444 



ZOOLOGY 



SECT 



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



ccelomic epithelium (jmr 



.., . peri). The cuticle (cut 

 Aestsr.ford 4 . , . . . . 



is a thin chitmous layei 



which exhibits an iri- 

 descent lustre due to the 

 presence of two intersect- 

 ing systems of fine lines 

 it is perforated by numer- 

 ous minute openings, the 

 openings of the epidermal 

 glands. The epidermis 

 (cp) is very thin, except 

 on the ventral surface, 

 where it becomes consider- 

 ably thickened. It consists 

 of a layer of cells con- 

 taining numerous twisted 

 unicellular glands, which 

 are most abundant on the 

 ventral surface, particu- 

 larly near the bases of the 

 parapodia; on the dorsal 

 surface the epidermis 

 contains plexuses of fine 

 blood-vessels. The mus- 

 cular layers are two in 

 number an external, in 

 which the fibres run cir- 

 cularly (circ. ??t?^s), and an 

 internal, in which they 

 run longitudinally. The 

 latter is not a continuous 

 layer, but consists of four 

 bundles of fibres, two 

 dorso-lateral (dors. long, 

 mus) and two ventro- 

 lateral (vent. long. mus). 



Nereis has a well-de- 

 veloped system of vessels 

 filled with blood of a 

 bright red colour. A 

 main dorsal vessel (Figs. 

 350 and 351, 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 



FIG. 350. Nereis dumerilii. Semi-diagrammatic 

 view of the anterior portion of the body with the 

 dorsal body-wall removed, so as to show the ali- 

 mentary canal, the septa, the blood-vessels, and the 

 nephridia ; a portion of the intestine removed so as 

 to show the ventral blood-vessel and nerve-cord 

 which lie below, dors. vess. dorsal vessel ; gl. ceso- 

 phageal glands ; int. beginning of intestine ; ne. c.o. 

 nerve-cord ; neph. nephridia ; ens. oesophagus ; palp, 

 palp ; para, parapodia ; wrist, peristome ; perist. tent. 

 peristomial tentacles ; ph. pharnyx with its jaws ; 

 /ini-st. prostomhim ; tent, prostomial tentacles ; vent. 

 vess. ventral vessel. 



