64 ZOOPHYTES. 
from the surface; so that the live portion, could it be separated, 
would form a thin hollow hemisphere. The depth to which life 
extends, may, in general, be estimated from the diameter of one of 
the polyps; for in the Actiniz, as well as the Astraas and Caryo- 
phylliw, the depth (or height) often exceeds but little the diameter, 
and very seldom, in any species, three diameters. 
Even the branching Madrepores are usually lifeless along the axis 
of the branches; and in the Porites, whether forming a branch half 
an inch in diameter, or a glomerate mass of twenty feet, the polyps 
do not extend within, beyond two lines. ‘The interior is dead coral, 
the former animal tissues of which have dried up. 
The branching or columnar coral zoophytes are not only dead 
along the axis, but they become throughout dead at bottom, after 
attaining a certain height. The addition of an inch at apex is death 
to an inch below. Some Goniopores, which grow in columns, two 
feet or more in height, have a head of live polyps—a capital to the 
column—of only two or three inches. 
Upon this principle of growing and dying, depends the vast power 
and geological influence of the coral polyp. But a few lines in 
height themselves, they would otherwise be limited in their coral- 
making to as many inches at the most, and what is now styled the 
coral-garden, would be but a bed of mosses or incrusting lichens. 
Like the sphagnous moss of a peat-swamp, coral zoophytes continue 
growing at top, with none the less luxuriance, though supported on 
several feet of lifeless trunk. Death follows on “ wequo pulsat pede” 
up the stem of a zoophyte “regumque turres.” 
The nature of this dying process seems to be simply this: that 
circulation loses its activity below, as growth proceeds above, and, 
consequently, the parts dry up in the pores of the corallum. In the 
Astras, this takes place continuously, at the same rate as increase 
above, and produces a gradual change of the animal. In some Cya- 
thophyllide, the same process goes on interruptedly, as explained 
by Ehrenberg. The tissues of the polyp disappear at intervals from 
the sides, leaving a row of unoccupied cellules; and the animal 
afterwards goes on to increase from its contracted size, without refill- 
ing the cellules, which are, therefore, left vacant, though usually 
closed above at the time of the retraction. Thus the surface of the 
zoophyte becomes covered with eneircling ridges, and the corallum 
appears to consist of a series of inverted cones inserted one in the 
other. ‘There is a gradual transition from species, in which these 
