CORALS 149 



GENUS Madrepora 



Some species of these corals appear like branches, while others 

 have flat, low surfaces or fronds ; all of them are covered with 

 innumerable small cylindrical cups Each cup is the home of a 

 polyp, which secretes calcareous matter around its column, and 

 septa between its radial partitions. The corallum, when ex- 

 amined with a glass, looks like meshwork. Through the fine 

 openings of the porous surface, the polyps of the colony are 

 connected by interlacing tubes of animal substance (coenosarc). 

 The polyps of the madrepore colonies are small in comparison 

 with the connecting structure, which becomes very massive ; but, 

 although large, the corallum is fragile, on account of its perfo- 

 rated character and the exceeding delicacy of its parts. 



In species which form incrustations, new polyps arise from 

 tissue which spreads beyond the corallites. In some species cer- 

 tain polyps and corallites increase in length, growing longitudi- 

 nally, and continually form buds around the base. This manner 

 of growth leads to the formation of branches, while the former 

 manner gives lateral extension. Both of these forms of budding 

 may take place at the same time in the same colony. The mad- 

 repores are among the most common of the reef-builders. They 

 form reefs which extend for miles around the Florida Keys. 



M. cervicornis, the stag-horn coral. This is a branching species 

 which attains large size. Its manner of growth is as follows : At the tip 

 of each branch is an individual polyp, which is larger than those which 

 surround the branch. This is the original animal which started the 

 branch, and is the parent from which aH the others on the branch have 

 budded. The large terminal polyp buds around the base ; a surplus of 

 lime also collects at the base and clogs its tissues, so that it no longer 

 can perform the functions of life, and after a certain period the base 

 becomes dead matter. The polyp on the upper end continues to live 

 and rises above the excess of solid matter. At the same time it con- 

 tinues to form new buds. The buds become independent corallites and 

 secrete an excess of calcareous matter at their bases, which cements them 

 to the parent stock and increases the thickness of the branch. Thus, as 

 the original polyp constantly rises and buds, the colony assumes a stem- 

 like form, covered with numerous individuals. Certain polyps on the 

 main stem have the attributes of the parent animal. These start 

 branches, and so the process goes on, and in time the colony becomes 

 much branched and arborescent in form. In the living coral each little 

 polyp is like a minute sea-anemone, having a colored cylindrical body 

 surrounded on its upper disk with numerous tentacles. (Plate XLV.) 



