It Consists of two macrosepta, the other of two microsepta. The former is termed ventral and to it corresponds the single siphonoglyphe. The remaining pairs of mesenteries are disposed so as to constitute two zones, a dorsal and a ventral. The former always has, even in the youngest specimens, five pairs of mesenteries, including the directive pair. On each side of the latter there is a pair of the typical Zoanthidan structure, and then a pair, either of the typical structure or composed of two macrosepta. The first-named disposition is termed the microtype, the second the macrotype. The ventral zone contains a variable number of mesenterial pairs according to age, but all of the typical structure, with the exception of the directive pair. In both the dorsal and ventral zones the mesentery immediately adjoining the directive pair is a macro-septum. New pairs are intercalated only in the interseptal space on either side of the ventral directive pair. The tentacles are disposed in two circles; one set corresponding to the intra-septal chambers, the other to the inter-septal. Rotteken's muscle is well developed and embedded in the mesoglaea except in Palythoa. The mesoglaea is filled with islands or branching cords of cells derived from the ectoderm, and it is traversed by radial nucleated muscle-fibres. It also contains, except in the genera Zoanthus and Mammilifera, foreign bodies, - particles of sand, calcareous fragments, spicules of Sponges, shells of Foraminifera or Radiolaria. The sexes are united in Zoanthus, separate in the colonies of Epizoanthus and Palythoa: not known in the other colonial genera.
The animals are fixed and colonial or free. Of the former, Zoanthus has branched stolons; Mammilifera stolons with a tendency to form lamellae; Epizoanthus a lamellate base; Palythoa a band-like base, whilst in Corti-cifera the zooids are imbedded in a coenosarc. The stolons, lamellae, or coenosarc are traversed by a system of canals which communicate with the gastric cavities of the zooids at their bases. The colonies are fixed to stones except in the genus Epizoanthus, which grows either upon empty shells of Mollusca, on the rooting spicules of Hyalonema, or on a shell inhabited by a Hermit Crab, and then the substance of the shell is resorbed by the coenosarc. The free-living Zoanthidae comprise the genus Sphe-nopus, in which the body is drawn out into a hollow peduncle terminating in a disc sunk in sand, etc, and an unnamed dioecious genus with buds originating from the base of the peduncle.
(6) The Cereantheae have numerous complete mesenteries, not disposed in pairs nor multiples of 6. There is a single siphonoglyphe which determines a ventral aspect. Two median mesenteries, which correspond to this siphonoglyphe are minute, and from their position are termed directive. The mesentery on either side of these two is of very great length and reaches to the aboral pole. The remaining mesenteries diminish in size to the middle dorsal line, the spot where new mesenteries are added. There are no retractor muscles, but each surface of a mesentery has feebly developed transverse muscles1. The body is elongated and pointed aborally where it is perforated by a pore. It is covered by a tough investment of mucus, discharged nematocysts, and foreign bodies derived from the soil in which the animal lives immersed. The tentacles are in two circles, one marginal and long, the other circumoral and shorter. The former have a series of slit-like pores on the oral aspect. There is a sub-ectodermic layer of plaited longitudinal muscles, not present in any other Actiniaria, which thins away in the dorsal median line, denoted externally by a furrow. The animal is hermaphrodite.
Sterile and fertile mesenteries are said to alternate one with another.
So far as concerns the histological structure of Actiniaria, it agrees more or less closely with that of Tealia, described on pp. 241-42. The ectoderm sometimes secretes a cuticle, e. g. in Phellia among Hexactiniae, and in Zoanthus. The consistency and degree of development of the mesoglaea is liable to variation; it is homogeneous, with or without cells, or fibrillate. The muscle cells either lie evenly on the surface of the mesoglaea, or the latter is thrown into supporting folds or plaits, or may even completely inclose the cells. The endoderm cells of Cereanthus bear many cilia, not a single flagellum, as in Tealia.
The Antipatharia are degenerate, but the different genera in various degrees. The ectoderm of the base secretes an organic or horny lamellate skeleton. Gephyra Dohrnii is the least modified. Its zooids are either isolated or in small groups united by their bases, and fixed by their basal plates to the stems of the Axiferan Isis which they embrace. The tentacles number about eighty and are disposed in circles. The mesenteries are numerous, and bear each a mesenterial filament. Gerardia forms colonies growing upon the stems of Gorgonia, on rocks, shells, etc. There are twenty-four tentacles, a large and a small alternating, and twenty-four mesenteries all complete. The zooids are united by a reticulum of vessels which communicate with the interseptal spaces. Antipathes is also colonial. The basal skeleton is erect, and carries rows of lateral branches, sometimes branched in turn. It is tubular, and the tube crossed from place to place by partitions; it is covered with small spines, and is inclosed in an epithelial ectodermic (?) sheath. The zooids are disposed in a line on the branches, usually on the aspect looking upwards, i. e. towards the apex of the stem. They have six short tentacles. The zooid is elongated in the axial plane of the branch, the mouth transversely to that axis.