1 3 8 REEF-B UILDING CORALS. PART in. 



as the Astrsea, which constitutes a portion of the reef- 

 building corals of the tropical seas. They form groups, 

 in which the whole of the polypes, except their starry 

 summits, are soldered or pasted together by a living 

 viscous substance, consolidated by carbonate of lime, 

 abstracted from the sea-water, so that the resulting 

 coral frequently becomes a rounded mass, the surface of 

 which is more or less covered with stars, which may be 

 circular or a-ngular, large or small, deeply set or promi- 

 nent, according to the genera or species, both of which 

 are exceedingly numerous. In fact the forms produced 

 vary according as the buds spring from the base of the 

 polypes, from the sides of the cylindrical body, from 

 the summit or disk, from the limits of these three parts, 

 or from the whole animal. In all these varieties the 

 buds are the result of a superabundance of vital activity 

 in the part. When the buds proceed from the sides of 

 the polypes the corals are rounded masses ; but when they 

 spring from the disk or cups of the star, the consequence 

 is the death of the parent polypes, and the development 

 of a new layer of living individuals above the dead ones. 

 No part of the new polypes is seen except their stars, 

 their bodies being enclosed in the common tissue. As 

 this process may be continued indefinitely, the coral may 

 increase to any size ; but the size becomes still greater 

 when successive buds are formed over every part of the 

 polypes, and when all the successive generations are 

 soldered together by the common tissue. In every case 

 the polypes are alive only on the surface where they have 

 free access to light, heat, and air, which is furnished by 

 the sea- water in which they live. 4 



In the reef-building corals the living viscous substance 

 that covers the surface and connects the polypes into a 

 mass, is in process of time so completely consolidated by 



4 ' Histoire des Corallines,' par Professeur Milne-Edwards. 



