190 MANUAL OF ZOOLOGY SECT. 



on its ventral aspect is a transversely elongated aperture, 

 the aperture of the mouth. The segments of the body 

 differ little in external characters from one another through- 

 out the length of the worm. Each bears laterally a pair of 

 parapodia which in the living animal are usually in active 

 movement, aiding in creeping, or acting as a series of oars 

 for propelling it through the water. When one of the para- 

 podia (Fig. 107) is examined more attentively it is found 

 to be biramous, or to consist of two distinct divisions a 

 dorsal, which is termed the notopodium (noto), and a ventral, 



dors, cirr 



noto 



neuro 



"^^Bb^ 

 vent.cirr 



FIG. 107. Nereis dumerilii. A single parapodium magnified, ac, aciculum; 

 dors, cirr, dorsal cirrus; neuro, neuropodium; noto, notopodium; vent, cirr, 

 ventral cirrus. (After Claparede.) 



which is termed the neuropodium (neuro). Each of these 

 is further subdivided into several lobes, and each bears a 

 bundle of setae. Each of the bundles of setae is lodged in 

 a sac formed by in vagi nation of the epidermis, the setigerous 

 sac, and is capable of being protruded or retracted and 

 turned in various directions by bundles of muscular fibres 

 in the interior of the parapodium. In each bundle there 

 is, in addition to the ordinary setae, a stouter, straight, dark- 

 coloured seta (ac), the pointed apex of which projects only 

 a short distance on the surface ; this is termed the aciculum. 

 The ordinary setae (Fig. 108) are exceedingly fine, but 



