The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes.

[10] This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.

[12] The group Ecdysozoa is supported by many morphological characters, including growth by ecdysis, with moulting of the cuticle – without mitosis in the epidermis – under control of the prohormone ecdysone, and internal fertilization.

The most notable characteristic shared by ecdysozoans is a three-layered cuticle (four in Tardigrada[17]) composed of organic material, which is periodically molted as the animal grows.

The ecdysozoans lack locomotory cilia and produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes.