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 an 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, 
