434 



ZOOLOGY 



SECT. 



^8 



intersecting systems of fine lines ; it is perforated by numerous 

 minute openings, the openings of the epidermal glands. The epi- 

 dermis (ep) is very thin, ex- 

 cept 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. mus), 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. 

 356 and 357, 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, 



rte.cc 



FIG. 356. 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. co. " -,-, ^'A A r rr n P nnntraH- irvne 

 nerve-cordl; neph. nephridia ; as. oesophagus ; palp, \ U n Q 



palp ; para, parapodia ; peritt. peristome; perist.tmt. * which are of a peristaltic 

 peristomial tentacles ; ph. pharynx with its jaws ; - , 

 prcest. prostomium ; tent, prostomia 1 tentacles ; vent. . Character Waves OI COn- 



* traction 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 



