MARINE WORMS 



187 



together as to almost entirely cover the surface. The average 

 diameter of these spirals is only about a sixteenth of an inch, and 

 many are so small that a lens is necessary to discern their shape. 

 In general form they closely resemble some of the small species of 

 Planorbis shells that are so common in our ponds and streams, 

 but these latter are the shells of freely moving molluscs, and are 

 generally of a brownish colour. 



FIG. 123. THE SEA MAT (Flustra) 



The minute worms that live within the tubes in question belong 

 to the genus Spirorbis, and are very similar to those of the 

 Serpulce, and their pretty plumed gills may be seen with a lens 

 when a cluster of the tubes is placed in a shallow vessel of sea 

 water. A sharp tap on the table on which the vessel rests will 

 cause the little creatures to suddenly retire into their homes, the 

 entrances to which may then be seen to be closed by an operculum. 



