14 



MOLLUSCA. 



these setse are collected into special groups, either 

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

 more or less elevated papillte. Fig. 15 represents the 

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

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

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

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

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



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

 sensitive, and is generally provided with minute setae, 

 especially on the tentacles, or as in Lameliibranchiata 

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

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

 the skin, have been ascertained to divide into a plexus 

 of fibrils. 



Fig. 16.— Diagrammatic seclion through a papilla of touch of Onchidium (after Semnert 

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

 b, enlarged epithelial cells ; b', oraiuary epithelial cells ; c, cellular body ; d. cells ; 

 n, nerve. 



In Onchidiiim, a genus of slugs, Semper describes as 

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



