25 



arms and j^iuniiles to their extremities. The ambulacra] 

 grooves are lined with an epithelium composed of fusiform 

 ciliated cells (PI. IV., figs. 42 and 43), beneath which 

 there is a well-marked band of longitudinally disposed 

 nerve fibres [amb. nv.), from which the inner faces of the 

 marginal tentacles are innervated (fig. 41). The band 

 is thickest in the median line of the groove and thina 

 out gradually towards the sides thereof. The filiform 

 basal ends of many of the overlying epithelial cells 

 penetrate the nerve band to the basement membrane upon 

 which it rests (figs. 42 and 43). Scattered between the 

 epithelial cells, just above the nerve band, are many bi- 

 polar and tri-polar cells, which are not improbably in 

 continuity with the epithelial cells and nerve fibres, and 

 similar cells occur in small numbers in the nerve band 

 itself, A layer of nerve fibres extends from the circum- 

 oral ring for some distance along the alimentary canal 

 (PI. lY., fig. 47; PL v., fig. 52, sh.nv.f.), but is not 

 readily traceable beyond the oesophagus. 



The Deeper Oral system is situated, like the fore- 

 going, on the oral side of the disc and of the arms, but is 

 sub-epithelial in position, and consists of a circum- 

 oesophageal ring with cords radiating from it (PI. Y., 

 fig. 52), lodged in the connective tissue of the tegmen 

 calycis and of the ventral faces of the arms. The circum- 

 oesophageal ring [d. o. nv. r.) is of irregular thickness, and 

 is not always readily traceable throughout a series of 

 sagittal sections of the disc. Of the nerve cords which 

 radiate from the ring Cuenot has described five principal 

 pairs, which are said to run one on either side of the five 

 ambulacral grooves of the tegmen calycis and from thence 

 traverse the arms in the same relative position. The 

 present writer has found the tracing of these nerves a 

 matter of very considerable difiiculty. In individual 



