242 EDWIN S. GOODRICH. 



pharynx to segments 8 to 14, and the front edge of the shield 

 {v. retr.) on either side of the nerve-cord. A retracted specimen 

 cut in half vertically (fig. 18) shows these muscles plainly ; 

 dorsally and ventral) y they are attached to the smooth areas 

 interrupting the intersegmental grooves on the outer surface 

 of the worm. They are the chief retractor muscles of the 

 anterior region of the body ; the sharp bend in the dorsal 

 retractors (fig. 18) is due to the pressure of the viscera. This 

 figure also shows the position of the prostomium and pharynx 

 in a retracted specimen, and the nerve-cord bent back at a 

 sharp angle. It will be noticed that owing to the length of 

 the nerves running to the body-wall from the ventral surface 

 of the nerve-cord, the latter organ does not closely follow the 

 curve and folds of the body-wall, and thus is possibly saved 

 from injury during the rapid process of retraction. 



The fourth set of retractor muscles are seen in fig. 17, and 

 in fig. 19 a retracted Sternaspis cut horizontally above the in- 

 trovert. These are narrow muscular ribbons running from 

 the posterior margin of the fifth segment to the grooves 

 between the 7th, 8th, 9th, 10th, 11th, and 12th segments 

 (lat.retr.). 



It is obvious that all these muscles, by co-ordinate and suc- 

 cessive action, form a very perfect apparatus for the acrembolic 

 introversion of the first seven segments. 



Posteriorly the rectum is provided with paired dorsal re- 

 tractors (figs. 17 and 18, d. red. m.), and with paired ventral 

 retractors (figs. 17 and 19, v. red. m.), attached to the ventral 

 shield. 



One of the functions of the ventral shield seems to be to 

 act as a sort of fulcrum for the attachment of the main re- 

 tractor muscles. 



The ten large lateral bundles of chsetse set round the sides 

 of the shield are provided with protractor muscles, and with 

 peculiar slender retractors attached above the nerve-cord 

 {retr. lat. ch., fig. 19). No such muscles belong to the small 

 posterior bundles of chsetse as figured by Vejdovsky. They 

 have small retractors attached to the shield. 



