INTRODUCTION. 23 
by the 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 exothecal structures also; the exothecal layers, the ccoenenchymal 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 costa. 
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 mouths 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 cespitosa 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 Serpule and Nullipores. 
“When the tentacules are fully 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 
' Plate II, figs. 14, 16, 17. 
* ‘Hist. Nat. des Corall.,’ vol. ii, page 589 e¢ seg. See description of Plate II. 
