PHYLUM ANNI'LATA 



469 



part.s of each parapodiuni of a single worm, or in parapodia of 

 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 sette are quite short, projecting 

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

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

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

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

 settB, is lodged in a sac, the sefigcivus 

 sac (Fig. 369), formed by an invagina- 

 tion of the integument, and lined by 

 cells continuous with the epidermis. 

 Each seta is derived from one of these 

 colls, and is to be looked upon as a 

 specially developed part of the cutich? 

 of the general outer snface. The 

 setigerous sacs are usually provided 

 with protractor and retractor muscles, 

 by the action of which the set;e may 

 be thrust out or retracted. 



In addition to the setae the para- 

 podium bears very commonly certain 

 soft appendages of a sensory character, 

 the cir7-i (Fig. 347, dors, cirr., vent. 

 cirr.). There are usually both dorsal 

 and ventral cirri, the latter nearly 

 always much smaller than the formei-. 

 The cirri are usually filamentous, 

 sometimes jointed ; sometimes they 

 are laterally compressed and leaf-like. 

 In Pulynoe (Figs. 365 and 370) and 

 its allies certain of the parapodia 

 bear, instead of dorsal cirri, flattened 

 scales, the elytra {el.), richly supplied 



with nerves : these are sometimes looked upon as modified dorsal 

 cirri, but in some members of the group cirri and elytra occur 

 together on the same segment. 



In Sternaspis a ventral shield formed by a thickening of the 

 cuticle in the posterior region of the body bears a number of sette 

 round its edge. 



In the Oligochseta (Fig. 372) the parapodia are absent as pro- 

 cesses of the body-wall, and are merely represented by a small 

 number of short seta; each lodged in its sac; cirri are not 

 developed. In certain OligochEeta setiB are absent. 



The first segment or 2}'>'ostomium , together with the second or 



Fig. 367.— Chaetopterus. Natural 

 size of a. young specimen. A, an- 

 terior region of the body ; B, middle 

 region ; C, hinder region, c, peri- 

 stomial cirri; d, "sucker" ; e, tlie 

 great "wings"; /, the first of tlie 

 three "fans": lu, . month. (From 

 Benhani, after Panceri.) 



