2j6 
THE TANGANYIKA TROBLEM. 
ing of the halolimnic gastropods, Nassopsis , we find our- 
selves confronted with a type of organisation which, from a 
morphologist’s point of view, is at once important and 
unique. The radula of this form is somewhat like that of 
the genus Vivipara , but it is much more closely similar 
to that of a number of Littorinas ; in like manner, the 
nerves are arranged upon a plan which at once recalls the 
more primitive types of the Prosobranchiata and even the 
Rhipidoglossa themselves. To make this quite clear, I 
would, in the first place, point out that if we consider the 
disposition of the anterior ganglionic masses in the different 
groups of the Prosobranchiata, we find that in all the forms 
of the Rhipidoglossa, both diatocardiate, and monatocar- 
diate, and their derivatives, the pleural ganglia are below 
the oesophagus and closely applied to the pedal ganglia. 
And I have, consequently, spoken of this condition of 
the nervous system as being hypoat hroid. In Nassopsis , 
Vivipara , and some other forms of archi-ttenioglossa, 
the pleural ganglia are more or less half-way between 
the cerebrals above and the pedals below the oesophagus. 
The ganglia are scattered, and I have, consequently, 
spoken of this condition as being dvstenoid. In all the 
higher Tsenioglossa and their derivatives, such as the Pteno- 
glossa, the Toxoglossa, and the Rachiglossa, the pleural 
ganglia are intimately related with the cerebral ganglia, 
both the cerebrals and the pleurals forming an almost indis- 
tinguishable ganglionic mass. And, in consequence, we 
may speak of this condition of the nervous system as 
being ep iathroid w hen compared with the other two. 
Nassopsis, like Vivipara, appears thus to stand half-way 
between the primitive Rhipidoglossate types and the 
higher Ttenioglossa and their derivatives ; and this conclu- 
sion is further driven home by the presence in Nassopsis of 
