THE MORPHOLOGY OF THE MOLLUSC A. 41 



Rhipidoglossa. The ctenidia of Chiton are, in fact, re- 

 garded as modified epipodial cirri. The consequence of 

 this view is that while the mantle of Chiton and the mantle 

 of Pelecypoda are regarded as homologous, the mantle of 

 the Gastropoda is supposed to represent only a portion of 

 the mantle in these other forms, and its projecting rim, 

 similar as it appears to be in the two cases, is held to be a 

 new and secondary formation unrepresented in the Am- 

 phineura and Pelecypoda. 



Nowhere, however, do we find in Thiele's voluminous 

 writings any explanation of the anomaly which ought to 

 have occurred to him, that while in Chiton the anus is 

 "ventral," and lies well beneath the "epipodium" and the 

 last shell-plate, in operculate Gastropods the intestine opens 

 not only above the epipodium, but between the operculum 

 and the shell of the embryo — a relation which could only be 

 represented in Chiton, if Thiele's theories were correct, by 

 the situation of the anus between two of the shell-plates 

 upon the back of that animal ! 



The nervous system of the Mollusca is treated by Thiele 

 with a ruthlessness no less than that which is meted out 

 to the external organs of the body. Let us take the 

 Amphineura first. In this group, if the relations of the 

 nervous system in Chiton be taken as typical, we have 

 dorsal to the gut a great ganglionic nerve-ring whose lateral 

 components are usually referred to as the lateral or pleuro- 

 visceral cords. Connected anteriorly with the cerebral 

 enlargements of this nerve-ring is a pair of ventral or pedal 

 cords, connected with one another by a series of commis- 

 sures lying beneath the gut, and also with the lateral cords 

 by means of lateral connectives. The lateral cords inner- 

 vate the pallial sense-organs, gills, and viscera ; the ventral 

 cords the musculature of the foot. The lateral cords are 

 regarded by Thiele as the homologues of the lateral cords 

 or nerve-ring of the Turbellarians. and the ventral cords are 

 taken to correspond to the ventral longitudinal nerves of 

 the same forms. So far we find nothing either erratic or 

 original, for the same view has already been taken by Lang 



(16). 



