The innermost circle of tentacles is the oldest. There are in a few instances circum-oral and intermediate tentacles, i. e. tentacles placed round the mouth and between the mouth and marginal tentacles respectively.
The marginal sphincter muscle is rarely completely absent as in Corallimor-phus, but the degree of its development varies very much. In Tealia it is remarkably strong.
The mesenterial filament which borders the free edge of each mesentery consists typically of three processes - one median covered with thread-cells and gland cells, and one on either side of the median with ciliated cells. The two lateral processes are present at the peristomeal end of the mesentery while they die out gradually towards its basal end.
In some forms, e. g. Sagartia, there are long vermiform mobile filaments attached to the surfaces of the mesenteries - close to the basal end of some of the mesenterial filaments. These threads are the acontia. They can be protruded from the mouth or from a series of apertures - the cinclides - near the base of the animal. One surface of an acontium is loaded with many thread-cells. The cinclides are certainly in some cases, e. g. Sagartia, Adamsia, genuine preformed apertures. It is not certain however that accidental ruptures of the wall may not take place occasionally.
Tealia and some other forms have the sexes separate. The male and female elements are alike derived from the endoderm. They occur only on the tertiary mesenteries or mesenteries of a lower order. They are situated between the mesenterial filament and the tentacular muscle. Many Sea-Anemones produce living young, resembling in shape the parent form.
It has been proved that some Anemones possess the power of multiplying by 'scissiparity.' A small portion of the limbus grows and separates itself from the parent, taking away a part of at least four mesenteries.
The tissues present in Tealia are an ectoderm, a supporting lamina or meso-dermic layer, and an endoderm. The latter clothes the whole coelenteric space, central and peripheral, including its extension into the tentacles.
The ectoderm consists of (i) ciliated cells, with flat triangular bases; (ii) gland cells; (iii) sense-cells which end internally in fine filaments connected to a nervous plexus; (iv) cells containing thread-cells or nematocysts, perhaps similarly connected; (v) muscle and ganglion cells placed sub-epithelially. Both (iii) and (iv) are provided with a fine tactile bristle. The muscle- and ganglion-cells lie next to, or are actually imbedded in, the supporting lamina, the latter especially in the peristome. The ectoderm is unilaminar in the wall, base, and stomodaeum. In none of them does it possess muscle-cells; and in the two former no ganglion cells. The mesodermic layer or supporting lamina is laminated; is thickest in the wall and base, and contains fibrils and stellate or fusiform cells. The supporting lamina of the mesenteries is continuous with that of the wall. The endoderm is unilaminar, its cells are flagellate, and the majority possess basal muscle-fibres. These fibres are disposed in a circular direction on the wall, peristome, stomodaeum, and the base where they are sometimes wanting.
They are longitudinally disposed on the intra-septal surfaces of the mesenteries; transversely on the inter-septal. The ridge forming the tentacular muscle is produced by folds of the supporting lamina on which the muscle-fibres lie. A parieto-basilar muscle running obliquely from the side of the wall to the base is developed similarly from the transverse muscle layer on the inter-septal surfaces. These two muscles, tentacular and parieto-basilar, are reversed in position on the surfaces of the directive septa. The endoderm also possesses gland cells and sense-cells. Ganglion cells with nerve fibrils have been found in it, but not so numerously as in the ectoderm of the peristome. Sense-cells and nerve-fibres are found most plentifully in the median process of the mesenterial filaments and the acontia.
Ectodermal muscle-cells occur in some forms on the wall and stomodaeum, ganglion cells on the wall and base.
The surface of the body, as in Tealia, is sometimes roughened by tubercles, which are local elevations of the supporting lamina. The coloured 'marginal spherules,' which are found outside the tentacular circle in Actinia mesembryan-themum, etc, are evaginations of the whole body wall. Their ectoderm is thickened, and contains many thread-cells. Gland cells are especially numerous near and round the base of these bodies, which must be regarded as batteries of thread-cells.
Unicellular Algae, the well-known 'yellow cells,' live in the endoderm cells of many Actinians, but not in Tealia. They are never found in those Actinians which have a red colour, only in those which are colourless, e.g. Aiptasia diaphana, etc, or in the colourless parts of a coloured Actinian, as in the tentacles of Ceriactis aurantiaca. The association between the two organisms is known as Symbiosis.
The term 'Symbiosis ' was first used by the botanist, De Bary, to denote the association together inter se of different animals, or of different plants without reference to the character of the association. The term has also been used in the same manner by Hertwig. It is now very generally restricted to mean the association together of two different organisms which are physiologically the complements of one another. Such an association may be formed between two plants, or between a plant and an animal.
Among plants the group of Lichens consists of an assemblage of forms in which a fungus grows in the thallus of an alga. Both fungus and alga are capable, at least in some instances, of living and growing independently of one another. A single alga may act as host to a large number of different fungi; and vice versa, a single fungus may affect a number of different algae. In some instances the alga grows more vigorously when surrounded by the hyphae of the fungus; in others the hyphae have been observed to penetrate the algal cells and destroy their contents after the manner of a parasite. Another remarkable instance of symbiosis among plants is the growth of a fungoid mycelium round the rootlets of certain Phanerogams - Orchidaceae; Monotropa; the Cupuliferae, e. g. oak, hazel, beech; Abietinae, Salicaceae, and Betulaceae, e. g. alder, birch.