NOTES ON THE ANATOMY OF STERNASPIS. 241 



not reach right round, but leave a narrow smooth strip on the 

 mid-dorsal and mid-ventral regions. The posterior bundles 

 of chgetse are set round the lateral and posterior edges of the 

 ventral shield; the lateral bundles are held by small conical 

 parapodia. 



The anterior end of the body is extruded (pleurecbolically) 

 by the contraction of the circular layer of muscles in the hind 

 body. These muscles are disposed in a thin layer, interrupted 

 dorsally and ventrally at the smooth strips already mentioned 

 (fig. 16f d. a. and v. a.). The inner layer of longitudinal 

 muscles, passing from one intersegmental groove to another, 

 lines the body- wall within (figs. 17 — 19, lonff, m.). The muscles 

 which serve to move the large anterior chsetae are disposed 

 as follows (figs. 17 — 20) : — The chsetse are bound together at 

 their inner ends with connective tissue into three bundles on 

 each side. A short thick muscular band joins these three 

 bundles together, and a similar band attaches them to the 

 posterior end of the pharynx. A larger strand of muscles runs 

 forwards to the body-wall near the base of the prostomium, 

 passing along the side of the pharynx (fig. 20). A slender 

 band of muscle, starting from the inner end of the outer bundle 

 of clisetse, passes almost horizontally outwards to the body-wall, 

 where it is attached at the hinder limit of the fifth segment 

 {lat. m.). This muscle draws the ends of the chaetse inwards 

 and towards the body-wall, whilst the other bundles just 

 described push them outwards and towards the pharynx. 

 Each bundle of chsetse is, moreover, provided with proper 

 protractor muscles {protr.). The retractor muscles are joined 

 together in two large bundles {retr.) (not three, as figured by 

 Vejdovsky and Rietsch), which pass backwards to their point 

 of attachment on the anterior edge of the ventral shield on 

 either side. Not only the chsetse, but also the whole anterior 

 region of the body, are withdrawn by these powerful muscles. 



Large strands of longitudinal muscles extend dorsally from 

 behind the prostomium to the median dorsal region, from the 

 anterior seven segments to segments 8 to 14 (figs. 17 and 18, 

 d. retr.). Similar strands extend ventrally from beneath the 



