188 



COMPA PA TI VE A XA TOM Y 



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

 dmary degree, and are of the greatest significance for classification. Set* are 

 wanting only m a few Chcetopoda, as in the EnchytrccUUc (Anachata) and in 

 brachiobdella among the Oligockceta, in the so-called Archianndida (Polygordius 

 'odrilus, etc.), and in the Tomopteridv. In Chvtogastcr the dorsal rows of seta' 

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

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

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

 parts, the setre being arranged in Pcrichccta in 

 a ^single row round the segment. The setre 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 Oligochceta 

 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. Scr- 

 pididce) 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- 

 ages (Fig. 124). These are the cirri, 

 unsegmented or segmented filaments, one 

 of which, in the simplest cases, occurs on 

 each parapodium. We can thus dis- 

 tinguish dorsal and ventral cirri. The 

 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 



Fi<;. ii>4. Eunice limosa (after 

 Ehlers). Anterior and posterior ends 

 of the l>ody ; dorsal side, fa, Unpaired 

 feeler;//*, paired feelers; a, eyes;/', 

 feeler-cirri; /;, gills; pc, dorsal para- 

 podial cirri; p, parapodia; c, anal 

 cirri. 



