SPONGES AND CORALS 



guished from the Coelenterates, they have many super- 

 ficial resemblances to corals, and often share the same 

 habitats. Sponges are found in great abundance along 

 coral reefs in tropical waters, in fact such reefs have 

 corals for their main structure and sponges are one of the 

 most outstanding features. Sponges and their extraord- 

 inary natural behaviour will therefore be better under- 

 stood if an account of them is preceded by an examination 

 of corals. 



The sponges we handle in our bathrooms are skeletons. 

 The world's coral reefs consist of skeletons of countless 

 millions of polyps, so that sponges and corals have that 

 in common — they are skeletons of living creatures. In the 

 vertebrates (including man himself) the skeleton form is 

 based upon a backbone. Corals have ''all over" skele- 

 tons : the creatures live inside structures which branch in 

 various directions. Crustaceans like crabs wear their 

 skeletons outside like armour. Sponges are interpene- 

 trated by their skeletons. When they die the hard 

 structure remains : in the corals such skeletons build up 

 the reefs; but in the sponges the structure (although 

 tougher and firmer than the gelatinous ground-sub- 

 stance) can be softened considerably so that the substance 

 is kind to our skins. 



True corals may be roughly divided into two kinds. 

 There is the simpler, more primitive type of coral, form- 

 ing a single calicle or coralite, as in the early Palaeozoic 

 cup-corals and certain existing species which live buried 

 in mud or extend in deep cold water over the sea-bed, 

 never rising to the surface. These corals are found in all 

 seas, from Greenland to the tropics. 



The reef-building corals are more complex, and occur 

 in encrusting masses. But both types are polyps with 

 skeletons which interpenetrate them, strengthening their 

 bodies while they are alive, having fulfilled their rein- 

 forcement functions, remaining in the sea after the polyps 

 have died. 



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