88 INTRODUCTION TO ZOOLOGY chap. 



arranged in the form of two spirally curved plates (Fig. 42). 

 Between the bases of these two plates is the mouth. 



Serpula secretes its own tube, which is formed 

 ■ of a calcareous substance given out by the body ; 

 usually several of these white limy tubes are found together. 

 These worms live in rather deep water, with the curved tubes 

 attached to some rock at their lower ends, but with their 

 upper larger ends free. From the upper end projects a pair 

 of often brilliantly coloured, scarlet or blue, fan-like plates of 

 gills, which are kept continually moving, driving a current of 

 water, with the food it contains, into the mouth. When the 



Fia. 44. — Pomatoce,ros triguetra. 



Several attached to a rock. : 



worm withdraws into its tube, the mouth of the tube is closed 

 by a special club-like structure called the operculum (seen 

 projecting between the gills in Fig. 43). 



Pomatoceros triquetra, another very similar 



' form which secretes a curved limy tube, is 



far more common than Serpula. It lives in shallow water 



between tide marks'. ' The gills and operculum resemble those 



of Serpula. 



Spirorbis is another very common, but much 

 piror IS. gjjj^jjgj. £qj.jjj . j|.g ij^tle white spirally coiled tubes 



are very frequently found attached to bladderwrack seaweed 

 and to the shells of other sea creatures. 



