252 
COMPARATIVE ZOOLOGY. 
Polyps dying below as new ones are developed above. The 
living part of an Astrcea is only half an inch thick. The 
growth of the branching Madrepore is about three inches 
a year. The prevailing color of the Coral Polyps is 
green; and the usual size varies from that of a pin’s head 
to half an inch, but the Mushroom-coral (which is a single 
individual) may be a foot in diameter. 
Corals are of two kinds: those deposited within the tis¬ 
sues of the animal ( sclerodermic ), and those secreted by 
the outer surface at the foot of the Polyp ( sclerobasic ). 
The Polyps producing the former are Actinoid, resem¬ 
bling the Actinia in structure. 131 The skeleton of a single 
Polyp (called corallite, Fig. 95) is a copy of the animal, 
except the stomach and tentacles, the earthy matter being 
secreted within the outer wall and between each pair of 
partitions. So that a corallite is a short tube with vertical 
septa radiating towards the centre. 132 A sclerobasic Coral 
is a true exoskeleton, and is distinguished by being smooth 
and solid. The Polyps, having eight fringed tentacles, are 
situated on the outside of this as a common axis, and are con¬ 
nected together by the fleshy ccenosarc covering the Coral. 
(1) Sclerodermic Corals.— Astrcea is a hemispherical mass 
covered with large cells. Meandrina , or “ Brain-coral,” 
is also globular; but the mouths of the Polyps open into 
each other, forming furrows. Fungia , or “ Mushroom- 
coral,” is disk-shaped, and differs from other kinds in be¬ 
ing the secretion of a single gigantic Polyp, and in not 
being fixed. Madrepora is neatly branched, with pointed 
extremities, each ending in a small cell about a line in 
diameter. Forties, or “ Sponge-coral,” is also branching, 
but the ends are blunt, and the surface comparatively 
smooth. Tubipora, or “ Organ -pipe coral,” consists of 
smooth red tubes connected at intervals by cross-plates. 
The Astrcea, Meandrina, Madrepora, and Forties are the 
chief reef-forming Corals. They will not live in waters 
