PHYLUM ANNULATA 



469 



parts of each parapodium 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 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 sur- 

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

 setae, is lodged in a sac, the setigerous 

 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 

 cells, and is to be looked upon as a 

 specially developed part of the cuticle 

 of the general outer suface. 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 para- 

 podium bears very commonly certain 

 soft appendages of a sensory character, 

 the cirri (Fig. 347, dws. 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 ; sometimes they 

 are laterally compressed and leaf- like. 

 In Polynoe (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 seta? 

 round its edge. 



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

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

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

 developed. In certain Oligochaeta seta? are absent. 



The first segment or prostomium, together with the second or 



FK;. 3(i7. Chaetopterus. Natural 

 size of a young specimen. A, an- 

 terior region of the body ; B, middle 

 region ; C, hinder region, c, peri- 

 stomial cirri; it, "sucker" ; , the 

 great " wings " ; /, the first of the 

 three "fans"; /H, mouth. (From 

 Benham, after Panceri.) 



