The eumetazoa (Eumetazoa, Butschli, 1910) (from the Greek εὖ [eu], good/true + μετά [half], above + ζῷον [zóon], animal), also known as Diploblasts, Epitheliozoa, or Histozoa, represent a subkingdom to which the vast majority of animal organisms belong. In them, the remarkable histological differentiation into true tissues has always suggested a phylogenetic position above the previous groups, such as poriferous or mesozoa. Compared to these, they developed from the beginning defined mouth opening and well delineated organs, which makes them a grouping of likely monophyletic origin. We distinguish the most primitive eumetazoa with originally radiate or secondarily biradial symmetry (radiate) and the monophyletic line of bilaterians with bilateral symmetry.
Some zoologists believe that the planula larva of cnidarians represents the level of organization of a likely ancestral eumetazoan, diblastic but at the same time similar to a ciliated protist. According to proponents of the planuloid theory, it was these ancestral mesoplanulae that evolved a third leaflet of embryonic cells. In fact, in parallel to the acquisition of bilateral symmetry, the same evolutionary line did a further fundamental achievement: the third cell sheet interposed between the ectoderm and endoderm, called mesoderm, not homologous to the acellular layers of radiata, which comes to characterize the new triblastic (or triploblastic) organization, proper to all bilaterians. The importance of the mesoderm is remarkable because, from the cavitation and compartmentalization of this, the evolution has led to the formation of the coelom and organs related to it, as we will see in the most evolved phyla. Probably the mesoderm of the precursor animals of the bilaterians was an ectomesoderm similar to that of some current cnidarians, therefore of ectodermal derivation, which gradually differentiated.
Before the advent of modern biomolecular techniques, the reconstruction of the phylogeny of animals without skeletal parts was rather difficult. To date, the planuloid theory seems to be plausible.