238 THE INVERTEBRATA 



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 

 notopodia of the tenth chaetiferous segment, carry these strings of 

 mucus to the cup-shaped organ where they accumulate to form a ball 

 of food which is carried forward in a dorsal groove to the mouth. 



Fig. 1 77 . Diagram of Pomatoceros triqueter in its tube. Original . The aperture 

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

 by vertical lines {th.), 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 fact that 

 the tube is composed of successive rings is indicated in the neighbourhood of 

 the aperture (ann.). ap.tb. aperture of tube ; pr. prostomium ; not. notopodia ; 

 neur, neuropodia; op. operculum; ten. tentacle. 



The burrowing Polychaeta 



Arenicola marina (Fig. 178) is the type of a burrowing polychaet and 

 it has a rounded cross-section like an earthworm. In its division of the 

 body into regions, the modification of the parapodia, and the internal 

 anatomy it resembles the tubicolous worms. The prostomium is much 

 reduced, however, without any appendages and there is an eversible 

 pharynx, covered with minute papillae, which is the organ for loco- 

 motion through the sand as well as for feeding. In general form it thus 

 resembles an earthworm ; the chief obvious difference is the presence 

 of gills and parapodia. It is divided into three regions : the anterior, 

 consisting of the peristomium, an achaetous segment, and six seg- 



