320 Scientific Proceedings, Royal Dublin Society. 



walls, on the other hand, continue into the pavement epithelium 

 investing the exterior of the mantle, of which they are demon- 

 strably nothing else than an extension. This is most plainly 

 shown in very young processes to be met with near the point 

 margin of the mantle, and which consist of but a few cells ; the 

 terminal cell being of a hemispherical form with the rounded 

 surface outwards, and evidently nothing more than an enlarged 

 epithelial cell. 



The ccecal tubes of Waldheimia cranium are, therefore, epider- 

 mal outgrowths, with a large granular invaginated terminal cell, 

 which at one end is continued into a nerve fibril, and at the other 

 covered by a transparent chitinous layer, separating it from all 

 external influences likely to serve as stimuli, except that of light. 

 It would therefore appear that the transformation of this form of 

 radiant energy into nervous disturbance is the function of the 

 so-called ccecal processes. Still further investigation is however 

 necessary before this view can be fully adopted, especially as a 

 serious objection to it exists in the absence of anything like 

 pigment in the terminal cells. Better material than that at my 

 disposal may, however, throw further light on this and on other 

 points which in my sections remain obscure. 



