INTRODUCTION. v 



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 epidermic scleren- 

 ehyma 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 Sertulariae ; but in 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 loculi. These vertical laminae, to which we shall exclusively 

 apply the name of septa, cover the upper surface of the wall when this spreads out in the 

 form of a disc (as in Fungiae) ; 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 trabiculse branching off from their inner edge, and forming a spurious 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 staminas which in most flower§ 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 caning the costa 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 costae. 



The cavity thus circumscribed by the walls of the corallum, and subdivided by the 



