POLTCELETA. 375 



gills, and also of the setae embedded in prominent parapodia, which 

 serve as aids to swimming. The internal organization, however, is 

 in no way more complicated than that of the Oligochceta. Neverthe- 

 less all these distinctive characters may be less and less marked, 

 and, indeed, so 'completely vanish that it is difficult to draw a sharp 

 line between the Oligochceta and the Polychceta. The parapodia 

 (Capitettidce) and also the setae (Fomopteridce) may be wanting. 



In rare cases, bundles of setae are present on all the segments behind 

 the head ; they are however arranged in a single row and embedded 

 in a single pair of ventral retractile parapodia in each segment. 



FIG. 30-1. Head and anterior body segments of Nereis Dumerilii (after E. ClaparMe). O, 

 Eyes ; P, palps ; Ct, tentacular cirri ; -BT, pharyngeal jaws. 



This arrangement, which is found in Saccocirrus and its allies, pro- 

 bably represents the primitive state, especially as in these animals the 

 character of the nervous system, which lies in the ectoderm external 

 to the dermal muscular envelope, and of the sense organs, which are 

 reduced to two simple tentacles upon the cephalic lobe and to ciliated 

 pits, indicates lower and more primitive conditions. 



In another and very remarkable type, Polygordius Schn. and 

 Protodrilus Hatsch., not only parapodia and setae but also the 

 external segmentation are wanting. The segmentation of this 

 achsetous and externally unsegmented worm is entirely confined 



