POLYCHAETA 



271 



abdomen and sometimes by sharp rhythmic contractions and ex- 

 pansions of the body which pump the body in and out of the tube. 

 The great development of the dorsal bands of longitudinal muscle 

 seen in a transverse section of a serpulid (Fig. 186 C) is characteristic 

 of the tubicolous worm. Another typical modification seen in the 

 serpulids and sabellids is the median ciliated groove, which starts from 

 the anus, runs along the ventral surface of the abdomen, turning on 

 to the dorsal surface when the thorax is reached. It serves to conduct 

 the faeces to the mouth of the tube. 



Fig. 188, 'D[2igv2i.m oi Pomatocer OS triqueter \n Its tube. Original. The aperture 

 of the tube is represented in black : the top and base of the tube are shown 

 by vertical lines {tb.), the sides not represented so that the thorax can be seen 

 within. The collar {col.) is shown by stippling, folded back over the top and 

 sides of the tube ; and the thoracic membrane also by stippling. The collar is 

 transparent showing the prostomium and the lip of the tube beneath. The 

 fact that the tube is composed of successive rings is indicated in the neigh- 

 bourhood of the aperture (ann.). ap.tb. aperture of tube; neur. neuropodia; 

 not. notopodia; op. operculum ; />r. prostomium; fen. tentacle. 



Chaetopterus (Fig. 187 B) is probably the most modified of all 

 tubicolous worms. It lives in a parchment-like tube which is U- 

 shaped with at least two apertures. There is a peristomial collar as in 

 other tubicolous worms, but the tentacles are a pair of rudimentary 

 processes. A very complicated mechanism exists for obtaining food, 

 which can be observed by taking a live Chaetopterus from its tube and 

 replacing it within a glass tube of the same calibre in an aquarium. 

 The worm fits very loosely in its tube and there is plenty of room for 

 a current of water to sweep through from end to end. Such a current 

 is maintained by the rhythmical oscillation of the fans (fused noto- 

 podia) of the middle region. Food particles contained in the current 

 are entangled in mucus secreted by the dorsal surface of the anterior 

 region, and ciliated currents, working in grooves in the enlarged 



