The exclusively aquatic phylum Cnidaria is represented by polyps such as sea anemones and corals, and by medusae such as jellyfish. A polypoid or a medusoid cnidarian is a radially or biradially symmetrical, uncephalized animal with a single body opening, the mouth. The mouth is surrounded by tentacles studded with microscopic stinging capsules known as nematocysts that are the agents of offense and defense. The possession of intrinsic nematocysts is the defining characteristic of the phylum (Hessinger and Lenhoff 1988); nematocysts are the most diverse and widespread of three types of cnidae (cnidos = thread) -- hence the preferred name of the phylum.

Cnidarians are diploblastic - that is, the body and tentacles consist of two cell layers, the endoderm (sometimes referred to as the gastrodermis) and the ectoderm (the epidermis). Between the two cell layers is the mesoglea, which ranges from little more than a glue to bind the layers (for example, in Hydra) to the vast bulk of the animal (for example, in jellyfish of Class Scyphozoa). The body encompasses a single sac-like body space, the coelenteron (koilos = cavity; enteron = intestine), which communicates with the surrounding medium through the mouth. The less preferred name of the phylum, Coelenterata, is based on this attribute. The coelenteron (also termed the gastrovascular cavity) serves for gas exchange and digestion.

All cnidarians are carnivorous, with cnidae and tentacles active in prey capture. Because polyps are typically sessile, and only some medusae possess sensory structures (the most sophisticated occur in the Cubozoa; Pearse and Pearse 1978), cnidarians are generally believed to be passive predators, feeding on prey items that blunder into their tentacles. Some cnidarians can absorb dissolved organic matter directly from seawater (e.g. Schlichter 1975), but it is not known how widespread this ability is. Living within the tissues of anthozoans of many species and hydrozoans and scyphozoans of a small number of species are unicellular algae from which the animals derive reduced carbon (Shick 1991). Dinoflagellate symbionts, termed zooxanthellae, are by far the most common algal symbionts; they are exclusively marine. Green algal symbionts, termed zoochlorellae, occur in both marine and freshwater cnidarians.

The text-book depiction of the typical cnidarian life cycle is an alternation between a medusa and a polyp (termed metagenesis), the former the sexually reproductive stage and the latter the asexual stage. In fact, an attribute of the entire class Anthozoa is the absence of a medusa. At least some individuals of all anthozoan species form gametes; those of some species may reproduce vegetatively as well. The other three classes -- Cubozoa, Hydrozoa, and Scyphozoa -- are often grouped as the "Medusozoa" because the medusa phase is present in them all. Indeed, the medusa dominates the life cycle of members of the classes Cubozoa and Scyphozoa (Cubozoa was formerly considered an order of Scyphozoa, and some specialists still consider it as such). Life cycles of the Hydrozoa are the most diverse in the phylum: although the polyp is the more conspicuous and persistent stage in most taxa, some lack the medusa phase, whereas others lack the polyp phase. Hydra, which is used in many textbooks to illustrate the phylum, is utterly atypical: a hydrozoan, it lacks a medusa, it has aggregations of gametogenic tissue that function as gonads, and it is among only a handful of freshwater cnidarian species.

The cnidarian larva is the planula, a pear-shaped, entirely ciliated animal. In the "typical" cnidarian life cycle, male and female medusae spawn freely into the sea, where fertilization occurs and a planula develops. At metamorphosis, the planula settles on and attaches to the substratum, where it metamorphoses into a polyp. The primary polyp produces additional polyps asexually, by budding, stolonic outgrowth, or some other process, to form a clone or a colony. At the appropriate time, determined perhaps by size of the colony or environmental conditions, rather than or in addition to polyps, medusae are produced asexually (in Cubozoa, each polyp metamorphoses into a medusa). They are released to take up a pelagic existence and the cycle begins anew.