PHYLUM ANNULATA 



191 



stiffish, chitinous rods, of which two principal kinds are 

 recognisable ; 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 appendage, the dorsal cirrus 

 (Fig. 107, 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 appen- 

 dages, the anal cirri, similar in char- 

 acter 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 coslom, body cavity, or peri- 

 visceral cavity — filled with a fluid, the 

 ccelomic fluid. The space is divided by a series of transverse 

 partitions or septa passing inwards from the body-wall to the 

 walls of the alimentary canal opposite the grooves between 

 the segments, and thus dividing the ccelom into a series of 

 chambers, each of which corresponds to one of the seg- 



FlG. 108 



Nereis dumerilii. 

 Sets highly magnified. 

 (After Claparede.) 



