146 MARINE INVERTEBRATES 



ORDER MADREPORARIA 



STONE- OR REEF-CORALS 



The coral resembles the sea-anemone and is a polyp con- 

 structed on the radial plan (page 113). It lives in colonies, but, 

 unlike the hydroid colonies, each polyp of the community is a 

 complete organism, and in the reef -building corals all the indi- 

 vidual polyps of a colony are alike. The home of each animal is 

 called a corallite, and the aggregation of many corallites is a 

 corallum. 



The corallite is composed of carbonate of lime secreted by the 

 polyp, and, broadly speaking, may be called a skeleton. The 

 secretion forms a basal plate and radiating partitions between 

 the mesenteries in the cavity of the animal, and also surrounds 

 the polyp like a cup. In some species this is a solid substance ; 

 in others it is like a network through which the animal sub- 

 stance (ccenosarc) of the zooids of the colony is connected, as in 

 Madrepora. 



The polyps reproduce by budding and by self-division (fission), 

 in a manner similar to that of sea-anemones. The way in which 

 the budding or the fission takes place determines the shape of 

 the colony, or corallum, which has a great variety of forms. In 

 some species the budding is confined to certain individuals of the 

 colony. In this case the branched forms result. When growth 

 takes place by fission, hemispherical masses are formed, which 

 are often perfectly symmetrical, as in the so-called brain-corals. 

 In Astrcea the polyps are inclosed separately, but in Meandrina 

 fission is confined to the upper half of the polyps, so that a com- 

 plex polyp is formed, with several mouths opening into a common 

 stomach, making long serpentine furrows on the corallum. 



Some of the genera of the second subdivision, the alcyonarians, 

 differ materially from the ordinary idea of corals, as their frame- 

 work is of a horny and more or less flexible material (chitin). 

 Of these are the sea-fans, sea-whips, and sea-pens. The organ- 

 pipe coral of the same subdivision is an example of an exceed- 

 ingly fragile lime structure, while the red coral, Corallina rubrum t 



