CORALS. 



707 



Fi{), 16. — Brain Coral (Maeandrina). 



a s cf the colony never forming more than a thin layer over the 



The skeleton of the Brain Coral (Fig. IG), which is distinguished by the 

 systems ot ridges and furrows winding over its surface, is built up in 

 successive layers like that of the Star Coral, 

 but the polyps multijily in a different 

 manner. Each long furrow denotes the 

 former home of a large polyp, which formed 

 a whole row of new mouths without dividing 

 up into new individuals. Here, therefore, 

 we have incomplete division of the 

 individuals. This furrow was fringed by 

 tentacles which rose above the ridges. After 

 a certain stage of elongation of the polyp was 

 reached, the body completed itself, as it 

 were, with a new row of tentacles between two of the mouths, and so a new 

 individual was started. 



While budding and division play such a large part in increasing the 

 number of polyps forming a colony, each new colony itself is started by 

 another method of reproduction. Eggs, which form on 

 the internal partitions or septa of the polyp, give rise to Development of 

 minute larvae that swim about, first within the body of Corals, 



the parent, and then in the open water, sometimes for as 

 long as two months. Then they attach themselves by one end to a rock or 

 some other surface, and begin to assume the polyp shape, tentacles and 

 mouth developing at the free end of the body, and the hard parts form as in 

 the parent animal. Such polyps, by budding and division, found new 

 colonies. 



All the skeletons we have as yet mentioned are more or less massive or 

 stony, as is usually the case with the skeletons of the six-rayed polyps (hence 

 the name stony corals), but some colonies of this division 

 secrete horny instead of chalky skeletons. The skeletons The Black Corals 

 of the Black Corals or Antipatharia form branching tufts {Antipatharia). 

 or trees of horny substance. The surface of the branches 

 appears smooth and polished, and, in some of the larger species, resembles 

 ebony. These corals grow to a great height as single trunks, or as tree- 

 like growths ; sometimes the separate branches fuse to produce networks. 

 In such coral skeletons there are none of the traces of the former presence 

 of polyps such as we see on the surface of the stony corals, because the 

 branched stem is merely the central axis of the living colony that produced 

 it and once covered it with a soft crust. Out of this crust the individual 

 polyps protruded at intervals. The hard, rod-like skeletons of the Black 

 Corals are used by natives in various parts of the world for dagger handles, 

 necklaces, or mouthpieces for pipes, on account of their toughness and 

 susceptibility to polish. 



The eight-rayed polyps differ in several ways, too technical for us to enter 

 upon, from the six-rayed polyps. The eight tentacles of the individual 

 polyps are toothed or feathered, and form a more 

 imposing crown than do the simple cylindrical tentacles 

 of the six-rayed order. The hard parts produced by 

 themj in the majority of cases, take the form of a central 

 axis like that found in the Black Corals, but here of chalk or horn, or of 



The EigKt-rayed 

 Corals (Octaclhiiii). 



