256 MARTIN F. WOODWAUD. 



We have already in dealing with the different organs 

 suggested that certain of them presented primitive characters, 

 — for example, the eye, the spiral cEecum in the stomach, and 

 still more notably the characters of the nervous system. 



The morphology of the nervous system has already been 

 dealt with at length by Bouvier and Fischer, especially with 

 reference to the relation of the pleuro-pedal cords to the 

 origin of the visceral connectives from the conditions seen in 

 Chiton. Personally, however, I do not think Pleurotomaria 

 thi'ows any fresh light on this branch of inquiry. This, 

 nevertheless, does not rob the nervous system of the mollusc 

 of all interest, for, as we have already seen, in the very 

 uniform disti-ibution of the nerve-cells through the connec- 

 tives and commissures, and the consequent practical absence 

 of distinct ganglia, we have retained in Pleurotomaria, no 

 matter what view we take of the origin of the molluscan 

 nervous system, a very primitive feature. 



The second point of interest, which taken in connection 

 with the above yields to no other feature in the anatomy of 

 Pleurotomaria in its importance, concerns the position of 

 the pleural centres. In the Gastropoda the pleural ganglia 

 may be defined as the accumulations of nerve-cells related 

 to both the cerebral and pedal ganglia, and giving origin to 

 the visceral connectives, these being the only constant 

 features presented by the pleural centres. In position the 

 pleural centres may vary from one close to the cerebral 

 ganglia, as exemplified by the majority of the Monotocardia, 

 to one close to the pedals as in Haliotis and Trochus, but 

 in each case the visceral nerves arise direct from these 

 ganglia. In Pleurotomaria, however, these nerves arise 

 as we have seen from the connective joining the cerebral with 

 the pleuro-pedal cords, so that if Bouvier and Fischer are 

 correct in their localisation of the pleural centres at the 

 anterior end and on the upper surface of the pleuro-pedal 

 cords, we should have the very peculiar and absolutely unique 

 condition of the visceral nerve arising from the cerebro- 

 pleural connective quite independent of the pleural centre ; 



