THE ANATOMY OP PLEUROTOMARIA BEYRICHII. 257 



but, as I have pointed out above, althougli there is a certain 

 amount of concentration of nerve-cells in this region, yet it 

 is mainly in the ventral half of the pleuro-pedal cords, i. e. in 

 the region of the pedal ganglion. Moreover, while the nerve- 

 cells are distributed along the whole pleural connective, yet 

 they are distinctly concentrated to a small extent round the 

 origin of the visceral nerve sufficient to give it a slightly 

 orange colour, an appearance which distinctly suggests the 

 localisation of the pleural centre at this point, a condition 

 which would be in harmony with what we find in many other 

 Gastropoda. From these considerations I am forced to the 

 conclusion that the pleui'al ganglion, such as figured by 

 Fischer and Bouvier, does not exist, and that a distinct pleural 

 ganglion has not yet evolved in Pleurotomaria. Never- 

 theless we can distinguish a pleural centre in the point of 

 origin of the visceral nerves, and it is here that a pleural 

 ganglion would form bj'- an aggregation of nerve-cells, sup- 

 posing a form were to arise from Pleurotomaria possessed 

 of distinct ganglia. 



The above conclusion is of great importance in considering 

 the phylogeny of the T^nioglossa, for with the exception of 

 Cyclophorus and Am pull aria — two very aberrant archi- 

 Tsenioglossa, all the remaining members of the great tasnio- 

 glossate group exhibit a condition in which the pleural 

 ganglia are more nearly approximated to the cerebral ganglia 

 than to the pleuro-pedal cords. The connection of these 

 forms with the typical nervous system of the Diotocardia has 

 been sought in the Cyclophoridse and in the Trochidae, 

 but a careful consideration of both these well-known types 

 of nervous system will show that they are both specialised 

 along a different line from that characteristic of the Ttenio- 

 glossa, by a tendency of the pleural ganglion to mount up the 

 visceral nerve (see Bouvier and Fischer's diagram, figs, d and 

 e). On the other hand, in Paludina, the form which, so far 

 as its nervous system is concerned, appears to me to be the 

 only true archi-trenioglossan, the pleural ganglion giving 

 origin to the visceral nerve is little more than a swelling 



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