114 J. MATTHEWS ON THE HISTORY AND 



starry summits, are connected by a living viscous substance, 

 called C^enosarc, which, becoming consolidated by lime taken from 

 the sea water, becomes what is called Ccenenchyma, or Scler- 

 enchyma, so that the resulting Coral becomes a rounded mass, 

 yet varying in shape according to the genus or species. The 

 variety of compact or branching Corals far exceeds description, one 

 hundred and twenty species being inhabitants of the Red Sea 

 alone. 



The Millepora complanata, which, according to Mr. Moseley, 

 of the " Challenger," is one of the Hydrozoa, is the largest coral 

 known. This and the Porites, and one or two others, are the chief 

 reef-building corals. They cannot live at greater depths than from 

 15 to 25 fathoms, for light and abundance of air are essential, and 

 these decrease as the depth increases. In every case the polypes 

 are alive only as far as they have free access to light, heat, and air 

 contained in the water. The process of budding kills them also, 

 for, when the buds proceed from the discs, or cups, of the star {i.e., 

 are casspitose), the consequence is the death of the parent, and the 

 formation of a new layer of the living above the dead. They thus 

 increase vertically. Red coral was for a long time regarded as a 

 marine plant, but is now well known to belong to a group of polypes 

 called Alcyonaria, of which Alcyonium digitatum, an octo-corallum 

 of Huxley, may be taken as the type. Its polypes possess eight 

 fringed tentacles. If they possess sheaths, or cases, they are with- 

 out horizontal septa or partitions. The mesenteries and body 

 chamber are in fours, or multiples of four. And the corallum, if 

 any, is sclero-basic, a term which I will explain presently, and they 

 have abundant spicules in the sarcode. They are divided into 

 Alcyonidse, Tubiporidse (organ corals), Pennatulidas (which include 

 the sea pens, sea fans, cock's combs, etc.), and lastly the Gorgonidaa, 

 to which the red coral of commerce belongs. 



I have now to set before you, as clearly as I can, the nature of 

 the difference between this group and the reef-building corals, and 

 to do this intelligibly, I must recall your attention to the three 

 coverings of the Actinias and Hydrozoa, viz., the Ecto-derm, the 

 Meso-derm, and the Endo-derm, as, upon the development of the 

 corallum (for so the hard structure of the corals is called) in each of 

 these is founded their classification. 



In the reef-building varieties, and in many others of the branch- 

 ing kind, the hard structure is formed within the Meso-derm, and 



