As nouns the difference between metazoan and cnidarian

is that metazoan is any animal that undergoes development from an embryo stage with three tissue layers, namely the ectoderm, mesoderm, and endoderm. The term applies to all animals except the sponges while cnidarian is any of various invertebrate animals, such as jellyfish, hydras, sea anemones, corals and formerly sponges and ctenophores that belong to the phylum Cnidaria.

As an adjective metazoan

is having to do with animals that develop from an embryo with three tissue layers.