Thenaria (Mesomyaria) with irregular arrangement of tentacles in connection with the bilateral development of the younger mesenteries. After the stage with 12 pairs of mesenteries the later mesenteries arise bilaterally in each sector of the animal, so that the youngest mesenteries appear in the middle of the secondary exocoels. In each pair of the younger mesenteries one mesentery is larger than its partner.
Definitions and contained taxa are those of Oscar Carlgren as published in 1949 : Kungl Svenska Vetenskapsakadamiens Handlingar, Series 4, Volume 1, Number 1