Characteristic features of Phylum Cnidaria

• All are aquatic animals.
• Radial or biradial symmetry around an oro-aboral axis, but no head.
• Diploblastic, with an epidermis and a gastrodermis, and a less cellular or non- cellular, gelatinous mesoglea in between.
• A gastrovascular cavity with a single opening, the mouth; tentacles encircling the oral region.
• No coelom, respiratory system or separate excretory.
• Nave net, for diffuse conduction only, present. Some sensory organs also occur.
• No distinct muscle tissue, but epithelia - muscular system is present.
• Two forms of individuals - benthic polyps and pelagic medusae - occur. Polyps usually reproduce asexually, budding sexual medusae. Sexual reproduction in all medusae and some polyps, involving gametes. Cleavage holoblastic and indeterminate.