SECT, ix PHYLUM ANNULATA 193 



ments. 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 

 continued back into a pharynx (Fig. 109, ph). In the 

 pharynx are a number of very small dark brown chitinous 

 denticles, which are very regularly arranged. The posterior 

 part of the pharynx has very thick walls composed of 

 bundles of muscular fibres, which are concerned in the 

 movement of a pair of laterally placed chitinous jaws. 



Behind the pharynx the alimentary canal narrows con- 

 siderably to form a tube, the oesophagus (&s}, which runs 

 through about five segments to open into the intestine. 



The anterior part of the alimentary canal is capable of 

 being everted as a proboscis until the jaws are thrust forth 

 and thus rendered capable of being brought to bear on some 

 small living animal, or fragment of animal matter, to be 

 seized and swallowed as food. 



Into the oesophagus open a pair of large unbranched 

 glandular pouches, or cceca (gl), which probably are of the 

 nature of digestive glands. The intestine (int} is a straight 

 tube of nearly uniform character throughout, regularly con- 

 stricted between the segments. 



Nereis has a well-developed system of vessels filled with 

 blood of a bright red colour. A main dorsal vessel (Figs. 

 109 and no, dors, vess) runs from one end of the body to the 

 other above the alimentary canal, and is visible in places 

 through the body-wall in the living animal. It, as well as 

 the majority of the vessels, undergoes contractions which are 

 of a peristaltic character waves of contraction passing 

 along the wall of the vessel so as to cause the movement of 

 the contained blood. These peristaltic contractions are 

 more powerful in the case of the dorsal vessel than in that 

 o 



