gh three corallites of _Lophohelia prolifera_.
ez, Edge-zone.

D, Diagram illustrating the process of budding by unequal division.

E, Section through a dividing calicle of _Mussa_, showing the union of
two septa in the plane of division and the origin of new septa at right
angles to them.

(C original; the rest after von Koch.)]

A large number of corals, both aporose and perforate, are colonial. The
colonies are produced by either budding or division. In the former case
the young daughter zooid, with its corallum, arises wholly outside the
cavity of the parent zooid, and the component parts of the young
corallum, septa, theca, columella, &c., are formed anew in every
individual produced. In division a vertical constriction divides a zooid
into two equal or unequal parts, and the several parts of the two corals
thus produced are severally derived from the corresponding parts of the
dividing corallum. In colonial corals a bud is always formed from the
edge-zone, and this bud develops into a new zooid with its corallum. The
cavity of the bud in an aporose coral (fig. 18, A, C) does not
communicate directly with that of the parent form, but through the
medium of the edge-zone. As growth proceeds, and parent and bud become
separated farther from one another, the edge-zone forms a sheet of soft
tissue, bridging over the space between the two, and resting upon
projecting spines of the corallum. This sheet of tissue is called the
_coenosarc_. Its lower surface is clothed with a layer of calicoblasts
which continue to secrete carbonate of lime, giving rise to a secondary
deposit which more or less fills up the spaces between the individual
coralla, and is distinguished as _coenenchyme_. This coenenchyme may be
scanty, or may be so abundant that the individual corallites produced by
budding seem to be immersed in it. Budding takes place in an analogous
manner in perforate corals (fig. 18, B), but the presence of the canal
system in the perforate theca leads to a modification of the process.
Buds arise from the edge-zone which already communicate with the cavity
of the zooid by the canals. As the buds develop the canal system becomes
much extended, and calcareous tissue is deposited between the network of
canals, the confluent edge-zones of mother zooid and bud forming a
coenosarc. As the process continues a number of calicles are formed,
imbedded in a spongy tissue in which the canals ramify, and it is
impossible to say whe