INTRODUCTION. 23 



by tlie soft tissue, but as the dissepiment closes off the space beneath it the inferior 

 layer of membrane is absorbed, and finally is no longer to be noticed. This is the case 

 with the exotliecal structures also ; the exothecal layers, the coenenchymal cells, and the 

 perithecal cells, are formed by the membranes, and as the cells become closed the included 

 membrane is absorbed. All the granular and spiniform ornamentation of the scleren- 

 chyma is also formed in the soft tissues, and the more or less dense epitheca results from 

 the development of a tissue from the base of the corallum. 



This last is called the foot-secretion, and covers the results of the growth of the 

 membrane which develops the wall and costse. 



The deposit of earthy and inorganic matter in living corals is not, then, a simple con- 

 cretionary process, but is essentially a vital one ; it follows certain laws, and its extent 

 and amount depend on the nutrition of the individual. When the influence of the soft 

 tissue is no longer felt the hard parts become harder and denser and are subject to 

 various changes in their mineral condition. 



In those corals whose calices are not separate, but are continuous and running into 

 series, the tentacules, as a rule, are small, numerous, and are often partly hidden by a ridge 

 of membrane.^ There are several moutlis to the elongated and tortuous calices. 



The microscopic structure of the soft tissues of the Sclerodermic Zoantharia has been 

 ably studied by many observers, and the following extract from the description of the 

 soft parts^ of Cladocora ccespitosa by the late M. Jules Haime contains information 

 sufficiently exact for the present purpose. 



" The surface of the corallum is more or less convex. When extended the polypes 

 touch each other with the extremity of their tentacules, and when they are seen from 

 above there is no interval between them. The tentaculiferous disc is never more than 

 two or three millimetres above the calicular margin of the polyperites, and the lateral and 

 inferior continuation of the disc only descends one or two millimetres below the margin. 

 When a polyperite is cut longitudinally it will be readily observed that the soft tissues 

 are not prolonged much deeper internally in the visceral chamber, so that in the adult 

 coral, which is usually several centimetres long, only about five or six millimetres of its 

 upper part are covered by the soft tissues. This limited portion is bounded inferiorly by 

 the uppermost of the series of horizontal dissepiments. All the rest of the corallum 

 appears to be dead, and is ordinarily covered with Serpulse and NulHpores. 



" When the tentacules are fidly extended, the diameter of the circle formed by 

 their extremities is about one and a half times as large as that of the calice. The 

 margin of the calice is usually visible on account of the transparency of the soft parts 

 covering it. 



" The tentaculiferous disc is horizontal, but towards the middle of it there is a slight 



1 Plate II, figs. 14, 16, 17- 



2 ' Hist. Nat. des Corall.,' vol. ii, page 589 et seq. See description of Plate II. 



