THE HABITS AXD STRUCTURE OF ARENICOLA MARINA. 95 



of the first nephridia. There are, then, three diaphragms and not, as so 

 often stated, four; and, while affording valuable evidence of the extent 

 of the first and second chsetigerous somites, they do not help in deter- 

 mining the number of segments which compose the achsetous portion 

 following the prostomium 



Behind the last diaphragm the body-cavity is unsegmented up to the 

 base of the tail. The segmental arrangement of the organs, however, 

 can be recognised by taking the funnels of the nephridia as marking 

 the anterior ends of the somites. The slight amount of connective 

 tissue supporting the long afferent and efferent vessels (segmental 

 vessels) (PI. VII., Fig. 5) of the nephridia and gills, may be regarded as 

 the remains of the septa. Allied species of Arenicola fully confirm 

 this view. 



At the level of the thirteenth pair of notopodial sacs, the segmental 

 afferent and efferent blood-vessels, which have hitherto run nearly 

 parallel across the coelom, diverge. At the base of the tail, the connec- 

 tive tissue between them increases slightly in amount, septa forming 

 which are continued down to the end of the body (Fig. 5, C. 8p.). 



4. Musculature. 



The muscles of the body-wall are arranged in (1) an outer circular 

 sheath, subdivided in the anterior and middle regions of the body into 

 hoops, which cause the annulation of the skin ; and (2) an inner 

 longitudinal sheath of considerable strength and thickness divided by 

 the nerve- cord and lines of insertion of the notopodial sacs into three 

 parts, two ventrolateral and one dorsal (PL IX., Fig. 23). The inter- 

 muscular spaces are filled by ccelomic fluid, and are probably lined by 

 a delicate peritoneum. 



In the anterior region of the body there are a few circular muscle- 

 bands which are stronger and more obvious than the rest (Fig. 5, 

 M. Girc). 



The oblique muscles, which divide the coelom longitudinally into 

 three compartments, commence behind the thiid diaphragm, and 

 disappear at the base of the tail. These muscles are arranged in thin 

 broad bands, arising at the sides of the nerve-cord, and are inserted 

 right and left into the body-wall at the level of the notopodial sacs. 

 They partly cover the nephridia, and in some specimens a muscle-band 

 is attached to each nephrostome. 



