Langdon, Sense-organs of Nereis virens. 41 



by Simon around the ganglion cells of the sub-oesophageal ganglia of 

 the Hirudinea, exactly resemble the appearances found in Nereis. As 

 he was not able to find the wall of these ganglion cells, he could not 

 state positively whether the network lay upon the surface of the cell 

 or was embedded in its peripheral protoplasm. Direct protoplasmic 

 communication, however, between the nerve-basket and the ganglion 

 cell, he considers as existing by means of a second, perinuclear, basket 

 which is directly connected with the pericelullar one. In Nereis, 

 the wall of the ganglion cell may be plainly seen and is often stained ; 

 therefore it may be readily demonstrated that the apparent pericellular 

 basket lies outside the wall itself and so far as I have been able to see, 

 has no direct connection with the protoplasm of the enclosed cell. 

 The only appearance I have seen in Nereis, which might at first sight 

 be considered a perinuclear basket is one which proved to be the 

 coarse, densely stained, chromatin network of the nucleus of the gan- 

 glion cell. 



These diffuse sense-organs in Nereis virens are strictly homologous 

 with the epidermal sense-organ described in Lumbricus by Hesse ('95) 

 and myself (Langdon, '95) and also with those recently described in 

 Axiothea and Clymene by Lewis ('98). The chief differences between 

 the sense-organs in Lumbricus and those in Nereis are that in Nereis 

 the cells of any one organ are smaller and fewer in number than in 

 Lumbricus ; in Lumbricus each cell bears but a single sense-hair, while 

 in Nereis some of the peripheral processes branch and thus bear two 

 or three; the slight concavity in the underside of the cuticula in Lum- 

 bricus is very much exaggerated in Nereis by the increase in thickness 

 of the inner layer of its cuticula so that there results the ovoid cavity 

 which is so characteristic a mark of the diffuse sense-organ of the lat- 

 ter form ; in Lumbricus the external surface of the cuticula over each 

 sense-organ is elevated, while in Nereis it is depressed ; and in Nereis 

 the perforated membrane appears to be formed only from the inner cu- 

 ticular layer instead of from both, as in Lumbricus. 



In Lumbricus, which entirely lacks any special tactile appendage, 

 the diffuse sense-organs are apparently the only means the animal has 

 of receiving tactile impressions. Living as it does in a comparatively 

 dry burrow or crawling over the surface of the ground, it may be that 

 it would more easily receive these impressions without injury to the 

 rest of the body if the sensory hairs were borne on on elevated area. 

 Since the surface of Nereis, whether in or out of its burrow, is always 

 bathed by the sea-water, the sense-hairs, whatever their position, would 

 also always be surrounded by this medium ; stimuli conveyed through 



