1.- Cnidocytes present, typically housing stinging organelles called nematocysts.
2.- Entirely aquatic, some in freshwater, but most marine.
3.-Radial symmetry or biradial symmetry around a longitudinal axis with oral and aboral ends; no definite head.
4.- Two types of individuals: polyps and medusae.
5.- Adult body two-layered (diploblastic) with epidermis and gastrodermis derived from embryonic ectoderm and endoderm, respectively.
6.- Mesoglea, an extracellular matrix (“jelly”) lies between body layers; amount of mesoglea variable; mesoglea with cells and connective tissue from ectoderm in some.
7.- Incomplete gut called gastrovascular cavity; often branched or divided with septa.
8.- Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells.
9.- Extensible tentacles usually encircle mouth or oral region.
10.- Muscular contractions via epitheliomuscular cells, which form an outer layer of longitudinal fibers at base of epidermis and an inner layer of circular fibers at base of gastrodermis; modifications of plan in hydrozoan medusa (independent ectodermal muscle fibers) and other complex cnidarians.
11.- Sense organs include well-developed statocysts (organs of balance) and ocelli (photosensitive organs); complex eyes in members of Cubozoa.
12.- Nerve net with symmetrical and asymmetrical synapses; diffuse conduction; two nerve rings in hydrozoan medusae.
13.- Asexual reproduction by budding in polyps forms clones and colonies; some colonies exhibit polymorphism.
14.- Sexual reproduction by gametes in all medusae and some polyps; monoecious or dioecious; holoblastic indeterminate cleavage; planula larval form.
15.- No excretory or respiratory system.
16. No coelomic cavity.
Integrated principles of zoology / Cleveland P. Hickman, Jr. ... [et al.]. – 14th ed. p. cm.