456 



ZOOLOGY 



SECT. 



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

 or deric epithelium, a dermis, muscular layers with associated con- 

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

 membrane, the peritoneum or ccelomic 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. i r 



typh 



cut 



e/iid 



Cll~C.tr MX 



It in a. tti.ua 



neph 



set 



FIG. 359. Immbricus, transverse section of the middle region of the body. circ. IHH.S. layer of 

 circular muscular fibres ; cccl. ccelome ; cut. cuticle ; (tors. v. dorsal vessel ; epi-i.l. epidermis ; 

 ext. nepk. nephridiopore ; hep. layer of chloragen cells ; Imui. mus. longitudinal muscle ; 

 neplt. nephridium ; nepkrost. nephrostome ; n. co. nerve-cord; fet. seta;; sub. 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 thickened, 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. mus] 

 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 



