLEM. 267 



or Tanganyician, type of structure ; and lastly, in like 

 manner, Spekia and Nassopsis are quite distinct from all 

 the other members of the halolimnic series, and consequently 

 form a Spekian and Nassopsian form of organisation, which 

 we shall have to consider. We have, in short, six different 

 types of gastropodean structure, the affinities of which it 

 is desirable to attempt to determine and come to some 

 conclusion as to what they are. It will have become 

 apparent to anyone who has read the preceding descriptions, 

 and who at the same time also happens to be au fait with the 

 existing researches concerning the anatomy of the proso- 

 branchiate mollusca in general, that it is a highly remarkable 

 fact that all these halolimnic forms, notwithstanding their 

 wide structural diversity, present the same peculiarities 

 in the form and arrangement of their alimentary parts. 

 Thus they all present a short, straight oesophagus, a 

 similarly, and curiously coiled intestine, which leaves a 

 stomach with two chambers, the anterior chamber containing 

 in each case a crystalline style. Such a similarity of their 

 alimentary parts might conceivably mean one of two things. 

 It might be due to similarity of physiological conditions 

 which affect them all in Lake Tanganyika, or it might be due 

 to all of them having diverged from ancestral forms which 

 possessed such an arrangement of their alimentary parts. 

 That this curious similarity in the alimentary tract in all the 

 halolimnic gastropods is not due to physiological agencies, 

 i.e , is not the product of similar conditions acting to produce 

 the same kind of stomach in diverse organisms, is, I think, 

 capable of being made quite clear. In the first place, 

 the halolimnic gastropods live under widely different con- 

 ditions, some being shore-dwellers, and thriving in the 

 scorching tropical sunshine, and the rough surf which 

 usually beats upon the rocky shores of Lake Tanganyika, 



