SECT, vi THE SENSORY ORGANS 85 



originally assumed it to be the remains of a frontal 

 cirrus (as shown in Fig. i) smoothed off to facilitate 

 swimming. We have, however, discovered that its 

 functions are entirely excretory (see Section IX., on 

 the excretory organs, and Appendix III.). 



Of the original four anal cirri of the Annelid, two 

 are retained and two are rudimentary. Those 

 retained have developed a ringed cuticle for the 

 greater part of their length, and so far are covered 

 with setae also arranged more or less in rings, those 

 on the inner side being longer than those on the 

 outer. The tips of these cerci are thin-skinned, and 

 function as tactile papillae ; this is indicated by 

 shading in the drawing of L. Spitzbergensis given as 

 frontispiece. 



The two rudimentary cirri are reduced to papillae 

 on the dorsal surface of the anal segment ; they are 

 thin-skinned, and surrounded anteriorly by a rampart 

 of thorns ; from the centre of each rises a long branched 

 tactile hair. 



The whole body is covered with hairs. We have 

 found at least four kinds. 



(i) There are very fine hairs in groups of two and 

 three ; they are apparently longer (ca. 4^) on the inter- 

 segmental membranes than on the harder parts of 

 the cuticle (ca. 2/jJ). We have found them chiefly on 

 the exposed abdominal segments. It is very doubtful 

 whether they are sensory ; their great numbers and 

 minute size render it probable that they serve to 

 prevent the attachment of other organisms which 

 might hinder free locomotion. On the other hand, 



