Source: Wikimedia Commons — Animal diversity showing representatives of major phyla

Cues / Keywords | Notes

Classification Criteria | Animals are classified based on six fundamental criteria: | 1. Level of body organization (cellular → tissue → organ → organ-system) | 2. Body symmetry (asymmetrical / radial / bilateral) | 3. Number of germ layers (diploblastic = 2; triploblastic = 3) | 4. Nature of body cavity (acoelomate / pseudocoelomate / coelomate) | 5. Segmentation (metameric or absent) | 6. Presence of notochord (notochord = chordate)

Symmetry types | Radial: Cnidaria, Ctenophora, adult Echinodermata — can be cut into equal halves by ANY plane through central axis | Bilateral: All phyla from Platyhelminthes onward — only ONE plane (sagittal) gives mirror halves | Asymmetrical: Most Porifera — no plane of symmetry

Germ Layers | Diploblastic (2 layers — ectoderm + endoderm only): Cnidaria, Ctenophora | Triploblastic (3 layers — ectoderm + mesoderm + endoderm): Platyhelminthes onward | No true germ layers: Porifera (cellular organization)

Coelom Types | Acoelomate — no body cavity: Platyhelminthes | Pseudocoelomate — false cavity (between mesoderm and endoderm): Aschelminthes | Coelomate — TRUE cavity (mesoderm lines both body wall AND gut): Annelida, Arthropoda, Mollusca, Echinodermata, Chordata

Mnemonic — Phyla Order | "Poor Creatures Cry Poorly And Arrive At Many Exotic Homes Cheerfully" | Porifera → Cnidaria → Ctenophora → Platyhelminthes → Aschelminthes → Annelida → Arthropoda → Mollusca → Echinodermata → Hemichordata → Chordata

Summary

The Animal Kingdom classification uses six key criteria applied progressively. The most tested criteria are coelom type (Platy = acoelomate; Ascha = pseudocoelomate; Annelida onward = coelomate) and germ layers (only Cnidaria and Ctenophora = diploblastic). The phyla sequence reflects increasing evolutionary complexity.