Externally they resemble ordinary sea-anemones, but there is only one ciliated groove, the sulcus, in the stomodaeum, and the mesenteries are arranged on a peculiar pattern.
The first twelve mesenteries are disposed in couples, and do not differ from those of Actinia except in size.
The subsequent development is peculiar to the group. New mesenteries are formed only in the sulco-lateral exocoeles.
The mesenteries are numerous, and the longitudinal muscles, though distinguishable, are so feebly developed that there are no musclebanners.
In this larva four pairs of mesenteries having the typical Edwardsian arrangement are developed, but the fifth and sixth pairs, instead of forming couples with the first and second, arise in the sulcar chamber, the fifth pair inside the fourth, and the sixth pair inside the fifth.