400 ZOOLOGY SECT. 



slightly hooked. On the dorsal side of the parapodium is a short 

 cylindrical, tentacle-like appendage, the dorsal cirrus (Fig. 319, 

 dors. cirr.\ and a similar, somewhat shorter, appendage, the ventral 

 cirrus (vent, cirr.), is situated on its ventral side. The last seg- 

 ment 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 apertures, 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 coelome, body cavity, or peri- 

 visceral cavity filled with a fluid, the ccelomic fluid. The walls 

 of the coelome (Fig. 322) are lined with a thin membrane, the 

 ccelomic epithelium or peritoneum, of which the outer layer that 

 lining the body- wall is the parietal layer (par. pefi.\ that cover- 

 ing the outer surface of the alimentary canal the splanchnic or 

 visceral layer (vise. peri?). The space is divided by a series of trans- 

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

 wall of the alimentary canal opposite the grooves between' the 

 segments, and thus dividing the coelome into a series of chambers, 

 each of which corresponds to one of the segments. These parti- 

 tions are not complete, spaces being left around the alimentary 

 canal and elsewhere, through which neighbouring chambers 

 communicate. 



The mouth leads into a wide cavity, the l)uccal cavity, con- 

 tinued back into a pharynx (Fig. 321, ph.) These two chambers 

 extend through the peristomium and the first to the fourth seg- 

 ments of the body. They are lined with a tolerably thick cuticle, 

 continuous with a similar layer lining the outer surface of the body, 

 and in the pharynx are a number of very small dark brown chitinous 

 denticles, which are very regularly arranged. The posterior part 

 of the pharynx (dentary region) has very thick walls composed of 

 bundles of muscular fibres, which are concerned in the movements 

 of a pair of laterally placed chitinous jaws. Each jaw is elongated 

 in the direction of the long axis of the body, rounded' at the posterior 

 end or base, where it is embedded in muscle, pointed at the apex, 

 which is strongly incurved ; the inner edge is divided into a 

 number of strong serrations or teeth : the whole jaw might be 

 compared to a pruning-hook with its cutting edge deeply serrated. 



Behind the pharynx the alimentary canal narrows considerably 

 to form a tube, the oesophagus (ces.), which runs through about five 

 segments to open into the intestine. 



Running backwards and inwards from the wall of the peristomium 



