The cells of the Flustrx and Escharce are disposed side by side upon the same plane, so as to form a broad leaf-like polypary, which is in the former genus of a coriaceous or horny texture, but in the latter so completely calci fied as to resemble the skeletons of the Litho phvtous Polypes. The individual cells (fig. 59), which are so extremely minute that they require a microscope for their examination, vary in shape in different species, and gene rally have their orifices defended by project ing spines, or sometimes by a movable oper culum or lid, which answers the same puryose as the setm of Bow?rbanhia, by dosing the entrance during the retracted state of the animal. The growth of these polyparies, which are thus densely populated, is effected by the progressive addition of new cells around the circumference, those occupying the margin being of course the most recently formed, and, indeed, the latter are not unfre quently found inhabited by the living animals, whilst in the older or central ones the original occupants have perished.
The facts observed by Dr. Milne Edwards* relative to the mode of formation of these cells possess a high degree of interest, and materially support the views already given concerning the organised nature of the skele tons of zoophytes in general; proving that the calcareous matter to which their hardness is owing is not a mere exudation from the surface of the animal, but is deposited in the meshes of an organised tegumentary mem brane, from which it can be removed with facility by means of extremely dilute muriatic acid. When so treated a brisk effervescence is produced, the cells become flexible, and are easily separated from each other; but they are not altered in form, and evidently consist of a dense and thick membrane, forming a sac, in which the digestive organs of the ani mal are contained. In this state the opening of the cell has no longer a defined margin, as it seemed to have before ; but, as in the case of the Tubipora musica, described in a pre ceding page, the membranous cell is found to be continuous with the tentacular sheath. We see, therefore, that in these creatures the shell is an integrant portion of the animal itself, not a mere calcareous crust moulded upon the surface of its body, being, in fact, a portion of the tegumentary membrane, which, by the molecular deposit of earthy matter in its tissue, becomes ossified, something like the cartilage of the higher animals, with out ceasing to be the seat of nutritive move ment. It is evident likewise that what is usually called the body of the Bryozoon constitutes, in fact, but a small portion of it, principally consisting of the digestive appa ratus.
As to the operculum, destined to close the entrance of the tegumentary cell, it is merely a lip-like fold of the skin, the marginal portion of which acquires a dense consistency by in terstitial deposit, while at the point where it is continuous with the general envelope it remains sufficiently soft and flexible to form a sort of hinge.
The tegumentary sac, deprived of its car bonate of lime, seems to be formed of a to mentous membrane, covered, especially upon its inner side, with a multitude of cylindrical filaments, disposed perpendicularly to its sur face, and closely crowded together. It is in the interstices left by these fibres that the calcareous matter appears to be deposited ; for if a transverse section be examined with the microscope the external wall is seen not to be made up of superposed layers, but of cylinders and irregular prisms arranged per pendicularly to the axis of the body.
But the above are not the only arauments adduced by Milne Edwards in proof thpat these polyparies are maintained in vital connection with the animal. On examining the cells at different ages it is found that after they are completely calcified they undergo material changes of form.
This examination is easily made, seeing that in many species the young sprout from the sides of those first formed, and do not separate from their parents ; each skeleton, therefore, presents a long series of generations linked to each other, and in each portion of the series the relative ages of the individuals are indicated by the position which they occupy. It is sufficient, therefore, to compare the cells situated at the base, those of the middle por tion, those of the young branches and those placed at the very extremities of' the latter. When examined in this manner it is seen that not only does the general configuration of the cells change with age, but also that these changes are principally produced upon the external surface. For instance, in the young cells of Eschara cervicornis, the subject of these observations, the walls of which are of' a stony hardness, the external surface is much inflated, so that the cells are very distinct and the borders of their apertures prominent ; but by the progress of age their appearance changes, their free surface rises so as to extend beyond the level of the borders of the cell, and defaces the deep impressions which marked their respective limits. It results that the cells cease to be distinct, and the polypary presents the appearance of a stony mass, in which the apertures of the cells only are visible.