The build up of oxygen (or similar high-energy molecule, such as chlorine on some worlds) enables the evolution of far more complex life forms. The start of this classification sees the arrival of large amounts of sponges, algae and plankton, and by the end complex life forms such as jellyfish, crustaceans and others have arrived.
The ecosystem is beginning to fill out now, with complex organic life forms. Very little has evolved hard shells, so jellies and simple plants are about all that is available. The Metazoan classification covers a wide range of evolutionary periods, so whilst some worlds may have little more than algae, others may have more complex forms of resource available.
Metazoa are any type of tiny multi-celled organisms capable of movement, generally millimetres in size. They are more complicated than Protozoa and tend to fade into insignificance as more complicated life forms arise. They are still present in large numbers on such worlds (and include plankton), they just aren't a great resource for humans.
Sponges are any type of aquatic creature that filters water through its body to find nutrients. They are normally sessile, and often lack specific body features. Sponge-like creatures with skeletons, spines or other features have been observed though.
Echinoderms are named after a class of Earth creatures that include starfish and sea cucumbers. They are complex organisms centimetres in size.
Kelp is any type of large aquatic plant life. They are generally sessile, though can be free floaters. They may be related to algae, though this isn't always the case.
Jellies are amorphous aquatic creatures with no hard parts.