Actinia, one of the commonest genera of the sea anemones, which affords a good type of the structure of that group. A. equina, "the Beadlet," is one of the commonest British species; it occurs attached to rocks, stones, and even crabs, all round the coast. It consists of a fleshy cylinder one to four inches in diameter and one inch in height. Its firm adherence to the object on which it lives is secured by its flat base, the disc; in the centre of the upper end of the cylinder is the mouth surrounded by rings of tentacles. The mouth leads to a short digestive tube, the stomodaeum; this is open below to the body cavity and is held in position by radiating membranes, the mesenteries; upon these are the reproductive organs. Its only method of defence is the shooting out of minute barbed threads. Actinia should be compared with Alcyonium, from which it differs mainly in that the mesenteries occur in multiples of six instead of eight and that the tentacles are not fringed.