PHYLUM ANNULATA 



44;} 



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 

 of which, and the pressure of the ccelomic fluid, this anterior part 

 of the alimentary canal can be everted so as to form a proboscis 

 (Fig. 349), and thus the jaws are thrust forth and rendered 

 capable of being brought to bear on some small living animal or 

 fragment of animal matter, to be seized and swallowed as food. 

 The eversion is arrested at a certain point by means of a muscular 

 diaphragm passing from the wall of the buccal cavity to that of 

 the first body-segment. The proboscis is withdrawn again by a 

 retractor sheet of muscle, which passes inwards and forwards to be 



FIQ. 349. Nereis diver Sic olor, x 4. Head with buccal-'region everted. A, dorsal view ; 

 B, ventral view, a, prostonrium ; B, everted buccal region ; c, c', peristomial tentacles, 1, 2, 3, 4 ; 

 (7, denticles ; e, eyes ; E, lower lip ; P, palp in A, entrance to pharynx in B ; J, jaw ; 

 2', prostomial tentacle ; I, peristomium ; 11, parapodium of first body-segment. (From the 

 Cambridge Natural History.) 



inserted into the wall of the alimentary canal at the junction of 

 the pharynx and oesophagus. 



Into the oesophagus open a pair of large unbranched glandular 

 pouches, or cceca (Fig. 350, #/), which probably are of the nature of 

 digestive glands. The intestine (int) is a straight tube of nearly 

 uniform character throughout, regularly constricted in each segment 

 -the constrictions becoming much deeper towards the posterior 

 end of the body. The part of the intestine which lies in the last 

 segment is termed the rectum. 



The wall of the alimentary canal (Fig. 351) consists (1) of the 

 visceral layer of the coelomic epithelium (vise, peri) ; (2) of a layer of 

 longitudinal muscular fibres (long, mus) ; (3) of a layer of circular 

 muscular fibres (circ. mus) ; and (4) of the enteric epithelium 

 (cnt. ep), consisting of close-set, long, narrow cells. To these 

 layers is superadded in the buccal cavity and the pharynx an 

 internal chitinous cuticle, continuous with that of the general 

 outer surface. 



Developmentally the buccal cavity and the pharynx constitute 

 the stomodceum, the rectum the proctodceum, the rest of the alimen- 

 tary canal the mesentcron. 



The wall of the body consists of a cuticle, an epidermis or 



