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Body Layers

2 layers: Ectoderm and Endoderm (skin and gut).

3 layers: Ectoderm, Mesoderm, and Endoderm.

3 layers: Ectoderm, mesoderm, and endoderm.

Symmetry

Radial symmetry (identical around a central axis).

Bilateral symmetry (mirror-image halves).

Bilateral symmetry (evolves to radial in some, e.g., echinoderms).

Gut Structure

Single opening for intake and output (incomplete digestive system).

Complete digestive system (separate mouth and anus).

Complete digestive system (separate mouth and anus).

Nervous System

Noncentralized nerve net (no brain, just scattered nerves).

Centralized nervous system (brain and nerve cord).

Centralized nervous system (dorsal, hollow nerve cord).

Coelom (Body Cavity)

No true coelom (only a gastrovascular cavity).

True coelom (body cavity lined by mesoderm).

True coelom (body cavity lined by mesoderm).

Embryonic Development

No mesoderm layer.

Mesoderm forms muscles, organs, and other structures.

Mesoderm forms muscles, organs, and other structures.

Movement

Mostly sessile or slow-moving (supported by water).

Active movement (locomotion and predation).

Active movement (locomotion and predation).

Examples

Cnidarians (e.g., jellyfish, corals), Ctenophores (comb jellies).

Flatworms, arthropods, mollusks, annelids, vertebrates.

Chordates (e.g., fish, mammals), Echinoderms (e.g., sea stars).

Special Features

Tentacles for capturing prey; radial symmetry aids in stationary lifestyles.

Bilateral symmetry aids in directional movement and predation.

Deuterostome development (anus forms before mouth in embryos).