WORMS 241 



else (in the tube-dwelling forms) crowded on the head 

 of the animal. These Polychaetes may be found in 

 most unexpected places, one species living in the top- 

 most coils of a dead shell inhabited by a hermit-crab, 

 another in the groove on the underside of the common 

 sandstar, and yet another clinging closely to the under- 

 side of a species of sea-cucumber. The tube-building 

 forms display an endless variety in their habitations. 

 The tough parchment-like tubes of Chsetopterus and 

 Hyalinoecia (Fig. 176) are buried in sand or mud, others 

 are made of sand or broken shells, some roughly put 

 together, and some built up with exquisite neatness 

 and symmetry. Many species secrete calcareous tubes 

 of various shapes, usually fastened to stones or shells, 

 and closed by a stopper which the inhabitant carries 

 attached to his head. The feathery plumes which 

 many of these tube-dwellers bear as gills are among the 

 most attractive sights of an aquarium, where alone they 

 can be seen to full advantage (Fig. 177). Perhaps the 

 most graceful are those of Sabella and its allies, whose 

 tubes, built largely of mud, have the appearance and 

 texture of india-rubber piping. Though most of the 

 free-living Polychaetes are unable to swim, there are a 

 few entirely pelagic species ; and some of the normally 

 creeping forms, when the breeding season arrives, 

 become entirely altered in appearance, developing 

 swimming-lobes along the sides of their bodies, and come 

 to the surface in immense swarms. The " Palolo " of 

 Pacific islands, which, leaving its shelter in the coral 

 reefs, suddenly appears in countless numbers on the 

 shore, is a well-known instance of this change of habit. 

 The huge group of the Crustacea is one which perhaps 

 attracts more attention than any other. In number 

 of species it far exceeds any other group of marine 

 animals, but most of these are of very minute size 



16 



