Classification of Kingdom Animalia

Every criterion covered in section 2.1 — organisation level, germ layers, symmetry, coelom, and segmentation/notochord — combines into one nested classification scheme rather than acting as five separate, unrelated checklists. Kingdom Animalia first splits at the top into Subkingdom Parazoa, the sponges, which never advance beyond cellular-level organisation and true tissues, and Subkingdom Eumetazoa, every animal that does build true tissues.

Eumetazoa itself splits into two grades. Grade Radiata covers the diploblastic, radially or biradially symmetrical, acoelomate animals — Cnidaria and Ctenophora — which are capped at tissue-level organisation because they lack mesoderm. Grade Bilateria covers every triploblastic, bilaterally symmetrical animal with organ-system organisation, which is then split into two Divisions by the fate of the embryonic blastopore during gastrulation. …