Langdon, Sense-organs of Nereis virens. 19 



I, Figs. I and 2) and the greater number of them branch once 

 or twice (Plate I, Fig. 4 and Plate II, Fig. 35) ; thus the space 

 occupied by such a bundle just beneath the cuticula is greater in 

 diameter than that at the base of the bodies of the epidermal 

 cells. The entire space is usually somewhat funnel shaped, and 

 the peripheral processes do not always completely fill it. In 

 the latter case, it is clearly seen that the surrounding epidermal 

 cells form an actual stiff-walled cavity. As this was observed 

 in living as well as in prepared material, this cavity is clearly a 

 normal structure (Plate I, Figs. 2, 3, 5 A, 13 A, 13B and 16). 



In the cuticula of a cirrus, over each of these ultimate ends 

 of the peripheral bundles, is a characteristic area which resem- 

 bles more that over the sense-organs of Lumbricus (see Cerfon- 

 taine, '90 and Langdon, '95) than that over the sense-organs of 

 the body epidermis of Nereis. As seen in surface views such 

 areas do not show the delicate striations of the rest of the cuti- 

 cula. Near the center of each of these areas are from 5 

 to 20 minute irregularly grouped pores through which the sen- 

 sory hairs pass to the exterior (Plate I, Fig. 6). In longitudinal 

 sections of the epidermis, it is seen that the outer layer of the cu- 

 ticula is apparently lacking over this area — the perforated mem- 

 brane through which the sensory hairs pass, in the cephalic cirri as 

 well as in the body-wall, appears to be formed entirely from the 

 inner cuticular layer (Plate I, Figs. 5 A and 16). In the cirri, 

 however, the absence of the external layer of the cuticula does 

 not produce an external depression such as is found over a diffuse 

 sense-organ in the body epidermis. The outer surface of the 

 perforated membrane in the cephalic cirri is generally more or 

 less convex so that the surface of the cuticula over one of 

 these areas is either plane or actually elevated, rarely concave. 

 The perforated membrane of a sense-organ in the cephalic cirri 

 is only i fx thick and the inner cuticular cavity beneath it is but 

 2 )x deep. 



In the minute canals which pierce this perforated mem- 

 brane, lie the ultimate tips of the peripheral processes, each 

 bearing a stiff sensory hair which normally, as may be seen in 

 living cirri, projects some distance above the cuticula (Plate I, 



