456 



ZOOLOGY 



The body-wall (Fig. 359) consists of a cuticle, an epidermis 

 or dcric epithelium, a dermis, nmscular layers with associated con- 

 nective-tissue, and, lining the inner surface, a thin cellular 

 membrane, the -peritoneum or ecelomic epithelium. The cuticle (cut.) 

 is similar to that of Nereis, and has a similar iridescent lustre ; it 

 is perforated by numerous minute apertures. The epidermis 

 consists, except on the clitellum, of a single layer of cells 

 elongated in the vertical direction : many of these cells have the 

 character of unicellular glands ; many others are sensory cells, 



dors. '■• 



veiii- V 



suh.n.vcss 



Fir;, 359, — Iiumbricus, transvL;v.se section of the middle region (if tlie body. Cnx. mm. layer of 

 circular inuscular fibres ; ad. coilome ; fot. cuticle ; doi-a. v. dorsal vessel ; epid. epidermis ; 

 cxt. neph. uephridiopore ; hep. layer of chloragen cells ; lonrj. mu/i. longitudinal muscle ; 

 neph. nephridiuni ; nephrost. nephrostome ; n. co. nerve-cord ; fi^t. setai ; suh. n. vess. sub- 

 neural vessel ; typh. typhlosole ; vent. v. ventral vessel. (After Marshall and Hurst.) 



and are connected by fine nerve-fibres with the nerve-cord. On 

 the clitellum the epidermis is thicketied, and blood-vessels extend 

 between the cells. Below the epidermis is a layer of connective- 

 tissue, the dermis. The muscular fibres which make up the 

 greater part of the thickness of the body-wall are arranged in two 

 principal sets — a layer of circularly arranged fibres (circ. m.us) 

 situated externally, immediately below the dermis and a layer of 

 longitudinally ■ arranged fibres [long, mus) situated internally. 

 The circular layer is interrupted at all the intervals between 

 the segments ; the longitudinal layer is interrupted along 



