ORDER ACTINOIDEA. 63 
zoophyte, arising from the fact that the coral secretions take place 
only from the lower parts of the polyps. 
It is hence apparent, that by this single difference in growth, the 
same mode of budding may produce either massive forms—globular 
or ramose—or a branched zoophyte, in which each branchlet is the 
erowth of a separate polyp. The former mode of growth produces 
what may be styled aggregate zoophytes, as isexemplified in figure 23, 
and in the various genera just referred to. The latter gives rise to 
segregate zoophytes, the polyps being separate from one another, 
excepting a basal connexion. ‘The coralla, in the latter case, may be 
described as calicularly branched. ‘This subject will be farther illus- 
trated when treating of the modes of budding. 
6. 61. Germ-polyps differ essentially in their mode of increase,—a 
process intimately connected with that of budding. The adult ani- 
mal, commencing a zoophyte, sometimes seems to raise itself on the 
coral it secretes, and, although but a fraction of an inch in height 
itself, gives rise to stems many times its own length. ‘This is the case 
with the species represented in figure 24, in which only the tips of 
the branches, for a line or less, are alive. The part below dies as 
growth proceeds above, and so growth and death go on at equal pace till 
the small polyp is finally supported upon a long pedicel of dead coral. 
This is styled an acrogenous* mode of increase. In the example 
referred to, and many others among coral zoophytes, there is no con- 
nexion whatever between the several polyps of the group, except for 
a short time after a bud first starts, the process of growth causing 
after awhile a complete separation between each bud and its parent. 
Other species are incapable of this indefinite upward growth ; 
and these consequently scarcely exceed their adult size in height, 
above the point from which they start into existence. Yet many 
of these give origin to large zoophytes. Instead of increasing upward, 
they extend themselves laterally, or m¢den by growth and budding. 
This result may be connected both with /ateral and terminal budding, 
as shown in figures 27 and 33. 
62. The singular process of dying below, while upward increase 
goes on, is illustrated by nearly the whole class of coral zoophytes, 
and may receive here some farther illustration. 
An Astrea dome, twelve feet in diameter, although solid coral 
through its interior, is alive for only half or three-fourths of an inch 
* From axpov, top, and yevvaw, to increase. 
