PHYLUM ANNULATA 



431 



Sometimes the setse are quite short, projecting little beyond 



the parapodia, and are 



hook-like or comb-like. \ 



Usually each bundle 



contains, in addition to 



the ordinary setee, a 



stouter straight simple 



seta, which scarcely 



projects on the sur- 

 face : this is termed 



the aciculum. Each 



seta or each bundle of 



seta? is lodged in a sac, 



the setigerous sac (Fig. 



339). formed by an 



invagination of the in- 

 tegument, 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 sur- 

 face. The setigerous 



sacs are usually pro- 

 vided with protractor and retractor muscles, by the action of 



which the seta? may be thrust out or retracted. 



In addition to the seise 

 the parapodium bears very 

 commonly certain soft ap- 

 pendages of a sensory char- 

 acter, the cirri (Fig. 319, 

 dors, cirr., vent, cirr.) There 

 are usually both dorsal 

 and ventral cirri, the latter 

 nearly always much smaller 

 than the former. The cirri 

 are usually filamentous ; 

 sometimes jointed ; some- 

 times they are laterally 

 compressed and leaf-like. 



Fio. 33n.-Section of the setigerous sac of an Oligo- I 11 Pofynde (FlgS. 336 and 



chsete. 61, setigerous sac; // 2 , supplementary 340) aild its allies Certain 



follicle with seta ; e, deric epithelium ; Im, longi- /* i i i 



tudinal muscles of body wall ; m, m, muscles of OI the parapodia bear, 



the setigerous sac; rm, circular muscular layer , i f i 



of body wall. (From Hatschek, after Vejdovsky). instead OI dOr^ai Cim, 



Fio. 338. Setae of various Polychseta. (From Claparede.) 



