X PHYLUM ANNULATA 446 



ments of the worm oil the surface of the ground or in the interior 

 of its burrow. The setao in the clitelluni, and those in the neigh- 

 bourhood of the genital apertures, are much slenderer than the rest. 

 Along the middle line of the dorsal surface, from about the eleventh 

 segment backwards, is a row of small apertures, one at the line of 

 division between each contiguous pair of segments : these, which 

 are termed the dorsal pores, perforate the body-wall and open 

 internally into the coelome. Through these ccelomic fluid is capable 

 of being discharged, covering the surface with a thin layer which 

 may protect the worm from desiccation or from contact with 

 irritating substances. On the ventral surface are two rows of 

 minute apertures a pair on each segment the excretory apertures 

 or nephruliopores. On the ventral surface of the fifteenth segment 

 (Fig. 363, 15) is a pair of slit like apertures with somewhat tumid 

 lips, the male reproductive apertures ; and on the segment imme- 

 diately in front the fourteenth are two smaller 

 rounded apertures, the female reproductive aper- 

 tures. In the intervals between the ninth and 

 tenth and tenth and eleventh segments are two 

 pairs of small pores, the openings of the recep- 

 tacula seminis. 



The body-wall (Fig. 365) 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, and are con- 

 nected by fine nerve-fibres with the nerve-cord. On the clitelluni 

 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 longi- 

 tudinally arranged fibres (long, mus) situated internally. The 

 circular layer is interrupted at all the intervals between the segments ; 

 the longitudinal layer is interrupted along a series of longitudinal 

 lines, so as to be divided into seven bundles. 



The setae (Fig. 364) are lodged in sacs, the setigerous sacs (see 

 Fig. 375), lined by a continuation of the epidermis. In the region 

 of the body in which the reproductive organs are lodged some of 



