PHYLUM ANNULATA 



433 



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

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

 (Fig. 355), 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 The proboscis is withdrawn again by a 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 

 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. 356, gl), 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 



<L 



FIG. 355. Nereis diversicolor, x 4. Head with buccal region everted. A, dorsal view ; 



'B, ventral view, a, prostomium ; B, everted buccal region ; c, c', peristomial tentacles, 



1, 2, 3, 4 ; d, denticles ; e, eyes ; E, lower lip ; P. palp in A, entrance to pharynx in B ; 



./, jaw ; T, prostqmial tentacle ; 7, peristomium ; II, parapodium of first body-segment. 



(From the Cambridge Natural History.) 



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. 357) consists (1) of the 

 visceral layer of the ccelomic 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 

 (ent. 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 mesenteron. 



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

 deric epithelium, muscular layers, and the parietal layer of the 

 ccelomic epithelium (par. peri). The cuticle (cut) is a thin chitinous 

 layer which exhibits an iridescent lustre due to the presence of two 



VOL. i. F F 



