270 J. H. ASH WORTH. 



brain. The epithelium lining the inner or terminal portion 

 of the organ is strongly folded, and suggests that this section 

 of the organ is to a certain extent eversible. Possibly the 

 small papilla noticed by Sars in S. inflatum (1846, fig. 21), 

 and by Hansen in S. (?) parvum (1882, p. 34, and pi. v, 

 fig. 8), may be the nuchal organ only partially withdrawn. 

 The retraction of the nuchal organ is effected by a small 

 muscle arising from the body-Avall at a point about midway 

 between the notopodial and neuropodial sacs of the first 

 chaBtigerous segment, and inserted into the inner end of the 

 organ {Nuc. Uetr., fig. 15). 



Dorsal, Ventral, and Anal Cirri; Setse. 



As pointed out in the section dealing with the ventral 

 uerve-cord, there is a pair of nerves given off in each seg- 

 ment, supplying among other structures the parapodial cirri 

 and the setal sacs. The nerves to the cirri spread out be- 

 neath the external epithelium. The nerves which supply the 

 cirri send fibres to the setal sacs neai- their mouths; these 

 may be traced for a short distance along the sacs towards 

 their inner ends, but owing to their small size they are soon 

 lost from view. They probably end among the bases of the 

 setse, as Retzius^ has shown for Arenicola. 



The anal cirri are abundantly supplied with nerves by a 

 pair of trunks given off near the termination of the cord. 

 Each cirrus is an epidermal outgrowth, along the whole 

 length of which there is an axial nervous strand. The nerve 

 is surrounded only by a single layer of epidermal cells. 



16. The Lateral Sense Okgans. 



These are the most interesting sense organs of Scali- 

 bregma. Levinsen (1883, p. 133) noticed the prominent 

 lips guarding the depression into which the sense organ is 

 withdrawn, but he mistook the structure for an aperture 

 "probably sexual. ^^ Theel (1879, p. 49) observed a papilla 

 ' '13iolog-. Foreii. Forliaiuli.,' Band iii, Hefie 4—6, p. 85, 1891. 



