Solitary Hexacorallia; adults live in tubes buried in soft substrata, larvae known as arachnactis, are planktonic and free-swimming. Column in adults rounded at the aboral end, elongated and very contractile. Tentacles are of two types: a series of long marginals at the periphery of the disc, and a series of short labials at the entrance to the actinopharynx. Both series may be arranged in pseudocycles (not true cycles as the mesenteries are unpaired), one marginal and one labial tentacle arising from each lateral mesenteric chamber. A single siphonoglyph, generally considered to be dorsal, is present. Longitudinal muscles are absent or only weakly developed on the mesenteries, contraction being effected by a powerful layer of ectodermal longitudinal muscle in the column wall; hence the tentacles cannot be retracted into the column. No sphincter muscle is present. Mesenteric filaments usually forming a double strand, at least on some parts of the mesenteries. Larvae are variable in shape and may lack labial tentacles. The cnidom consists of spirocysts, atrichs holotrichs, p- and b-mastigophores.
The mesenteries of cerianthids, apart from the single pair of dorsal directives, are not paired but are coupled, new couples arising in the ventral multiplication chamber. All mesenteries are perfect but vary greatly in length generally the younger ones are shortest, the older ones often reaching the aboral end of the column. Usually adjacent mesenteries vary in length and development and may be grouped in quartets or duplets of regularly recurring characteristics. Various structures may occur on the mesenteries: acontioids are organs resembling acontia which occur on the lower part of the mesenteries; cnidorhagae are rounded projections bearing numerous nematocysts, sometimes these occur in bunches on stalks, then being termed botrucnids. Craspedonemes are outgrowths of the mesenterial lamella over which runs a convoluted section of the double mesenteric filament. In some genera the siphonoglyph is continued as a grooved structure connecting the directive mesenteries, the hyposulcus; below this a further continuation, the hemisulcus, may occur separately on each directive mesentery.
An adult cerianthid lives in a long tube which it constructs from numerous discharged nematocyst threads, mucus and various foreign materials. The texture of the tube is soft and felt-like, with a smooth, very slippery lining. The tube is buried in mud, sand or gravel with the thicker distal end protruding above the substratum. Occasionally it may be situated beneath stones or in rock crevices. The tentacular crown is expanded from the top of the tube but when disturbed the anemone contracts very fast and retreats to the bottom of the tube, which is usually very much longer than the animal.
Larval cerianthids, known as arachnactis larvae, are seasonal planktonic predators. Few of these larvae have been positively correlated with adult forms and for this reason a separate nomenclature of adults and larvae has arisen. Many more larval species than adults are known and, although this may reflect the comparative ease of collecting - compared to adults, it is possible that m some species neoteny occurs. A few species are known to develop ripe gonads while still planktonic and in these the adult stage, if such exists, has not been identified.
The Ceriantharia is a small order comprising three families with about eight adult and 32 larval genera. The relationships of these are poorly understood, in part due to differing interpretations of the characters and their terminology by various authors. Arai (1965), in an effort to standardize this terminology, published a comprehensive glossary of these terms. Den Hartog (1977a) suggested an outline for a new classification based on nematocysts as well as the more conventional characters. In an interesting discussion he placed in perspective the relative values of certain characters whose importance may have previously been exaggerated and proposed two new suborders, Penicillaria and Spirularia. He also showed that the family name Arachnactidae McMurrich, 1910 has priority over the many other names which have been applied to this family in the past (Arachnanthidae, Acontiferidae, Acontioidiferidae, Cnidonemiae).