Q.What are the four characteristics common to most animals?

The five basis-of-classification criteria — organisation level, germ layers, symmetry, coelom, and segmentation/notochord — combine into one single nested classification tree rather than five separate, unrelated checklists.

Kingdom Animalia first splits into Subkingdom Parazoa (the sponges — cellular-level organisation, no true tissues) and Subkingdom Eumetazoa (every animal with true tissues). Eumetazoa splits into Grade Radiata (diploblastic, radially/biradially symmetrical, acoelomate — Cnidaria and Ctenophora) and Grade Bilateria (triploblastic, bilaterally symmetrical, organ-system-level animals). Bilateria then splits into two Divisions by the fate of the embryonic blastopore: Division Protostomia (blastopore becomes the mouth), containing three coelom-based subdivisions — Acoelomata (Platyhelminthe …