Animals are classified using several key criteria that reflect their structural organization and evolutionary relationships. The four main criteria used in animal classification are:
Animals are first divided based on whether they possess true tissues:
During embryonic development, animals form primary germ layers that give rise to all body tissues:
Symmetry describes how the body parts are arranged around a central point or axis:
| Type | Description | Examples |
|---|---|---|
| Asymmetry | No symmetry; irregular body shape | Sponges (Porifera) |
| Radial Symmetry | Body parts arranged around a central axis; any plane through the centre divides the body into equal halves | Cnidaria, Echinoderms |
| Bilateral Symmetry | Only one plane divides the body into two mirror-image halves (left and right) | Most animals (Platyhelminthes, Arthropods, Chordates) |
Bilateral symmetry is an adaptation for directional movement (motility) and is associated with cephalization — the concentration of sensory organs and nervous tissue at the anterior (head) end.
The body cavity (coelom) is a fluid-filled space between the body wall and internal organs. Animals are classified into three groups:
Coelomate animals are further divided based on embryonic development:
| Feature | Protostomes | Deuterostomes |
|---|---|---|
| Blastopore fate | Becomes the mouth | Becomes the anus |
| Cleavage type | Spiral and determinate | Radial and indeterminate |
| Coelom formation | Schizocoelous | Enterocoelous |
| Examples | Molluscs, Annelids, Arthropods | Echinoderms, Chordates |
Spiral cleavage: Cells divide at oblique angles and sit in the furrows of lower cells. Radial cleavage: Cells divide symmetrically and sit directly on top of one another.