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



protoplasmic medulla and a contractile cortex which stains 

 more deeply. 



The oblique muscles, which divide the body-cavity into 

 three portions, vary considerably in their development in 

 the various species. In this respect A. cristata agrees 

 most closely with A. marina (see dissection of A. cristata, 

 fig. 30, and compare with that of A. marina, PL 2, fig. 5, of 

 our former paper, 1898). In this species the oblique muscles 

 commence behind the third diaphragm, and are present 

 to the end of the tail. They are usually thin, moderately 

 broad bands, arising at each side of the nerve-cord, and 

 inserted right and left into the body- wall at the level of 

 the setal sacs. They partially cover and bind down the 

 nephridia. 



In A. Clap are dii these muscles are present in the 

 same region, but are not so well developed, and they bind 

 down tbe nephridia to a far less extent. They are omitted 

 from the drawing of the dissection in fig. 26. 



Oblique muscles are absent from the anterior portion of 

 A. ecaudata and A. Grubii, in front of the second or third 

 gills. Behind this point they are feebly developed, the 

 muscle bands being very thin and narrow (about j — g mm. 

 wide), but are present to the end of the body. The absence 

 of oblique muscle bands in the nephridial region is a most 

 striking difference between these two and the other three 

 species of Arenicola. In consequence of this absence of 

 oblique muscles in these two species, the nephridia with their 

 nephrostomes, and, in the case of A. ecaudata, the ovarian 

 processes or spermatic sacs, are clearly visible on opening the 

 body (PL 25, fig. 45). 



7. General Anatomy of the Internal Organs 

 (figs. 26, 30,44,45). 



The general arrangement of the organs, as seen after open- 

 ing the body-cavity by a dorsal and median incision, is in all 

 species similar to that of A. marina, except that in i^^^J 



