154 PROCEEDINGS OF THE ACADEMY OF [1897. 



injury after it has withdrawn into its shell, which it does upon the 

 slightest disturbance. 



There are two body regions — thorax and abdomen. The pro- 

 stomium and peristomium are not sharply separated from the suc- 

 ceeding metameres, but are coalesced into a buccal somite. The 

 mouth is circular in form and is placed between two semicircular 

 plate-like areas. The anus appears as a mere slit. 



The metameres arise through a process of cell-division from the 

 primitive layer of the undifferentiated mesoderm. The constriction 

 of the metameres begins in the trochophore stage of the worm. The 

 regions or layers of a metamere consist of the following : epithelium 

 or ectoderm, body sac (taking the place of the solid primitive mes- 

 oderm), intestinal epithelium or endoderm and mesenchyme cells 

 scattered between these layers. 



The parapodia are slightly developed. The upper bear hair-like 

 setse ; the lower consist of transverse ridges. In the adult worm 

 there are three groups of falciform seta? placed on either side of the 

 anterior thoracic region, each group containing from 6 to 12 setae. 

 These seta? arise from setigerous glands which are folliclesformed from 

 proliferation of the outer epithelial cells. Each seta is developed 

 from one of several structural cells. In its growth it breaks through 

 the follicle and extends from here over the surface of the surround- 

 ing epithelium. The follicle also projects into the body cavity and 

 is furnished with muscle fibres. 



Between the intestine and body-wall is found a cavity filled with 

 fluid ; this coelum or body cavity arises from the mesoderm by 

 cleavage. It is divided into as many consecutive chambers as there 

 are metameres (14 to 20). 



In the alimentary system three divisions may be distinguished, 

 fore-gut, mid-gut and hind-gut. The pharynx is non-protrusible. 

 The fore-gut proceeds from the stomodeum of the larva and its 

 epithelium is of ectodermal origin ; it is very much reduced in size. 

 The mid-gut is developed from the mesenteron of the larva ; its 

 epithelium is endodermal. This portion of the alimentary tract is 

 the principal digesting portion, and presents two regions, the ante- 

 rior or stomach intestine is wide, while the posterior is long and 

 narrow. The hind-gut is short and is developed from the procto- 

 deum of the larva. 



The enteric canal is surrounded for its whole length by a blood 

 sinus. Each branchial filament is penetrated by one blood vessel. 



