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and is inserted into the body wall just behind the level 
of the sixth chetigerous annulus. This cord and the 
other rudimentary septa are all placed at the boundary 
of the segment to which they belong. 
Between the intestine and the body wall in the tail 
there is a comparatively small coelomic cavity crossed by 
septa, which are much closer in the anterior than in the 
posterior caudal region, especially in young specimens, 
in which the anterior tail segments are very short. 
(For a description of the alimentary canal, the 
vascular system and the nephridia, see below.) 
MUSCULATURE. 
There is a small amount of connective tissue between 
the epidermis and the well-developed underlying muscula- 
ture of the body wall, which consists of a layer of circular 
muscles below which are seen the bands of longitudinal 
muscle fibres. The latter, which are covered by the thin 
celomic epithelium, abut upon the cceelomic cavity (see 
figs. 24, 35, 36 and 54). In the anterior region of the 
body there are usually a few circular muscle bands which 
are stronger and more obvious than the rest (fig. 23). 
The thin bands of muscle seen in dissections arising 
at the sides of the nerve cord and inserted right and left 
into the body wall at the level of the notopodial sacs are 
the oblique muscles which are very characteristic of 
Polycheta. They commence behind the third diaphragm 
and extend to the base of the tail. They divide this region 
of the celom into three longitudinal compartments, a 
dorsal median portion containing the alimentary canal, 
and two ventro-lateral portions containing the nephridia 
(fig. 36). The oblique muscles partially cover the 
nephridia and one of the bands is usually attached to the 
