188 



COMPARATIVE ANATOMY 



CHAP. 



The shape, nuniher, and arrangement of the setse vary in details to an extra- 

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

 wanting only in a few Chcetqpoda, as in the Eiuhytrceidce (Anachceta) and in 

 BracMohdella among the Oligochceta, in the so-called Archiannelida {Polygordius, 

 Protodrilus, etc. ), and in the Tomopterid/z. In Chcetogaster the dorsal rows of setse 

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

 In the stationary PolycTueta each ventral bundle of setse is developed in the form of 

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



parts, the setfe being arranged in Perichmkc in 

 a single row round the segment. The setfe may 

 disappear in certain regions of the body. They 

 are specially reduced in number in the Schhtridce 

 (see systematic review). 



The bundles of setee 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 

 Fohjchcda. 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. Ser- 

 ^^ pulidca) 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- 

 FiG. 124. -Eunice umosa (after ages (Fig. 124). These are the eirri, 

 EMers). Anterior and posterior ends unsegmented or Segmented filaments, one 



of the tody; doi-sal side, /a, Unpaired ^f ^y^j^ -j^ ^j^^ simplest CaseS, OCCUrS On 

 feeler ; fp, paired feelers ; a, eyes ; /c, ' ^TT i t 



feeler-cirri ; t, gills ; pc, dorsal para- each parapodlUm. We can thUS dlS- 



podiai cirri; p, parapodia; ac, anal tinguish dorsal and Ventral cirrl. 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 vnth 



