STRUCTURE OF CORALS. 115 



in the intercellular substance of that layer of the body, and also of 

 the mesenteric partitions, which form the loculi. Thus the calcifica- 

 tion of the base and side walls of the body forms the cup (or calyx), 

 while, in the centre, the columella is formed by the coalescence 

 of the edges of the completed vertical mesenteric plates, called the 

 primary, but sometimes independently. In like manner the " Pali," 

 i.e., posts, or props, arise from those mesenteric plates, which do not 

 reach the stomach — the secondary. These varieties are called 

 sclerodermic, and the classification thus founded is comparatively 

 easy to understand. They have what is called an " endo- skeleton. ," 

 But the Gorgonidae-Isidse, and the rest of that class, which includes 

 Red Coral, present at first sight greater difficulties. In all these, 

 the Corallum is deposited outside the meso-derm, and although it 

 forms the support (or axis) of the creature upon which the soft 

 parts are spread, and thus appears to be inside it, it really is only so 

 because the soft tissues, or sarcode, clasps it around. This axis is 

 therefore outside the base of the actinozoon, and it is said to be 

 Sclero -basic — to have an Exo-skeleton, like that of a crustacean. 

 It arises thus — the ciliated embryo swims away, base forwards, 

 affixes itself by its base to a suitable spot, and then begins to secrete 

 corallum from it. Budding next occurs, and a branching coral is 

 in due time the result ; but no calyx is ever formed by or in the 

 corallum, the body of the polyj)e being retracted when required, into 

 the soft parts which surrounds the axis, as the bark does a tree. 

 Whatever the shape then, they may thus be distinguished from the 

 Sclero-dermic kinds. Red Coral is, however, according to some 

 observers, in part sclero-dermic, since it has spicules inside the meso- 

 derm also. There seems, indeed, to be no absolute line of demarka- 

 tion between the two classes. 



I will now try to give you a short account of the various kinds of 

 reefs which are the result of the life of these animals. They are — 



1st. Coral fringes, which lie very close to the land, and in 



shallow water. 

 2nd. Encircling reefs, lying in somewhat deeper water, and 



further from the shore around an island. 

 3rd. Barrier reefs, much further from the shore, about 60 or 70 



miles off, and a hundred yards to a mile wide. That on 



the N.E. coast of Australia extends along the coast 



more than a thousand miles in length. 



