THE ANATOMY OP PENTASTOMUM TERETIUSCTJLUM. 15 



as to cause it to project from the body-wall. In this action 

 it is assisted by the two following muscles. 



(d) Ventral Protractor. — This is the equivalent of the 

 m. protractor ventralis of Stiles, and the adductor basis 

 unci of Hoyle. Its fibres are attached to the posterior end 

 of the basal joint, and pass outwards and slightly downwards 

 to be inserted into the body-wall. Its action is to pull the 

 whole apparatus outwards, and to cause the point of the hook 

 to be pulled slightly towards the median line. 



(e) Dorsal Protractor. — This is the equivalent of the 

 m. protractor dorsalis of Stiles, and of the protractor 

 basis unci of Hoyle. It is a strongly developed muscle, and 

 arises from the posterior end of the basal joint, nearer to the 

 ventral surface than the last-named muscle. Its fibres run 

 upwards and forwards to be inserted into the dorsal surface of 

 the head, and its action is to pull the hook apparatus slightly 

 outwards, but more still to cause the point of the hook to move 

 towards the ventral surface. 



(/ and ^) Internal and External Retractors. — These 

 are equivalent to the mm. retractores externi et interni 

 of Stiles, and form a series of muscular bands, which arise partly 

 from the ventral posterior end of the basal joint and partly 

 from the basal chitinous thickening. They form strong bands 

 running backwards, the internal ones towards the middle line, 

 the external ones towards the side of the body, to be inserted 

 into the body- wall. Their action is that of retracting the 

 whole apparatus. 



(A) Ventral Retractor. — This is equivalent to the m. 

 retractor or bursse ventralis of Stiles. It arises from the 

 sides of the basal thickening of the hook invagination, and 

 passes outwards to be inserted into the body-wall. It acts as 

 a retractor. 



5. Alimentary Canal. 

 (Figs. 27, 33, 50, 59.) 

 The following five divisions may be distinguished in the 

 alimentary canal : — (1) The mouth with the oral papilla or 



