CORALS. 



119 



lime for vital functions takes place continually, and with equal pace the live portion of 

 the original individual mounts higher and higher until a branch, similar to that with 

 which we started, is eventually formed. It can thus be seen that there is in the 

 branch of coral a larger individual, which is the original polyp with which the 

 colony starts its growth, and smaller individuals, or lateral buds, which have from time 

 to time, as the first polyp grows, budded out on its base and sides. When, as often 

 happens, a lateral branch is protruded froqi the sides of another branch, we simply 

 have a bud which partakes of the character of the larger individuals rather than its 

 neighboring lateral buds. It grows by the same laws as the branch from which it 

 originates, and as in the first, so in this, small lateral buds which resemble those on the 

 parent branch are freely formed upon it. 



A second species of Madrepora, although departing widely in general form from 

 the first, is closely allied to it in generic characters. This species is known as pal- 

 mata, and instead of a branching habit, occurs in flat, massive slabs, on the upper 





mi 



Figs. Ill and 112. — IIeliastrcea Jieliopora, traiu-coral, -with and without fleshy parts. 



surface of which the different individuals are irregularly distributed. Blocks of such 

 fragments generally grow at greater depths than M. ceroicornis, and when water- 

 worn form the so-called coral boulders which make the foundations upon which the 

 reef rests. They play no small part, by their solidity, in the successful resistance of 

 these islands to the encroachments of the sea. 



Some of the most massive genera of Actinoid corals are those known as the 

 "brain-corals," or "brain-stones." There are several genera which are commonly 

 confounded in this nomenclature, and all such have many similar anatomical peculiari- 

 ties. The genus Diploria assumes a hemispherical shape, varying in size from a few 

 inches to several feet in diameter. Its external surface is covered 

 with serpentine furrows, which recall vividly the convolutions of 

 the brain, and probably suggested the name of brain-stones for 

 these corals. Over the curved surface of a live brain-stone is 

 stretched the soft organic parts of the coral, while in the super- 

 ficial furrows lie the stomachs, which have a similar serpentine 

 form to the convolutions in which they lie. Upon the surface 

 of the brain-stone, arranged in lines, are found rows of mouths, 

 each opening into that stomach or part of the stomach which lies 

 just beneath them. 



Another genus allied to Meandrina and Diploria is known as Manicina. This 

 genus is not commonly as large as either of the former, and does not have the brain- 

 like shape. Its upper surface, however, has the same furrows as the genera above 



Fig. 113. — Three 

 mouths of Heli- 

 aatrma, enlarged. 



