The members of this family are commonly known as Sea-anemones, and are distinguished by having no corallum, or a spurious one, by being rarely compound, and by having the power of locomotion.
The body of a Sea-anemone (fig. 70, a) is a truncated cone, or a short cylinder, termed the "column," and is of a soft, leathery consistence. The two extremities of the column are termed respectively the "base" and the "disc," the former constituting the sucker, whereby the animal attaches itself at will, whilst the mouth is situated in the centre of the latter. Most Sea-anemones fix themselves by the base to some foreign object - a stone or a living animal - but others (Peachia and Edwardsid) bury themselves more or less completely in the sand. In a few cases (Cerianthus and Peachia) the centre of the base is perforated, but the object of this arrangement is unknown. Some forms, again (Minyas, Nautactis, Oceanactis, &c), are oceanic in their habit. Between the mouth and the circumference of the disc is a flat space, without appendages of any kind, termed the "peristomial space." Round the circumference of the disc are placed numerous tentacles, usually retractile, arranged in alternating rows, and amounting to as many as 200 in number in the common Actinia. The tentacles are tubular prolongations of the ectoderm and endoderm, containing diverticula from the somatic chambers, and often having apertures at their free extremities. The mouth leads directly into the stomach, which is a wide membranous tube, opening by a large aperture into the general body-cavity below, and extending about half-way between the mouth and the base. The wide space between the stomach and column-wall is subdivided into a number of compartments by radiating vertical lamellae, termed the "primary mesenteries," arising on the one hand from the inner surface of the body-wall, and attached on the other to the external surface of the stomach. In reality the mesenteries are arranged in pairs, the chamber between each pair opening above into the cavity of a tentacle. As the stomach is considerably shorter than the column, it follows that the inner edges of the primary mesenteries below the stomach are free; and these free edges, curving at first outwards and then downwards and inwards, are ultimately attached to the centre of the base. Besides the primary mesenteries, there are other lamellae which also arise from the body-wall, but which do not reach so far as the outer surface of the stomach, and are called "secondary" and "tertiary " mesenteries, according to their breadth. The reproductive organs (fig. 35) are in the form of reddish bands, which contain ova and spermatozoa, and are situated on the faces of the mesenteries. Most of the Adiniae are dioecious - that is to say, the same individual does not develop both ova and spermatozoa; but some forms are monoecious. The free edges of the mesenteries below the stomach are thickened, and constitute puckered and convoluted cords ("craspeda"), which are richly furnished with thread-cells. Attached also to the free edges of the mesenteries are sometimes found long thread-like filaments likewise crowded with thread-cells. These peculiar structures ("acontia") appear to be organs of offence and defence, as they can, on irritation, be rapidly shot forth from the mouth, as well as from certain minute orifices in the body-wall ("cin-clides") which appear to be specially intended for their emission. As regards their nervous system, nerve-cells and anastomosing nerve-fibres are stated to be present in the base (Martin Duncan), and may exist in other parts of the Sea-anemones, whilst the pigment - masses at the bases of the tentacles in some forms appear undoubtedly to be rudimentary organs of vision.
Fig. 70. - Morphology of Actinidae a Actinia rosea; b Arachnactis albida.
The embryo of the Actiniae is a free-swimming ciliated body, at first rounded, but afterwards somewhat ovate. The rudimentary mouth is soon marked out by a depression at the larger extremity; thread-cells appear as a layer in the ectoderm; a fold is prolonged inwards from the mouth to form the digestive sac; and the primitive tentacles are at first two in number, but are rapidly increased to six.