Langdon, Sense-organs of Nereis vtrens. 39 



fore in any series of animals arranged to illustrate the passage of the 

 sensory cells from the epidermis to the central nervous system, Nereis 

 would come before, not after, Lumbricus. 



Racovitza ('96) states that in Nereis dumeriU "la surface desdeux 

 antennes est h^riss^e de petits poils sensitifs " — probably the sensory 

 hairs born by the cells of the diffuse sense-organs. He did not inves- 

 tigate these cells. 



Hamaker ('98) is the only previous writer who has mentioned the 

 bipolar nerve-cells in Nereis virens itself. He figures and briefly men- 

 tions isolated bipolar nerve-cells which generally lie partly or wholly 

 beneath the epidermis of the body wall and the parapodia. He states 

 that the cells of the sensory fibers of the third parapodial nerve — a 

 nerve which receives the central processes from the diffuse sense-organs 

 situated in the dorsal parapodial cirri and the dorsal gill lobes — "lie far 

 beneath the hypodermis." As before stated, except for the doubtful 

 cases in certain regions of the parapodial cirri, I have always found 

 these bipolar sense-cells grouped into definite organs and, in all cases, 

 have always found the bodies of these cells situated in the epidermis 

 itself. In optical sections of living tissues or in thick microtome sec- 

 tions of methylene blue material, it is very difficult to perceive the un- 

 stained cells of a sense-organ and the limits of the epidermis. Even 

 in thin paraffin sections, I have found that I could not feel sure of my 

 results without the use of a secondary stain to define the limits of tis- 

 sues unstained by the blue. Hamaker does not state from what kind of 

 preparations his methylene blue figures are taken but his failure to use 

 a secondary stain would in itself account for his failure to perceive the 

 true arrangement and position of these sense-cells. Hamaker did not 

 perceive the modified cuticular area over each sense-organ, and although 

 he did not distinguish the sensory hairs as such, he noted that one of 

 the peripheral processes enlarged "just beneath the cuticula into a 

 small knob, from which a fine prolongation extended through the cuti- 

 cula." This "fine prolongation" is, of course, a sensory hair and the 

 "small knob" an artefact whose formation has partly retracted this hair. 

 There is a division of opinion concerning the final termination of 

 the central processes from the bipolar nerve-cells — i. e., from the cells 

 of the diffuse sense-organs — in Nereis. Retzius ('91) decided that these 

 fibers entered the central nervous system, divided into an anterior and 

 a posterior branch and each branch finally ended in an end-bush. Some 

 of his figures of these end-bushes very closely resemble pericellular 

 nerve-baskets. Among the branches of one of these end-bushes, he 



