14 



MOLLUSCA. 



these setse are collected into special groups, either 

 situated in cup- shaped depressions of the skin, or on 

 more or less elevated papillae. Fig. 15 represents the 

 anterior part of the body of a small fresh-water worm 

 (Bohemilla), and shows clearly the small cuticular, and 

 the larger tactile, hairs. In other cases, as in the feelers 

 and cirri of the Alciopidse, there are short, shining, 

 ovoid rods, to the base of which runs a nervous fibril. 



In the IVIollusca, also, the surface of the skin is very 

 sensitive, and is generally provided with minute setae, 

 especially on the tentacles, or as in Lamellibranchiata 

 (mussels, etc.), on the edge of the mantle. In some, the 

 snail for instance (Helix), the nerves, on approaching 

 the skin, have been ascertained to divide into a plexus 

 of fibrils. 



Fig. 16.— Diagrammatic section through a papilla of touch of Onchidium (after Semper). 

 a', a". Two layers of the cuticle ; a, biconvex thickened portion of the cuticle ; 

 h, enlarged epithelial cells ; h', ordinary epithelial cells ; c, cellulai- body ; d, cells ; 

 n, nerve. 



In Onchidium, a genus of slugs. Semper describes as 

 organs of touch (Fig. 16) certain slight elevations of the 



