THE MOLLUSnS OP THE GKEAT AFRICAN LAKES. 191 



tiicle, and aortic trunks there are the usual valves. The gill 

 of Nassopsis is of average length, very simple in structure, 

 and consists of a large number of low, broad, triangular 

 leaves, the apices of which are not produced into filamentous 

 processes, nor ornamented in any way. The osphradium is 

 long and simple; it lies in a groove at the base of the gill, and 

 shows no tendency to become pectinated or modified in any 

 way either before or behind (PI. 20, fig. 6, o. s.). 



The nervous system of Nassopsis is extremely interesting, 

 and is certainly one of the most archaic tseuioglossate types at 

 present known. The cerebral ganglia are widely separated 

 from one another (PI. 20, fig. 7, e.g.) ; and the pleural ganglia 

 are not only separated from the cerebral ganglia (fig. 10), but 

 quite below the oesophagus, the cerebro-pleural connectives 

 being consequently of great relative length. The supra-intes- 

 tinal cord springs directly from the right pleural ganglion, 

 passes up over the oesophagus, and carries the supra-intestinal 

 ganglion (fig. 7, sup. int. g.). From the left pleural ganglion 

 there passes a fine nerve towards the supra-intestinal ganglion, 

 which appears to form a dyaloneurous connection with a deriva- 

 tive of the supra-intestinal nerves. Towards the right the 

 subintestinal connective passes from the left pleural ganglion 

 beneath the oesophagus straight to the subintestinal ganglion 

 (fig. 7, sub. int. g.). This ganglion is directly connected with 

 the right pleural ganglion by a thick cord (fig. 7,x.), and the 

 nervous system is therefore strongly zygoneurous on the right. 

 Above, the cerebral ganglia give off a number of anterior 

 nerves, which are distributed to the buccal mass and the 

 parietes of the head. Among these there are conspicuous the 

 tentacular nerves which pass separately to the tentacles and 

 ocular papillae. The buccal ganglia are situated on the lateral 

 walls of the buccal mass, and are united to the cerebral ganglia 

 by connectives (PI. 20, fig. 7, b.g.). Near the origin of the 

 buccal nerves there arise two fine nerves, one from each cerebral 

 ganglion, which pass forward along the walls of the body, and 

 then bend down, uniting with each other below the mouth 

 (PI. 20, fig. 7, L.com.). This connection appears, therefore, 



VOL. 42, PART 2. NEW SERIES. K 



