48 PEARLS [CH. 



of the inorganic and organic worlds — are veritable 

 " marine biological stations " in themselves. Formed 

 largely of the remains of corals, foraminifera, sponges, 

 calcareous algae and sand, the}^ are burrowed into 

 and provide homes for tiny crabs and shy Crustacea. 

 Worm tubes, in which live beautiful tube-worms, 

 wind about the surface. A patch of a colonial 

 polyzoan, allied to those incrustations often found 

 on seaweeds round our coasts, and perhaps a dozen 

 or so pearl oysters hanging on by the byssus may 

 complete the catalogue of some of the more con- 

 spicuous organisms. 



That arch enemy of the mollusc all the world 

 over, the starfish, is certain to be found creeping 

 about amongst its fare, and sea urchins and sea 

 cucumbers (close relatives of the starfish and not 

 vegetables, as the name might imply) represent the 

 Echinodermata. 



The well-known seaweed-covered rocky beds of 

 our own coasts are very different. The seaweeds in 

 these tropical waters are enclosed in lime and might 

 well be mistaken for non-living things. In many 

 cases the plants live enclosed in the bodies of the 

 animals ! Indeed, in these tropical regions where 

 the nitrogenous compounds so essential to plant life 

 seem to be deficient, the vegetable cells have been 

 driven to live inside the animals. This phenomenon 

 — of plant animals — is represented of course in our 



