24 Journal of Comparative Neurology. 



sense-hairs are in definite groups, thus indicating the presence of 

 definite organs (Plate II, Fig. 36). 



In these parapodial cirri, the sensory hairs always project 

 a very long distance above the cuticula (Plate I, Fig. 27). In 

 a dorsal cirrus only 300 // wide at its base and having a cuticula 

 only 2 >w thick, the sensory hairs on its dorsal border, as seen 

 in living material, were 32 // long and those on its ventral bor- 

 der from 24 to 28 u. The length of these hairs decreased 

 toward the tip of a cirrus ; at the tip itself they were only from 

 4 to 6 jii long. 



It will now be seen that, with the exception of the doubt- 

 ful cases in the tips of the dorsal and ventral parapodial cirri 

 and the doubtful cases occasionally seen in the base of a dorsal 

 cirrus, the seiise-cells foiindin the appendages of Nereis virens, like 

 those found in the body -wall, are all grouped into definite sense-or- 

 gans. Moreover, sittce these tactile appendages are purely epidennal 

 outgi'owths, it ivill also be seen that all of their sense-organs, and 

 therefore all of their bipolar sensory cells are situated in the epi- 

 dermis itself. 



B. Study of the Living Diffuse Sense-Organs . 



The diffuse sense-organs of the cephalic and parapodial 

 cirri of Nereis virens are excellent objects for a study of living 

 nerve-tissue. If a cirrus be removed from a living worm, 

 mounted quickly in sea-water under a cover-glass, and exam- 

 ined immediately, with the oil immersion, the living nerve-cells, 

 nerve-fibers, and sensory hairs may be studied before any appre- 

 ciable change takes place in them. 



The living nerve-cells of the diffuse sense-organs are more 

 elongated and smoother in outline than are the same tissues 

 after fixation by reagents. Each cell always tapers more or less 

 gradually into both processes (Plate I, Fig. i2)-7-the rounded 

 form with abruptly attached processes which is so often seen in 

 sections does not appear in living material. As the tissue dies, 

 there appears a tendency for the entire cells to shorten and 

 widen under the action of surface tension. 



