44 Journal of Comparative Neurology. 



The sensory systems of these worms, as is known to be the case 

 in other worms, would then come into direct relation with the external 

 world — a condition which seems more probable than that this system 

 should receive stimuli through a thick layer of resistent cuticula. 



The nerve cells of the sensory system in the Cestodes and Trema- 

 todes closely resemble those of the diffuse sense-organs of Nereis. In 

 several cases they are figured so close together that it would not be sur- 

 prising if further research should prove that in some regions of the 

 body these cells are grouped into small yet definite sense-organs. 



Erode ('98) considers the metameric arrangement of the sense- 

 organs of Dero of importance as a proof of the colonial theory of met- 

 amerism. My study of the distribution of the sense-organs of Nereis, 

 especially when this distribution is compared with the form of the body 

 and the movements of the worm itself, has led me to the opposite con- 

 clusion. The distribution in a given part of the body is so dependent 

 upon the external form of this part and the form is so dependent upon 

 the method of locomotion that I have been led to adopt the theory of 

 Meyer ('91) that the metamerism of worms has been brought about by 

 a secondary segmentation due to muscular activities. A further sup- 

 port of this theory is found in Nereis in that the external and internal 

 metamerism do not agree with one another. It seems to me most prob- 

 able that in the primitive worm the diffuse sense-organs — or what is 

 probably their earlier form, isolated sense-cells — were evenly distrib- 

 uted over the general body and merely somewhat more numerous at 

 the anterior end. Then as greater size and complexity of body brought 

 metamerism with it through muscular activities, there became apparent 

 a tendency for the sense-organs to become more pronounced in the 

 more exposed portions of a given metamere and a tendency for the dis- 

 appearance of these organs in regions in which they were of less use. 

 There would thus be brought about a secondary arrangement of the dif- 

 fuse sense-organs in girdles or longitudinal lines. I would therefore not 

 lay stress upon the distribution of these sense-organs as a guide in trac- 

 ing lines of descent. 



V. Spiral Organs.* 



In the body epidermis, in the bases of the palps and ceph- 

 alic cirri, and in the gill-lobes of the parapodia, are found com- 



* These organs were called "ocular organs" in a brief r6sum6 which ap- 

 peared in Science (see Langdon, '97). I have since thought best to designate 

 them by a term that will apply to them even if further research should prove 

 my present conception of their function erroneous. 



