CORALS AND CORAL REEFS — VATJGHAN. 193 



and Meandrina on certain genera. One genus of corals in which 

 the corallum forms tall, more or less divided columns, has long, 

 winding series and is appropriately named Dendrogyra. In such 

 series the polyp mouths occur along longitudinal depressions, called 

 valleys, which may be narrow or wide, shallow or deep, and adjacent 

 valleys may be close together with very narrow interspaces or they 

 may be relatively far apart. 



It will be shown in remarks to follow that the growth-form is of 

 much importance in considering the relations of corals to the physi- 

 cal conditions under which they live. The flattish, cushion-shaped, 

 and hemispherical corals, that are attached by wide bases, have the 

 strongest structures; those corals composed of thick plates or thick 

 platelike branches rank next in strength; while those that form 

 thin, erect laminae and slender, long branches are the weakest. 

 Some corals that have rather strong skeletons need to be classed with 

 the corals with weak skeletons, so far as their habitats are con- 

 cerned, for they live either free on the sea bottom or are very weakly 

 attached. 



The corals so far considered are those known as the Madreporaria. 

 Their soft tissues secrete nearly pure white skeletons composed almost 

 entirely of carbonate of lime; there are pitlike calices or valleys in 

 the skeleton; and more or less distinctly radial septa are present. 

 The hard skeleton is called " coral " and this is the kind of " coral " 

 from which coral reefs derive their name. Before speaking of an- 

 other kind of coral, it will be stated that the tentacles of the Madre- 

 porarian corals are either simple (see pi. 17 and text fig. 2) or are 

 bifurcate or trifurcate — they are never pinnate; and it will also be 

 said that in the Madreporarian corals now living, the septa and 

 mesenteries are arranged on a plan of six or in multiples of six, ex- 

 cept where the plan has been obscured by fission. Because of this 

 arrangement of septa and mesenteries, this group of corals is called 

 Hexacoralla. Ages ago, geologically speaking, the predominant 

 corals had their septa arranged on a basal plan of four or multiples 

 of four and these have been called Tetracoralla, the other highest 

 subdivision of the Madreporaria. 



The Alcyonaria, constituting a group of corals of the 1 same rank 

 as the Madreporaria, comprise the precious coral, Corallium rubrum, 

 and other species from which jewelry is made, the sea fans, sea 

 feathers, and sea whips, some of which are among the most beauti- 

 ful objects in the ocean. The tentacles of these corals are pinnately 

 fringed or plumose, and, because their mesenteries and tentacles are 

 arranged on a plan of eight, they have been called Octocoralla. The 

 skeleton of the Alcyonaria is unlike that of the Madreporaria, in 

 that it usually consists of a horny axis, more or less completely cal- 

 cified, surrounded by horny material in which spicules are embedded. 



