262 THE TANGANYIKA PROBLEM. 



Natica lineata : they are present in the gills of Littorina, 

 but curiously enough they are not represented in Haller's 

 figure of the gill of Sigaretus neritoides (Lam.). 



The osphradium is lodged in a groove beneath the gill ; 

 it is simple, and slightly but distinctly tending to become 

 pectinated. At the outer extremity it is curiously bent 

 downwards and back, exactly repeating in this the condition 

 of the same organ in Sigaretus neritoides and Natica 

 lineata as represented by Haller. 



The nervous apparatus of >S. sonata is remarkable, the 

 whole completely simulating in every detail that of Lamel- 

 laria perspiciia, as figured and described by Bouvier and 

 Haller. 



The cerebral ganglia are united by a short but distinct 

 commissure, and are almost completely fused with the 

 pleural ganglia. The right pleural ganglion gives rise to a 

 nerve cord which passes upwards and directly across the 

 axis of the animal's body. It almost immediately expands 

 into a ganglionic mass, which represents the supra-intestinal 

 ganglion. 



This ganglion is in turn connected with the left pleural 

 by the commissure. The nervous system is therefore com- 

 pletely zygoneurous on the left. From the left pleural 

 ganglion a nerve cord passes almost parallel to and below 

 the supra-intestinal commissure to a ganglionic enlargement 

 which represents the sub-intestinal ganglion on the right. 

 This ganglion is in direct connection with the right pleural 

 ganglion by short connective. The nervous system is 

 therefore completely zygoneurous on both sides. All these 

 details could be made out by dissection alone, but the 

 animals were so small that, for confirmation, I resorted to 

 sections. In all essential details the nervous system just 

 described corresponds with that of Lamellaria. 



