They are classified on the basis of the following points:
The structural organization (body design)
Body symmetry: a) Asymmetrical (irregular shape)
b) Symmetrical (regular shape):
1. Bilateral symmetry
2. Radial symmetry
3. Spherical symmetry
Presence or absence of notochord: The animals having notochord develop a skeleton as in the vertebrates and the ones without having notochord have no skeleton as in invertebrates.
Presence or absence of coelom:
Coelomates: Each plan consists of three cell layers: endoderm, mesoderm and ectoderm.
Acoelomates: They have no cavity other than the gut. Also called solid worms.
Pseudocoelomates: They have a body cavity that is not completely lined with mesoderm.