260 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



constant movements of protraction and retraction of these 

 setse suggests that they may have a sensory function. 



The Otocysts (figs. 46, 49 and 50) are the best developed 

 sense organs of Arenicola. They may be seen in dissec- 

 tions close to the outer edge of the dorso-lateral portions 

 of the oesophageal connectives. Each is a vesicle com- 

 municating with the exterior by a narrow tube, the 

 external opening of which is situated in the peristomium 

 close to the point where the metastomial groove crosses 

 the first inter- annular groove (fig. 5). The otocyst is 

 placed at an angle to its tube and both are lined by a 

 very thin cuticle, best seen in old specimens. The 

 epithelial wall of the otocyst is thicker in old specimens 

 than in young ones, due to the elongation of the cells. In 

 specimens 65 mm. long the epithelium is about 25/x thick, 

 but it is twice as thick in specimens 250 mm. long. The 

 cavity of the otocyst does not, however, increase in the 

 same Avay, it is practically the same size in these two 

 specimens, its mean internal diameter being T2 to "13 mm. 

 (cf. figs. 49 and 50). The epithelium is composed of non- 

 eiliated sense cells and supporting cells. The sense cells 

 are not always easily recognisable, but in some prepara- 

 tions they may be distinguished by their fusiform shape, 

 their more deeply staining nuclei and by the possession 

 of delicate neurofibrillar. In favourable sections, stained 

 with iron hamiatoxylin, each sense cell is seen to be 

 traversed by a delicate, deeply-staining fibril which 

 terminates either just below or at the surface of the 

 cuticle. Similar cells and their fibrillar may also be seen 

 in the wall of the adjacent part of the tube of the otocyst. 

 The tube is lined by columnar cells among which are 

 gland cells ; the cells lining its proximal part (imme- 

 diately after it leaves the otocyst) are ciliated. The 

 epithelium of the distal part of the tube gradually merges 



