HABITS AND STRUCTURE OF AHENIOOLA MARINA. 11 



insertion of the notopodial sacs into three parts, two ventro- 

 lateral and one dorsal (PI. 4, fig. 23). The intermuscular 

 spaces are filled by coelomic 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. Circ). 



The oblique muscles, which divide the coelom longitudinally 

 into three compartments, commence behind the third dia- 

 phragm, 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. 



The musculature of the buccal mass consists of a strong 

 sheath of fibres derived from the longitudinal layer just behind 

 the first diaphragm. This sheath, which is loosely attached to 

 the proboscis by slips which run through the coelomic space 

 between the two structures (PI. 3, fig. 6, B. Sh.), is inserted into 

 the anterior part of the proboscis. Pressure of the coelomic 

 fluid at this point causes eversiou of the buccal mass, which 

 is withdrawn by the contraction of its muscular sheath. 



The prostomium is retracted by a small sheet of muscle 

 which arises partly from the longitudinal layer dorsally, and 

 partly from the muscular covering of the circumcesophageal 

 connectives ventrally, and it is inserted into the ventral surface 

 of the brain, and the ventral and hinder edge of the nuchal 

 organ (PI. 3, fig. 6, iVw. Tr.). 



The parapodial muscles are modifications of the longitudinal 

 layer. One, the retractor of each notopodium, is remarkably 

 long, reaching to the side of the nerve-cord (PI. 3, fig. 13^ 

 Rn.). The protractors (Pn.) of the notopodia are six to eight 

 in number, three to four being placed in front of, and three 

 to four behind, the setigerous sac. They arise from the body- 

 wall just below the dorsal longitudinal vessel, and are inserted 

 into the base of each sac. 



