ANATOMY OF PALUDBSTRINA 169 



3. Sense Organs. 



The statocysts are in close proximity to the posterior 

 surface of the pedal ganglia. The auditory nerve is very diffi- 

 cult to trace, and in a large number of preparations I have 

 only succeeded in finding one in which it is clearly seen. It 

 runs backward from the statocyst in close proximity to the 

 cerebro-pedal connective and ultimately becomes indistin- 

 guishable from the latter. The cysts contain each a single 

 moderately-sized otolith. 



In certain other Taenioglossa, e.g. Valvata (Bernard, 1), 

 Melania and Paludina (Pelseneer, 15), numerous small 

 otoconia replace the single otolith of Paludestrina, 

 Vitrella, and Bythinella. This diversity, which con- 

 trasts with the remarkable constancy in the number of otoliths 

 found in Teleostean fishes, might well supply a subject for 

 independent study both from the taxonomic and the physio- 

 logical point of view. 



The cysts are formed of an external layer of very thin 

 epithelium covering an internal layer of irregularly-shaped 

 cells. These are flattened and resemble rather those figured 

 by Bernard (1) for V a 1 v a t a . 



In Valvata Bernard found no cilia, while Garnault (8) 

 observed that in Cyclostoma they are very sparsely 

 developed. In Bythinella and Vitrella they are not 

 referred to. In P. ventrosa it is almost certain they are 

 absent. There is quite definitely no ordinary ciliated layer 

 and nothing more on the interior surface than occasional vague 

 clumps of unrecognizable tissue. How the otoliths in this case 

 stimulate the sensory layer is therefore uncertain unless the 

 latter has a general tactile sensibility. In Valvata Bernard 

 (loc. cit.) observed a sort of network formed of prolongations 

 from the membrane of the lining cells. 



The eyes do not call for special attention. They resemble 

 those figured by Henking for P. ulvae in general form, 

 though they differ in the closer approximation of the imier and 

 outer cornea. In Bythinella the space between the two 

 corneas is considerable and filled with connective tissue. In 



