There are, in addition to these groups, several genera of Actinians whose mesenterial arrangement differs from the normal type.
The third cycle comprises twelve couples, each formed in an exocoele between the primary and secondary couples, and so on, it being a general rule (subject, however, to exceptions) that new mesenterial couples are always formed in the exocoeles, and not in the entocoeles.
The Scleractiniae may best be divided into groups of families which appear to be most closely related to one another, but it should not be forgotten that there is great reason to believe that many if not most of the extinct corals must have differed from modern Actiniidea in mesenterial characters, and may have only possessed Edwardsian mesenteries, or even have possessed only four mesenteries, in this respect showing close affinities to the Stauromedusae.
The lower part of the free edge of every mesentery, whether complete or incomplete, is thrown into numerous puckers or folds, and is furnished with a glandular thickening known as a mesenterial filament.
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.
There are, however, several groups of Zoantharia in which the mesenterial arrangement of the adult differs widely from that just described.
The mesenterial pairs I, II and III are attached to the stomodaeum, and are called macromesenteries (fig.
M, Mouth; mf, mesenterial filament; ax, axis.
While the mesenterial couples belonging to the second and each successive cycle are formed simultaneously, those of the first cycle B are formed in successive pairs, each member of a pair being placed on opposite sides of the stomodaeum.
Form which has been studied all the eight mesenteries were present, but only two of them, namely the sulco-laterals, bore mesenterial filaments, and so it is presumed that they are the first pair to be developed.