Langdon, Sense-organs of Nereis virens. 13 



above the surface these sensory hairs normally project, but, 

 from what I have seen, I am inclined to think it is not very far 

 (Plate I, Fig. 9). 



The fact that the perforated membrane is found only in the outer part of 

 the thick inner cuticula and the only cells going to it are the bipolar nerve cells 

 would at first seem to indicate that it is formed once for all. If, however, the 

 outer surface of the cuticula of Nereis is either shed or worn away, there must 

 be some provision for the regeneration of the perforated membrane. It may be 

 that the epidermal supporting cells, acting through the thick cuticula, control 

 the formation of a new perforated membrane, or it may be that the nerve cells 

 themselves possess the power of forming cuticula. The latter explanation seems 

 to me the more probable one. Then the extreme thinness of the perforated 

 membrane as compared with the thickness of the general cuticula could be ac- 

 counted for by supposing that the nerve cells, being highly specialized for an- 

 other purpose, had retained the power of secretion in but a slight degree. The 

 above supposition must, of course stand or fall by a study of the embryological 

 development and of normal regeneration. Any difficulty in meeting the prob- 

 lems involved cannot, however, be taken as a proof that in Nereis or any other 

 worm, the cuticula is an unchangeable formation because in the Rhynchobdelli- 

 d«, which are known to shed the outer layer of the cuticula, there are epider- 

 mal sense-organs above each of which is a perforated membrane. 



In the methylene blue preparations, the processes in the 

 cuticular cavities are often variously distorted (Plate II, Figs. 

 30, 34 and 37), Sometimes a fiber is abnormally thick and 

 ends just under the perforated membrane in a small knob from 

 which one, two, or three finer processes — the sensory hairs — 

 pass through the canals of this membrane. The sensory hair 

 may be normal while somewhere in the course of the peripheral 

 process through the inner cuticular cavity there may be one or 

 more varicosities of various sizes, or the peripheral process may 

 appear normal while the sensory hair itself is swollen into a ball 

 which lies above the cuticula. When these varicosities exist in 

 the peripheral processes themselves, the withdrawal of the sen- 

 sory hairs, as will be seen later, is due to their formation. In 

 other cases it is due to a decided bending of that part of a peri- 

 pheral process lying in the cuticular cavity, caused, perhaps, by 

 the protoplasm of the fiber expanding or contracting more on 

 one side than on the other. 



In the methylene blue preparations that have been re- 

 stained by the cochineal, there are often seen in each organ a 



