Class Hydrozoa: Hydroids and hydromedusae: hydroids, hydromedusae. Medusae small. Cnidocysts epidermal.

• Order Hydroidea: Hydroids
  • Suborder Anthomedusae: Hydra
  • Suborder Leptomedusae: Polyps always colonial. Obelia, Sertularia
    • Order Milliporina (Hydrocorallina): Fire corals
    • Order Stylasterina
    • Order Trachylina: Gonionemus
    • Order Siphonophora: Polymorphic swimming or floating colonies. Physalia
    • Order Chondrophora: Colonial or specialized polypoid individuals (?). Velella
    • Order Actinulida

Class Anthozoa: Sea anemonies, corals, sea pens. Marine. No medusae. Cnidocytes both epidermal and gastrodermal. Mesenteries. Pharynx.

• Subclass Octocorallia: Octocorals. Eight mesenteries.
  • Order Gorgonacea: Sea fans, sea whips
  • Order Telestacea
  • Order Pennatulacea: Sea pens, sea pansies
  • Order Alcyonaea: Soft corals
  • Order Helioporacea
  • Order Stolonifera: Organ pipe corals (among others)
• Subclass Hexacorallia: Six mesenteries or multiples thereof. Sea Anemones and true corals
  • Order Actinaria: Sea anemones
  • Order Scleractinia: True or stony corals
  • Order Zoanthidea: Colonial sea-anemone-like animals
  • Order Corallimorpharia: Coral-like anemones
• Subclass Ceriantipatharia
  • Order Antipatharia: black corals
  • Order Ceriantharia: tube anemones

Class Cubozoa: Sea wasps and box jellyfish. Fast and dangerous.

Class Scyphozoa: Jellyfish. Medusoid stage predominates.

• Order Stauromedusae
• Order Coronatae
• Order Semaeostomae: Aurelia
• Order Rhizostomae

Body Plan	two layers: epidermis and gastrodermis. Middle layer (mesoglea, mesenchyme) is jellylike, varying in stiffness and thickness, having no, few, or many cells. Dimorphic: alteration of generations, with polypoid and medusoid adult forms (though one or the other form may be reduced to varying degrees). Radial symmetry. Gut is gastrovascular cavity, with one entrance (mouth/anus). Many forms colonial (shared tissues, gastrovascular cavity). Cnidocytes with nematocysts. No true "organs".
Locomotion	No differentiated muscle cells, but both epidermal and gastrodermal cells may have contractile extensions running in parallel along mesogleal surface.
Feeding and Digestion	Use of tentacles by individuals or specialized colonial polypoid hydroid polyps (dactylozooids) to capture prey, usually with nematocysts. Digestion extracellular, with secretion of enzymes into gastrovascular cavity by gastrodermis, then phago- and pinocytosis of resulting breakdown products. Undigested food expelled through mouth. Some use of zooxanthellae.
Sense Organs	Both "unspecialized" mechanoreceptive and chemoreceptive nerve endings in epidermis and gastrodermis. Special sense organs include statocysts and ocelli.
Nervous System	Nerve net. No centralization. Neurons are bare and nonpolar. Hydrozoans gap-junction (electrical synapses) except for effectors or net-net communication; Anthozoans and Scyphozoans have only chemical synapses. Normal action potentials. Conduction rates 50-100cm/sec.
Respiration and gas exchange	Cellular. Uses simple diffusion. Gas exchange at gastrovascular or epithelial surface. Some facultative anaerobiosis in some anemones which are frequently buried by sediments.
Circulation	No specialized system. Gastrovascular cavity may be branched (eg radial arms and ring canal of scyphozoan medusae; mesenteries of anthozoans) to supply sizeable bodies.
Osmoregulation	Cellular level. FW osmoregulation in Hydra is by actively producing a hypoosmotic fluid in the gastrovascular cavity (mechanism unknown), then periodically expelling it (functionally like a contractile vacuole). Ions are actively taken up from the medium by individual cells (mechanism unknown).
Reproduction	Asexual is by budding (solitary polypoids and some medusae. Colonial polypoids use gonozooids) or binary fission (some medusae). Sexual may be dioecious or hermaphroditic (though not self-fertilizing).