The order Cnidaria is divided into 4 classes.
1. Hydrozoa: They live in water. Polyp or medusa stages are seen in the life cycle. Mesoglia are cellless. Genitalia arise from ectoderm. Like- Hydra viridis, Obelia geniculata.
2. Scyphozoa: These are cup or bell or umbrella shaped animals. Medusa Dasha is the main one in the life cycle. Genitalia arise from endoderm. Mesoglia are large and composed of cells and fibers. For example, Aurelia aurita
3. Cubozoa: They are storm-like creatures. Their Medusa Dasa is the main one. For example, Metridium
4. Anthozoa: They are flower-shaped organisms. A polyp stage is present in the life cycle. Mesoglia are composed of fibrous connective tissue. Genitalia arise from endoderm. For example, Metridium senil, Gorgoni verrucosa.