THE TANGANYIKA PROBLEM. 
268 
while others are inhabitants of the quiet dark water which 
covers the deep door of the lake. In the second place — and 
it is this fact which will carry most weight — the type of 
alimentary canal which the halolimnic gastropods present 
is not confined to them, but is present in part or in its 
entirety in very widely divergent oceanic forms. We have 
only to compare the alimentary canal of Typhobia with that 
of Strombus to see that both are built entirely on the same 
plan. In each there is a straight oesophagus, a curiously 
bent intestine, a stomach with two chambers and a crys- 
talline style. Still further, we find that the possession of an 
anterior stomachic chamber and a crystalline style is not 
confined to the prosobranchiate mollusca ; it occurs also, 
and presents the same relationships in the Lamelli- 
branchiata , and, when we realise this fact, there can, I think, 
be little doubt, as the investigations of Collier, Huxley, 
Woodward and myself have tended to show, that the 
type of alimentary apparatus which is found in the Lamet- 
libranchiata, in Strombus , Pterocevas, Bithynia , and which 
now turns up again in every member of the halolimnic 
group, is indicative of a retention in them all of a primitive 
type of organisation which once belonged generally to the 
older molluscan forms.* 
As a group, then, and, taken as a whole, one of the most 
remarkable characters which the gastropods of the halolimnic 
series possess is, that they all present a similar and primitive 
character in the arrangement of their alimentary apparatus, 
and consequently appear to be a series of forms which 
retain the characteristics of a time when most gastropods 
* This view has been strikingly confirmed by a comparison recently made by one of 
my students and myself between the gastric apparatus of Spirula, Nautilus and Nassopsis, 
whereby it was shown that the style sac of the Gastropods is unquestionably present in 
the ancient Cephalopods as well. Pro. Roy. Soc., vol. 70, p. 231. 1892. 
