re the theca of one corallite ends and that of
another begins. In the formation of colonies by division a constriction
at right angles to the long axis of the mouth involves first the mouth,
then the peristome, and finally the calyx itself, so that the previously
single corallite becomes divided into two (fig. 18, E). After division
the corallites continue to grow upwards, and their zooids may remain
united by a bridge of soft tissue or coenosarc. But in some cases, as
they grow farther apart, this continuity is broken, each corallite has
its own edge-zone, and internal continuity is also broken by the
formation of dissepiments within each calicle, all organic connexion
between the two zooids being eventually lost. Massive meandrine corals
are produced by continual repetition of a process of incomplete
division, involving the mouth and to some extent the peristome: the
calyx, however, does not divide, but elongates to form a characteristic
meandrine channel containing several zooid mouths.

Corals have been divided into _Aporosa_ and _Perforata_, according as
the theca and septa are compact and solid, or are perforated by pores
containing canals lined by endoderm. The division is in many respects
convenient for descriptive purposes, but recent researches show that it
does not accurately represent the relationships of the different
families. Various attempts have been made to classify corals according
to the arrangement of the septa, the characters of the theca, the
microscopic structure of the corallum, and the anatomy of the soft
parts. The last-named method has proved little more than that there is a
remarkable similarity between the zooids of all recent corals, the
differences which have been brought to light being for the most part
secondary and valueless for classificatory purposes. On the other hand,
the study of the anatomy and development of the zooids has thrown much
light upon the manner in which the corallum is formed, and it is now
possible to infer the structure of the soft parts from a microscopical
examination of the septa, theca, &c., with the result that unexpected
relationships have been shown to exist between corals previously
supposed to stand far apart. This has been particularly the case with
the group of Palaeozoic corals formerly classed together as _Rugosa_. In
many of these so-called rugose forms the septa have a characteristic
arrangement, differing from that of recent corals chiefly in the fac