ORDER ACTINOIDEA. 53 
The secretion of a calcareous lamella or plate takes place between 
two visceral lamelle, but whether from the surfaces of these fleshy 
lamelle, or from a prolongation inward of the membrane forming 
the walls of the visceral cavity, has not been ascertained. The secre- 
tion does not always commence at the central plane of the plate, but 
often either side of it; for they are sometimes hollow within. In some 
species the spines of the surface are tubular, as in the Echinopora 
reflexa. We have observed, with regard to the visceral lamelle, ex- 
hibited in figure 15, } 25, that a large compartment alternates with 
three narrower; and also that the large calcareous lamelle in the Den- 
drophylliz and many other species, alternate with three smaller: it 
appears therefore that the larger rays of a cell are formed between a 
pair of large visceral lamelle, and the smaller in the narrower spaces 
which are intermediate. 
The calcareous secretions forming these corals are often deposited at 
successive intervals or as layers in the zoophyte. In a Madrepora the 
surface between the cells becomes covered with minute points by the 
continued secretions, and then a layer forms, connected with the pre- 
ceding by these points or columns. The interior usually becomes, 
afterwards, nearly solid by additional secretions. This variety of 
structure may be observed also in the Dendrophyllie; and even 
the compact species, in which there are no traces of cellules, will 
often show evidence of having been deposited in layers. I have seen 
it brought out with singular distinctness in a specimen half fossilized, 
the layers easily peeling off from one another. In many corals, how- 
ever, we fail to detect this deposition in layers. This is the case in 
the Astrea tribe. The Pocillopore, and some allied corals, have 
transverse plates crossing the cells internally, which are intermitted 
secretions from the lower part of the polyp; but no appearance of layers 
has been detected in the spaces between the cells. The Favosites 
and many Cyathophyllide are examples of similar interrupted secre- 
tions across the cells. 
47. Our explanations thus far, apply more especially to the Acti- 
naria. ‘The Alcyonaria secrete no rays within the cells, nor even 
striate the inner surface of the same, while rays or striating lamelle 
belong to the cells of all the Actinaria. Moreover, in the Alcyonia 
group, the secretions, instead of forming layers, constitute dissemi- 
nated granules or spicule, some of which are represented in figure 22, 
and there is every gradation, from the purely fleshy species to those 
which are quite firm, from the secreted calcareous grains. These 
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