The terms “sea jelly” and “jellyfish” usually refer to the adult phase (medusae) of animals from the Phylum Cnidaria, and to the comb jellies from the Phylum Ctenophora.

Phylum Cnidaria

The Phylum Cnidaria contains five classes, with sea jellies represented in four.
The Class Anthozoa is the exception; these are the corals and the sea anemones.
Sea jellies from Class Scyphozoa are often referred to as the ‘true jellies’ and contain many of the largest and most conspicuous animals that we commonly recognise as sea jellies. There are about 200 species of Scyphozoan jellies.
Class Cubozoa are the ‘box jellies’. Cubozoans have a cube-shaped bell with one or more tentacles that trail from the lower four corners of the bell. Many (but not all) Cubozoans contain potent toxins that can severely injure or even kill people. This group includes the dangerous ‘box jelly’, Chironex fleckeri, that has killed more than 70 people in Australia as well as the suite of species referred to as the “Irukandji’ jellies. Only about 50 species of Cubozoan jellies have been described.
Class Hydrozoa is, by far, the most diverse class of sea jelly, with more than 3500 species described. Hydrozoan sea jellies tend to be very small and are usually translucent.
Class Staurozoa are perhaps the most unusual types of sea jellies because they do not swim and spend their lives attached to the sea floor.

Phylum Ctenophora

Species within Phylum Ctenophora are more commonly known as comb jellies. Unlike cnidarian jellies, comb jellies do not sting and are not toxic. Comb jellies are easily recognised by their eight rows of combs (ctenes) that beat in time together and refract light, generating rainbow hues.
There are two classes of Ctenophora.
Class Tentaculata includes comb jellies that have tentacles.
Class Nuda includes comb jellies without tentacles.