52 Z.O1OPR HY EES: 
7 
The existence of surface-cells on a corallum, is due simply to this,— 
that the upper and interior parts of the animal do not secrete lime. 
In some species, these secretions are confined to the basal portions of 
the polyp below the visceral cavity, as in many Porites, and conse- 
quently these zoophytes have no cells to the corallum. The terms 
polypary, polypidom, given to coral, are, therefore, in every respect, 
inapplicable. 
Where cells occur, there is some appearance of a retreat into them 
by the contracting polyp. But it is only the upper part or disk of 
the animal that is thus withdrawn; and this happens only because the 
projecting part of the animal, on contraction, will necessarily collapse 
upon the solid part below, and into a cavity, if there be one. Figure 
la, on plate 6, represents one of the contracted coral-polyps, and 
some parts of figure 1, on plate 7, represent others. ‘There is here 
no retreat into a cell: indeed, from the external appearance, even the 
existence of coral within would hardly be suspected, so closely do 
the separate polyps resemble some of the Actin. In a Fungia, 
there is no cell whatever, but the small cavity at the centre below 
the mouth, and contraction produces no change in the appearance of 
the animal, except that the tentacles collapse and are not seen; the 
mouth falls a little, and the fleshy exterior, owing to the expulsion of 
the inflating water, lies somewhat more closely upon the coral plates 
beneath.* 
The calcareous secretions begin to form in the young animal 
after the last metamorphosis, which takes place when the animal 
leaves its free swimming state and attaches itself to some support. 
The rays of the cell in this state indicate the number of visceral 
lamelli, and generally of the tentacles; and as these increase in number, 
so also do the calcareous lamelle. It has been stated that in the 
growing Actinia new visceral lamelle are developed to fill up the 
enlarging intervals, and at such a rate that there is always the 
same number in a certain interval. The same is true of the calcareous 
lamelle ; there is a remarkable degree of similarity between the cell 
of a young and adult polyp; it is very rare that the lamelle in the 
former are more crowded than in the latter, and generally, when quite 
young, they appear more open. 
* Where the polyps have most perfectly the app@arance of withdrawing into a cell, as 
in the Sertularias, it is still no more the case than that a turtle retreats into its shell when 
it draws in its head, 
