IX 



PHYLUM ANNULATA 



191 



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, 102) are exceedingly fine, 

 but stiffish, chitinous rods, of which 

 two principal kinds are recognis- 

 able ; both have a terminal blade 

 articulating with the main shaft of 

 the seta by a distinct joint. On the 

 dorsal side of the parapodium is a 

 short cylindrical, tentacle-like ap- 

 pendage, the dorsal cirrus (Fig. lor, 

 dors, cirr.}, and a similar, somewhat 

 shorter, appendage, the ventral cirrus 

 (vent. cirr.\ is situated on its ventral 

 side. The last segment of the body, 

 the anal segment, bears posteriorly 

 a small rounded aperture, the anus 

 this segment is devoid of parapodia, 

 but bears a pair of appendages, the 

 anal cirri, similar in character to 

 the cirri of the ordinary segments, 

 but considerably longer. 



On the ventral surface, near the 

 bases of the parapodia, there is in 

 each segment a pair of very fine aper- 

 tures, the openings of the nephridia. 



The enteric canal is a straight tube running throughout the 

 length of the body from the mouth to the anus. Between 

 the outer surface of this tube and the inner surface of the 

 wall of the body is a considerable space the ccelome, 

 body cavity^ or perivisceral cavity filled with a fluid, the 



FIG. 102. Nereis dumerilii. 



Setse highly magnified. 

 (After Claparede.) 



