408 



ZOOLOGY 



SECT. 



consisting of close-set, long, narrow cells. To these layers is super- 

 added in the buccal cavity and the pharynx an internal chitinous 

 cuticle, continuous with that of the general outer surface. 



Developrn en tally 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 

 peritoneum referred to above. The cuticle (cut.) is a thin chitinous 



dors to/ 



i*en. long. THUS 



vent.vesg ne.co 



FIG. 322. Nereis dumerilii. Semi-diagrammatic transverse section of the: middle region of 

 the body : circ. mus. (external), circular layer of muscle of body-wall ; circ. mus. (internal), 

 circular layer of muscle of wall of enteric canal ; ccel. coelom ; cut. cuticle ; dorm. loiiii. //< as. dorsal 

 longitudinal muscles of body -wall ; dors. ves. dorsal vessel ; ent. ep. enteric epithelium ; ep. epider- 

 mis ; long. mus. longitudinal muscle of wall of enteric canal ; ne. co. nerve cord ; neph. nephri- 

 dium ; neur. set. neuropodial setae and aciculum with their muscles ; not. set. notopodial setae 

 and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri, parietal layer of ccelomic epithe- 

 lium ; vent. long. mus. ventral longitudinal muscle ; vent. ves. ventral vessel ; vise. peri, visceral 

 layer of peritoneal epithelium. 



layer which exhibits an iridescent lustre due to the presence of 

 two intersecting systems of fine lines ; it is perforated by numerous 

 minute openings, the openings of the epidermal glands. The 

 epidermis (ep.) is very thin, except on the ventral surface, where 

 it becomes considerably thickened. It consists of a layer of cells 

 containing numerous twisted unicellular glands, which are most 

 abundant on the ventral surface, particularly near the bases of the 

 parapodia ; on the dorsal surface the epidermis contains plexuses 

 of fine blood-vessels. The muscular layers are two in number 

 an external in which the fibres run circularly (circ. mus.), and an 



