52 ZOOPHYTES. 



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 the animal that is thus withdrawn; and this happens only because the 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 Actiniae. 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 

 lamellae, and generally of the tentacles; and as these increase in number, 

 so also do the calcareous lamellae. It has been stated that in the 

 growing Actinia new visceral lamellae 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 

 lamella? ; there is a remarkable degree of similarity between the cell 

 of a young and adult polyp ; it is very rare that the lamellae 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 appearance 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. 



