8 ARTHUR E. SHIPLEY. 



The fibres of the general body wall are arranged in an 

 external circular and an internal longitudinal layer^ separated 

 by an exceedingly delicate layer of oblique fibres. This latter 

 can only be seen in surface views^ as, owing to its extreme 

 thinness, it is difficult to detect in sections. 



The circular muscles commence behind the collar fold, 

 where they form a series of rings round the introvert, one 

 lying beneath each ring of hooks (fig. 1). Posteriorly to the 

 hook-bearing region the circular fibres form a continuous 

 sheath, which extends to the posterior end of the animal 

 (fig. 22). 



The longitudinal fibres form a complete sheath round 

 the introvert, commencing anteriorly just behind the attach- 

 ment of the collar. At the posterior extremity of the intro- 

 vert these fibres separate into longitudinal bundles, generally 

 about twenty-two in number, which run parallel with one 

 another down the trunk. In passing backward these bundles 

 gradually fuse with one another, and so become fewer and 

 larger, till near the '^tail^^ they form a series of projecting 

 ridges, giving to a section of the body-cavity in this region a 

 characteristic star-shaped appearance (fig. 13). At the poste- 

 rior extremity of the body the bundles finally unite. The lon- 

 gitudinal bands occasionally give off side branches, which pass 

 into the adjacent bands (fig. 22). 



The retractor muscles of the proboscis arise by a common 

 origin from a kind of dissepiment, stretching across the body 

 at the level of the origin of the mantle fold, and just behind the 

 skeletal tissue of the collar (fig. 9). Almost immediately after 

 their origin they split into two bauds, which pass backwards, 

 one on each side of the oesophagus, for about half its length. 

 Each lateral band then again divides into two branches, a 

 shorter dorsal and a longer ventral branch, which run to the 

 body wall, where they fuse with the adjacent bands of longitu- 

 dinal fibres. The ventral bands, being longer than the dorsal, 

 are attached to the body wall behind these, lying one on each 

 side of the nerve-cord, and being connected by the generative 

 ridge. The posterior ends of the retractor muscles are fan- 



