PHYLUM ANNULATA 



459 



different regions of the body. Some are exceedingly delicate and 

 hair-like, others needle-shaped, others compressed and sabre-like, 

 others bayonet-like. Very often there is a long, straight, narrow 

 part or handle with which is articulated a terminal blade, or 

 bayonet, or hook. Sometimes the setae are quite short, projecting 

 little beyond the parapodia, and are hook-like or comb-like. 

 Usually each bundle contains, in addition to the ordinary setae, 

 a stouter, straight, simple seta, which scarcely projects on the 

 surface ; this is termed the aciculum. Each seta, or each bundle of 

 setae, is lodged in a sac, the setigerous sac (Fig. 375), formed by an 



ristlenb 





FIG. .'576. Polynoe extenuata. Dorsal view of anterior extremity, dors. cirr. dorsal cirri; 

 el. elytra ; perist. tent, peristomial tentacles ; prcest. prostomium. (After Claparede.) 



invagination of the integument, and lined by cells continuous with 

 the epidermis. Each seta is derived from one of these cells, and is 

 to be looked upon as a specially developed part of the cuticle of 

 the general outer surface. The setigerous sacs are usually provided 

 with protractor and retractor muscles, by the action of which the 

 setae may be thrust out or retracted. 



In addition to the setae the parapodium bears very commonly 

 certain soft appendages of a sensory character, the cirri (Fig. 353, 

 dors, cirr., vent. cirr.). There are usually both dorsal and ventral 

 cirri, the latter nearly always much smaller than the former. The 



