432 ZOOLOGY SECT. 



the ventral cirrus (vent, cirr), is situated oil 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 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 ccelome, body -cavity, or peri- 

 visceral cavity rilled with a fluid, the ccelomic fluid, containing 

 amoeboid corpuscles. The walls of the coelome (Fig. 357) are 

 lined with a thin membrane, the peritoneum or ccelomic epithelium, 

 of which the outer layer that lining the body-wall is the 

 parietal layer (par. peri], that covering the outer surface of the 

 alimentary canal the splanchnic or visceral layer (vise. peri). The 

 space is divided by a series of transverse 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 partitions 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 buccal cavity, con- 

 tinued back into a pharynx (Fig. 356, 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 buccal cavity 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 priming-hook with its cutting 

 edge deeply serrated (Fig. 355, B). 



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 

 to the wall of the buccal cavity and pharynx are a number of 

 bands or sheets of muscle, the protractor muscles, by the contraction 



