Langdon, Sense-organs of Nereis virens. 65 



like invagination — an invagination which is easily seen in the 

 removed cuticula. To the cuticula in the base of each invagin- 

 ation is attached a large muscle which is probably one of the 

 muscles of the proboscis, and it is, I believe, the pull of these 

 muscles which has caused these invaginations: the two posterior 

 sensory organs have come to lie in these pockets through the 

 accident of their position. It appears to be generally true in 

 Nereis that muscles are often attached to the cuticula and that 

 such lines of attachments become the bases of grooves. These 

 posterior cephalic organs have been supposed by different 

 writers to serve every special sense except that of sight. Ra- 

 covitza ('96) concludes that they are olfactory organs. Their 

 structure, position, and innervation correspond to the so-called 

 olfactory organs of other worms in which undoubted otocysts 

 are also present (see Gamble and Ashworth's account of Areni- 

 cola marina) ; but the presence in Nereis virens of one and pos- 

 sibly two other pairs of cephalic organs of unknown function 

 renders necessary an extended study of the latter before the 

 function of any of them can be absolutely decided for this 

 worm. In any case the presence of a sensory area in an invag- 

 ination which is apparently due to muscular action gives a very 

 suggestive theoretical explanation of the primary cause of such 

 invaginations as are found in some otocysts — invaginations 

 which merely serve to increase the sensory epithelium in higher 

 animals in which the proboscis has disappeared. 



VII. Epidetmal Anchoring Cells. 

 I have given this name to certain epidermal cells which 

 seem to be intimately connected by one end with the cuticula 

 and by the opposite end with muscle fibers passing to the epi- 

 dermis. Each of these cells, when stained by methylene blue, 

 is seen to have a very slender somewhat cylindrical or prismatic 

 body, one, two, or three diverging basal processes, and a large 

 number of peripheral processes. (Plate II, Fig. 49). The 

 body of an anchoring cell extends from the cuticula centrally 

 about to the middle height of the epidermis and contains a 

 more deeply stained oval nucleus at or beneath the middle of 



