THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 185 



We may, therefore, conclude that the name Typhobia Horei, 

 as given by Smith,' stands rightly for but one species after all. 



The nervous system. — The nervous system of Typhobia, 

 both in its general relationships and in the details of its different 

 constituent parts, is almost entirely unique. Viewed from above, 

 it is at once obvious that there is a great condensation and 

 fusing together of the chief ganglionic masses, no commissure 

 being visible externally between the cerebral ganglia (fig. 5,5). 

 Each cerebral ganglion gives off anteriorly two sets of nerves, 

 one obliquely above the other (fig. 34, 1,S) ; the upper and 

 external arises from a prolongation of the ganglion comparable 

 to the " saillie labiale.'^ These nerves are distributed to the 

 tentacles and the eyes. From each cerebral ganglion below 

 the " saillie " there arises another set of buccal nerves, the two 

 innermost of which pass forward, enlarge into the buccal 

 ganglia (fig. 34, 18, and fig. 5, 1), and unite again below the 

 mouth. The remaining members of this set of nerves are dis- 

 tributed to the buccal mass, and to the parietes of the anterior 

 portion of the head and snout. Ganglionic cells extend along 

 the buccal nerve trunks as far as the buccal ganglia. Late- 

 rally each cerebral ganglion gives off a number of small nerves 

 distributed to the head (figs. 34, 19). Towards the posterior 

 upper surface of the cerebral ganglion the paired otocyst 

 nerves arise, and pass obliquely backward over the pleuro- 

 pedal commissures to the enormous otocysts (fig. 35, 6). 



Below, the cerebral ganglia are connected with the pedal 

 ganglia by the rather loug cerebro-pedal commissures (figs. 6, 

 and 35, 20). Immediately behind these there is found on each 

 side a second commissure, which at first sight appears to pass 

 from the cerebral ganglion also (fig. 35, 11). In reality this 

 is the pleuro-pedal commissure, the pleural ganglion being dis- 

 placed forward, so as to lie closely applied to and immediately 

 beneath the cerebral ganglia. In order to understand the rela- 

 tions of these ganglia it is necessary to examine several sections, 

 as they cannot be seen by the ordinary methods of dissection. 

 Fig. 9 represents a section taken through a point marked X in 

 1 ' Proc. Zool. Soc.,' loc. cit. 



