A, _Cerianthus solitarius_ (after A. Andres).
B, Transverse section of the stomodaeum, showing the sulculus, sl, and the arrangement of the mesenteries.
C, Oral aspect of _Arachnactis brachiolata_, the larva of _Cerianthus_, with seven tentacles.
D, Transverse section of an older larva. The numerals indicate the order of development of the mesenteries.]
The order ANTIPATHIDEA is a well-defined group whose affinities are more obscure. The type form, _Antipathes dichotoma_ (fig. 14), forms arborescent colonies consisting of numerous zooids arranged in a single series along one surface of a branched horny axis. Each zooid has six tentacles; the stomodaeum is elongate, but the sulcus and sulculus are very feebly represented. There are ten mesenteries in which the musculature is so little developed as to be almost indistinguishable. The sulcar and sulcular pairs of mesenteries are short, the sulco-lateral and sulculo-lateral pairs are a little longer, but the two transverse are very large and are the only mesenteries which bear gonads. As the development of the Antipathidea is unknown, it is impossible to say what is the sequence of the mesenterial development, but in _Leiopathes glaberrima_, a genus with twelve mesenteries, there are distinct indications of an Edwardsia stage.
A, Portion of a colony of _Antipathes dichotoma_.
B, Single zooid and axis of the same magnified. m, Mouth; mf mesenterial filament; ax, axis.
C, Transverse section through the oral cone of _Antipathella minor_, st, Stomodaeum; ov, ovary.]
There are, in addition to these groups, several genera of Actinians whose mesenterial arrangement differs from the normal type. Of these perhaps the most interesting is _Gonactinia prolifera_ (fig. 11, B), with eight macromesenteries arranged on the Edwardsian plan. Two pairs of micromesenteries form couples with the first and second Edwardsian pairs, and in addition there is a couple of micromesenteries in each of the sulculo-lateral exocoeles. Only the first and second pairs of Edwardsian macromesenteries are fertile, i.e. bear gonads.
The remaining forms, the ACTINIIDEA, are divisible into the Malacactiniae, or soft-bodied sea-anemones, which have already been described sufficiently in the course of this article, and the Scleractiniae (= Madreporaria) or true corals.