SEA-FISHERIES LABORATORY. 229 



and is inserted into the body wall just behind the level 

 of the sixth chsetigerous aimulus. 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 ccelomic 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 

 coelomic epithelium, abut upon the ccelomic cavity (see 

 figs. 24, 3-3, oij and 54). In the anterior region of the 

 body there are usually a few T 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 wail at the level of the notopodial sacs are 

 the oblique muscles which are very characteristic of 

 Polychseta. They commence behind the third diaphragm 

 and extend to the base of the tail. They divide this region 

 of the coelom 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 



