268 



PHYLUM ANNELIDA 



one. In Mynanida a long chain of sexual zooids is formed. In this 

 way a regular alternation of sexual and asexual generations may arise. 

 Some Polycha)ta dwell habitually within tubes, others are at least at 

 times active and free-living. The latter have usually well-developed 

 parapodia and sense organs, the anterior part of the gut may be 

 furnished with strong jaws, the body is more or less uniform, and the 

 worms are carnivorous. These forms are all included in the sub-order 

 Nereidiformia, which embraces such familiar animals as the common 

 sea-mouse (Aphrodite), with its mass of iridescent bristles covering the 

 sides of the body, the species of Nereis and Ncphthys, so common 

 under stones on the shore, and others equally remarkable for beauty of 



Fig. 145. — Free-living Polychaete {Nereis cultrifera). 



Note, as compared with Arenicola, the absence of gills, and tlio vvcll- 

 developed parapodia, which are absent from the peristomium (pe.), 

 or first true segment. The prostomimn bears eyes {e.), the small 

 tentacles (t.), and the large palps (p.) ; c, the four paired cirri, 

 borne by the peristomium ; a., the anus, with two long cirri. 



colour. The bright colours may be due to the iridescent cuticle or to 

 pigments. There are a few transparent pelagic forms, e.g. Tomopteris. 

 The sedentary forms lead a sluggish life within various kinds of 

 tubes — limy, sandy, papery, or gelatinous. They are not nearly 

 related, but possess in common certain adaptive characters, such as the 

 aggregation of gills, cirri, tentacles, and sense organs to the anterior 

 exposed part of the body ; the reduction of the parapodia, often used 

 solely for clambering in the tube ; the absence of " jaws," and the 

 habit of feeding on minute Algae or other organisms suspended m 

 water. Among these are included Serpula, which forms twisted limy 

 tubes outside shells and other marine objects ; the aberrant Sabellaria. 

 which often builds reefs of porous rock formed of the aggregated sandy 



