Langdon, Sense-organs of Nereis virens. 25 



Living nerve-fibers, while sometimes varying slightly but 

 always gradually in diameter, are comparatively cylindrical and 

 even in outline ; they never show any of the varicosities so pro- 

 nounced in fixed tissues. These varicosities, whether large or 

 small, are always artefacts and never appear until the tissue is 

 dying. They then form in such tissue whether it has been 

 treated with reagents or not. As the tissues die, the nerve- 

 fibers begin to shorten and widen. Occasionally this takes 

 place through some considerable portion of its length, causing 

 an abnormal thickening of the fiber through this part. Usually 

 a nerve-fiber is affected at numerous isolated but often adjacent 

 parts ; this causes the fiber to become finely beaded or coarsely 

 varicose, or even, if the contraction goes far enough, to become 

 broken up into a row of disconnected granules. The last con- 

 dition is more likely to be found in very delicate fibers. If a 

 pigment granule is present in the neuroplasm of a nerve-fiber, 

 a varicosity is apt to form around it. A slight but normal en- 

 largement of a nerve-fiber is apt, during post-mortem changes, 

 to become a large varicosity. 



The living sense-hairs are best studied in the parapodial 

 cirri. It can, be seen that each living sensory hair is of uniform 

 diameter throughout and has a bluntly rounded apex free from 

 any enlargements whatever (Plate I, Fig. 27). As particles hit 

 against the living hairs, the impulse given by the blow causes a 

 sidewise movement of the sensory hair which is struck. / 

 never saw anything that I could interpret as a nonnal zvithdrawal 

 of one of these hairs. Since the nerve-cells bearing these hairs 

 lie in the removed cirrus, it seems probable that the re- 

 moval of the latter does not cause any immediate disturbance 

 of the normal action of the peripheral processes of these cells. 

 As the tissues die, the sensory hairs are often withdrawn, but 

 always through the formation of varicosities in the peripheral pro- 

 cesses. Often the tips of the sensory hairs swell into a rounded 

 knob (Plate I, Figs. 4 and 24). Sometimes a hair can be seen 

 to form a knob at its apex and then to be slowly withdrawn 

 until this knob rests upon the cuticula (Plate I, Fig. 26). In 

 living tissue stained by methylene blue it can be seen, in such 



