CQELENTERATA. 243 
The members of a coral community are organically con- 
nected; each feeds himself, yet is not independent of the 
rest. We can speak of the individual Corals, a, 6, c, but 
we must write them down abc. The compound mass is 
“like a living sheet of animal matter, fed and nourished 
by numerous months and as many stomachs.” Life and 
death go on together, the old Polyps dying below as new 
ones are developed above. The living part of an Astrea 
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.“* 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 coralliteis a short tube with vertical 
septa radiating toward the centre.* 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 
connected together by the fleshy cwnosarc covering the 
coral. 
(1) Sclerodermic Corals— Astrea is a hemispherical mass 
covered with large cells. MJeandrina, or “ Brain-coral,” 
is also globular; but the mouths of the Polyps open into 
each other, forming furrows. wngia, or “ Mushroom- 
coral,” is disk-shaped, and differs from other kinds in be- 
ing the secretion of a single gigantic Polyp, and in not 
