10 F. W. GAMBLE AND J. H. ASHWORTH. 



of setal sacs occur, caused by the forward shifting of the upper 

 edge of the first diaphragm (fig. 5). The second and third are 

 inserted both above and below, opposite the second groove 

 behind the second and third chsetigerous annuli. Between the 

 first and second diaphragms, dorsal and ventral mesenteries 

 occur, supporting the corresponding vessels; and it will be 

 noticed that the dorsal mesentery ends in front, exactly where 

 the first diaphragm would be inserted if it corresponded with 

 the other two. The third diaphragm is perforated by the 

 funnels of the first nephridia. There are, then, three diaphragms 

 and not, as so often stated, four, and, while aff"ording valuable 

 evidence of the extent of the first and second chsetigerous 

 somites, they do not help in determining the number of seg- 

 ments which compose the achaetous 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 

 aff"erent and eff'erent vessels (segmental vessels) (PI. 2, 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 afi'erent and eff'erent blood-vessels, which have 

 hitherto run nearly parallel across the coelom, diverge. At 

 the base of the tail, the connective tissue between them in- 

 creases slightly in amount, septa forming which are continued 

 down to the end of the body (fig. 5, C. Sp.). 



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 



