The protruding portion of the polyp is very delicate, the internal viscera being, as it were, enclosed in a bladder formed of two very thin membranes in intimate union, so transparent as to permit a view of their arrangement. At the base of the body, where thickest, it coalesces with the base of the adjacent polyp; thus constituting the common cortical portion into which each animal retreats at will, by a process in many respects resembling that by which a snail draws in its horns. In the greater number of _Asteroidæ_ this common portion secretes carbonate of lime, which is deposited in the meshes of its tissues either in granules or in crystalline spiculæ, which imparts a solid consistency to the whole. The inner tissue meanwhile continues unaltered, being prolonged throughout the polypiferous lining of the cell, the abdominal cavity, and the longitudinal canals which permeate the whole polyp, as well as the tubular network with which the space between the canals is occupied. It is among these inner tissues that the buds or gemmæ are generated, by whose increase and evolution the polyp mass is enlarged, the shape and size depending on the manner in which the buds are evolved; for in some, as in the _Pennatulidæ_, determinate spots only have the appropriated organization, while in others, as in _Alcyonium_, the generative faculty appears to be undefined and more diffused. THE ACTINIARIA. Here we leave the group of polyps which form united families. The Sea Anemones, of which the _Actinia_ are the type, consist of _Zoanthaires_, which produce no corals, that is to say, of polyps whose covering remains always soft, and in whose interior nothing solid is produced. This order is usually divided into two families--the _Actiniadæ_, having the tentacles in uninterrupted circles, with no corallum, and the _Minyadinæ_, having globose bodies, and very short tentacula. The modern aquarium exposes the spectator to many wonderful surprises. Coiled up against the transparent crystal walls of the basin, he observes living creatures of the most brilliant shades of colour, and more resembling flowers than animals. Supported by a solid base and cylindrical stem, he sees them terminate like the corolla of a flower, as in the petals of the anemone: these are the animals we call _Sea Anemones_--curious zoophytes, which, as all persons familiar with the sea shore may have observed, are now seen suspended from the rocks, and presently buried at the bottom of the sea, or floating on its surface. These charming and timid creatures are also called _Actinia_, as indicating their disposition to form rays or stars, from the Greek ἀκτὶν, a ray.