WORMS, MOSS-POLVPS, SPONGES, ETC. 113 



vantage certain structures that are not present in 

 all the members of the class. These are the ' bird- 

 head processes' (avicularia), little snapping bodies 

 of the general shape of a bird's head, which are 

 found attached to the cells, but of whose real 

 functions we as yet know very little. That they 

 seize by a sort of instinctive movement little aquatic 

 objects has been long since demonstrated, but the 

 wherefore of this action still remains to be as- 

 certained, since the particles thus caught can- 

 not readily be turned to account by the animal. 

 Other forms have in place of these avicularia long 

 lashes or whips (vibracula), which are in almost 

 constant motion, and may, at least in part, serve to 

 keep the colony clean from adhering particles. The 

 movement of both of these or- 

 gans can be easily followed in the 

 field of the microscope. 



A beautiful rosette-formed poly- 

 zoan, Orisia ebumea, whose attach- 

 ment is the frond of the sea-weed, 

 is not rarely found scattered over 

 the sands; it is easily distinguished, 

 apart from its manner of growth, 

 by the calcareous or limy character of its ivory- 

 white habitations. Another white form {Pedicellina 

 Americana), with minute club-shaped individuals, 

 weaves a delicate tracery around the branches or 

 stems of other polyp colonies, hydroids, etc. 



10* 



