180 
constant movements of protraction and retraction of these 
sete 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 csophageal connectives. Hach 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 thick, 
but it is twice as thick in specimens 250 mm. long. The 
cavity of the otocyst does not, however, increase in the 
same way, it is practically the same size in these two 
specimens, its mean internal diameter being -12 to “13 mm. 
(cf. figs. 49 and 50). The epithelium is composed of non- 
ciliated 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 neurofibrille. In favourable sections, stained 
with iron hematoxylin, each sense cell is seen to be 
traversed by a delicate, deeply-staiming fibril which 
terminates either just below or at the surface of the 
cuticle. Similar cells and their fibrilla 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 
e 
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