POLYCHAETA. 



461 



curved hook (Terebelliformia, Sabelliformia). When the parapodia 

 are completely wanting, the setae project from the body -wall 

 (Capitella). 



K 



a. c d f / m 3 



FIG. 365. Setae of different Polychaeta (after Malmgren and Claparede). o, hooked setae of 

 Sabella crassicornis ; b, of Terebella Danielsseni; c, seta with spiral ridge from Sthenelais; 

 d, lance-shaped seta of Phyllochaetopterus ; e, of Sabella crassicornis ; f, of Sabella pavonis ; 

 g, composite, sickle-shaped seta of Nereis oultrifera. 



The appendages of the parapodia present a great variety of form, 

 and not unfrequently vary in the different parts of the body. Most 

 important are the cirri, which are attached to the dorsal and. ventral 

 surfaces of the parapodia. The cirri are for the most part filiform, 

 and sometimes pointed ; 



they may also be ringed. 1 , ft 



In some cases the dorsal 

 cirri are flattened out as 

 broad scale-like structures 

 the elytra (Fig. 366) 

 which constitute a protec- 

 tive covering to the back 

 (Apliroditidae). In other 

 cases the dorsal cirri* are 

 modified as branchiae, which 

 may be filiform, branched 

 and antler - like, comb - 

 shaped, or in tufts ; some- 

 times they are confined to 

 the middle region of the 

 body (Arenicola, Fig. 397), 



FIG. 366. Anterior end of Polynoe extunuata, the first 

 elytron on the left hand being removed (after 

 Claparede). The two setae of the oral segment aie 

 visible. El elytron. 



* It sometimes happens that cirri are found on branchiferous segments, in 

 which case the gills must be regarded as additional to cirri. 



