Langdon, Sense-organs of Nereis vircns. 43 



Cestodes, it is probably an abnormal appearance caused by the silver 

 nitrate deposit. All of these workers regard the end-knob of the peri- 

 pheral process as a normal structure. Zernecke states that above these 

 peripheral terminations in the Cestodes there is occasionally found a 

 depression in the cuticula but does not think there can be any normal 

 relation between the two. That is, according to all of these investigators 

 the peripheral terminations of the bipolar nerve cells in the Cestodes 

 and Trematodes are not external but lie in a small blind cavity which 

 penetrates only the inner one-fourth of the thickness of the cuticula. 



It seems to me probable, in comparing these peripheral termina- 

 tions with those of the diffuse sense-organs of Nereis, that this blind 

 cavity is but the lower part of such a modified cuticular area as has 

 been described in preceeding pages for Nereis^ . It often happens in 

 my preparations that the plane of the section is such that the ovoid 

 inner cavity is cut obliquely (Plate II, Fig. 37). Then not only will 

 the perforated membrane and outer cavity not appear in the sec- 

 tion, but the inner cavity itself will often appear to extend but a 

 short distance into the cuticula. In this case the appearance seen is 

 similar to that figured by Blochmann and his pupils in the Cestodes 

 and Trematodes and for some time I thought this appearance showed 

 the correct structure of the cuticula over a sense-organ of Nereis. Even 

 where the entire cuticular area appears in the same section, it is diffi- 

 cult without the use of a secondary stain in the methylene blue prepar- 

 ations to perceive the outer cuticular cavity even when the inner one 

 is clearly defined. 



The end-knob described by all these investigators as terminating 

 each peripheral process, I regard as probably a varicosity formed dur- 

 ing post-mortem changes — a varicosity whose formation caused the 

 withdrawal of the "Stiftchen" which they occasionally found project- 

 ing from it. I believe that this "Stiftchen" is a sensory hair and that 

 it normally passes through the cuticula and projects above the external 

 surface. When the "Stiftchen" is not seen it is probably sometimes, 

 as suggested by Blochmann and Bettendorf, unstained and sometimes 

 wholly withdrawn; in the latter case the end-knob would be the abnor- 

 mal swelling of the tip of the sensory hair so often found in Nereis. 

 (See Plate I, Fig. 4.) 



1 If the modified cuticular area over the sensory cells of the Cestodes proves 

 to be homologous with that in the cuticula of Nereis, it may be that this homol- 

 ogy will tend to prove that the cuticula in the first named form is homologous 

 with that of Nereis and that, therefore, the theory which claims the subcuticular 

 tissue as epidermal is the correct one. 



