INTRODUCTION. Vv 
certain parts of the surface of that membrane in a manner somewhat similar to that in which 
calciferous epidermis covers the skin of Crustacea and Mollusca. ‘This epidermie scleren- 
chyma constitutes the tissue which Mr. Dana has designated by the name of “ foot-secretion, ” 
and is the only anatomical element employed by nature in the formation of the common 
red Coral, and the horny tubes of Sertularize; but im most Polypidoms it is of secondary 
importance, and the structure is essentially made up with the dermic sclerenchyma, or 
ossified chorion. ‘The calcification of this tegumentary tissue always commences in the 
centre of the inferior part of the Polyp, and, spreading gradually, rises as the animal 
grows, so as to inclose the lower part of the gastric cavity, and to constitute a sort of cup 
or cell, which is sometimes broad and shallow, sometimes long and tubular. 
In general the fundamental part of these Corals corresponds to the parietes of the 
great gastric or visceral cavity of the Polyp, and forms what may be called the walls of the 
Polypidom. The basal disc, the spreading cup, or the columnar sheath so produced, very 
seldom remains in this simple condition, and in general soon gives rise to a certain number 
of laminate processes, which converge towards the axis of the body, and divide the central 
cavity into so many radiating Jocw/i. These vertical laminze, to which we shall exclusively 
apply the name of sepfa, cover the upper surface of the wall when this spreads out in the 
form of a disc (as in Fungi) ; but in general they are more or less completely inclosed in 
the cup-shaped or tubular cell produced by the growth of this wall around the visceral 
cavity, which pervades the body of the Polyp from top to bottom. In some Corals the 
septa remain free all along their inner edge ; in other species they adhere to a sort of central 
style or plate, which rises from the bottom of the same cavity, and which M. Ehrenberg 
has proposed calling the columella. The loculi, or interseptal spaces, are then completely 
separated ; and in many Polypidoms, where there is no true columella, the same result is 
produced by a greater development of the septa, which become united by means of 
irregular trabicula branching off from their inner edge, and forming a sowrious columella, 
the structure of which is usually loose and spongy. 
Other lamellar or styliform processes, quite distinct from the septa and the columella, 
are in some Corals interposed between these organs, and form around the central style a 
sort of circular palisade, somewhat like the stamina which im most flowers surround the 
pistil. These additional elements of the Polypidom have been designated by the name of 
pali, and form sometimes one, sometimes two or three, circular rows or coronets. 
In most Corals other lamellar or spiniform processes extend from the walls outward, and 
constitute the parts which we propose calling the cost of the Polypidom. In general they 
correspond exactly to the septa; and in many cases they seem to be mere prolongations of 
these organs through the sort of sheath formed by the walls. Sometimes, indeed, the walls 
themselves are no longer composed of a distinct, independent, calcified lamina, and are 
made up by a slight thickening and cementing of the septa along the line corresponding to 
the boundaries of the gastric cavity and the inner margin of the costi. 
The cavity thus circumscribed by the walls of the corallum, and subdivided by the 
