INTRODUCTION. 7 
Some simple forms have walls which are moderately stout superiorly and excessively 
thick and hard inferiorly, so as to encroach on the visceral cavity; this fillmg up of the 
lower part of the corallites is observed in some compound corals. It is very evident that 
the thickness and the hardness of the wall are determined by the nutrition of the coral ; 
but no defect in this will produce the perforate condition. 
Two series of wall-shapes are noticed,—one more or less horizontal and the other 
ranging from a shallow cup to a long cylinder in shape; the square, polygonal, and com- 
pressed outlines of some walls are either the result of pressure or are characteristic of the 
species. 
The horizontal wall produces shallow, disc-shaped corals; the septa arise from its 
upper and the coste from its lower surface. In some species the under surface is concave, 
so that the cup-shape is seen reversed. 
The second and commonest form may be slightly horizontal at first, and with growth 
the edges turn up and enclose the calicular cavity ; then any height, width, and contortion 
may result; the turbinate, subturbinate, conical, conico-cylindrical, tubular, and other 
forms, may thus arise. 
The wall forms the most important part in some corals, but only a secondary in others ; 
it may be uncovered externally by costa or by epitheca, or it may be in such close 
contact with neighbouring walls, in compound corals, as to become fused.’| The upper 
termination or margin of the wall is very visible when the septa are not exsert; and in 
compound corals, when the walls have become united, this margin may be sharp or broad, 
and variously marked. Usually the walls of neighbouring corallites (not fused together) 
are separated by a dense tissue, which is ornamented superiorly, and often traversed by 
coste. 
The wall occasionally gives out processes, and is often marked by growth-rings, con- 
strictions, and ridges. It is rarely symmetrical; for most simple corals are curved, 
twisted, or more or less compressed; and this is equally true as regards the compound. 
The base of the wall is often attached to foreign substances, and may be broad, even 
concave from rupture, or very delicate and pedunculate. The epitheca, where it exists, 
is generally more strongly developed over the base; the inner base is the floor of the 
visceral cavity. 
Septa.—The septa have been already noticed in a general manner; and it has been 
mentioned that they are developed between the mesenteric folds, and that they are 
localized in the intermesenteric or subtentacular spaces. 
The number of tentacules has a direct ratio to that of the septa and pali. 
The septa, in their simplest condition, are spiniform agglomerations of nodules, pro- 
jecting slightly into the calice from the wall,* and there is every imaginable variety 
1 Plate III, figs. 3—16; Plate IV, fig. 11. 2 Plate III, figs. 5, 6. 
