THE ANATOMY OF SCAM BREGMA INFLATUM. 269 



15. Sense Organs. 



The sense organs are (1) the epithelium of the prostomium 

 and tentacles, (2) the nuchal orgaa, (3) the lateral sense 

 organs, (4) the dorsal, ventral, and anal cirri. Probably also 

 the long capillary setae should be added to this list. There 

 are no eyes or otocysts. 



Prostomial Epithelium. 



The epithelium of the anterior and dorsal faces of the pro- 

 stomium and its tentacles consists of columnar cells, among 

 which slender fusiform sense organs may be distinguished. 

 The latter are generally seen in small groups, and their 

 slender tips are level with or project slightly beyond the 

 outer surface of the cuticle. The bases of these cells are in 

 intimate relation to either the cells of the brain itself or the 

 fibres of the nerves which supply the two tentacles. 



The Nuchal Organ (figs. 3, 15). 



On each side of the prostomium there is a narrow longi- 

 tudinal slit which leads inwards and downwards into the 

 blindly-ending nuchal organ. In sections the inner ends of 

 the two nuchal organs are seen lying close together near the 

 middle line, below and behind the brain. The character of 

 the epithelium lining the organ varies considerably. Near 

 the mouth of the depression the epithelial cells are short, 

 columnar, or cubical, and stain lightly, but towards the inner 

 end they rapidly increase in length, and here they are long, 

 narrow, columnar, and deeply-staining, and maiiy of them in 

 the terminal portion are ciliated. In some specimens there 

 is quite a sharp line of demarcation between the cubical and 

 elongate cells. In favourable sections sense-cells may be 

 seen among the columnar cells in the middle and inner por- 

 tions of the organ. From the inner ends of these fusiform 

 sense-cells slender fibrils may be traced to the adjacent 

 nerve, which is in continuity with the posterior lobe of the 



