188 



COMPARATIVE ANATOMY 



CHAP. 



The shape, number, and arrangement of the setae vary in details to an extra- 

 ordinary degree, and are of the greatest significance for classification. Setae are 

 wanting only in a few CJuetopoda, as in the Enchytrceidce (Anacfueta) and in 

 Brachiobdclla among the Oligockceta, in the so-called Archiannclida (Polygon? in*. 

 Protodrilus, etc.), and in the Tornopteridce. In Chtctoyaster the dorsal rows of setae 

 are wanting ; and in Saccocirrus also only one longitudinal row is found on each side. 

 In the stationary Polychceta each ventral bundle of setae is developed in the form of 

 a transverse row of short hooks. The bundles can break up into their component 



parts, the setae being arranged in Perichceta in 

 a single row round the segment. The setae may 

 disappear in certain regions of the body. They 

 are specially reduced in number in the Echiuridce 

 (see systematic review). 



The bundles of setae stand either 

 simply in the integument, or on special 

 elevations of the body wall, the so-called 

 parapodia (Fig. 158, p. 237). The 

 former is the case chiefly in the Oligochcda 

 and Echiuridce, the latter in most of the 

 Polychceta. The parapodia are well devel- 

 oped as strong rowing and locomotory 

 organs principally in the Errantia, while 

 they are'reduced in the Sedentaria, especi- 

 ally the ventral parapodia, which are for 

 the most part insignificant ridges carrying 

 hooks. In a few tube-worms (e.g. Scf- 

 pulidcc} the parapodia are entirely obliter- 

 ated, no doubt in adaptation to the 

 tubicolous manner of life. They are also 



wanting in the Archiannelida. We do 

 not always find separate ventral and 

 dorsal parapodia ; there is often on each 

 side only one row of parapodia. We 

 then, however, find in each parapodium 

 a dorsal and a ventral branch. Whether 

 the uniserial or the biserial arrangement 

 is the original cannot yet be decided. 



In the Polychceta the parapodia them- 

 selves again carry characteristic append- 



ac 



FIO. V24.- Eunice iimosa (after ages (Fig. 124). These are the eirri, 

 Ehiers). Anterior and posterior ends uiisegmented or segmented filaments, one 



of the body; dorsal side, fa, Unpaired Q ^j^ j n the simplest cases, OCCUrS Oil 

 feeler ; fp, paired feelers ; a, eyes ; fc, ,. , , , 



feeler-cirri ; fc, gills ; pe, dorsal para- each parapodium. We can _ thus CilS- 



podiai cirri; p, parapodia; ac, anal tinguish dorsal and ventral cirri. The 

 cirri - cirri may undergo the most varied trans- 



formations. Thus the dorsal cirri, or their lateral branches, frequently 

 become gills, which are often delicately branched and provided with 



