Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey.

Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic, that is, they develop from two embryonic layers. Nearly all (about 99 percent) cnidarians are marine species. Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers).

Cnidarians have two body plans — the free-swimming medusa and the sessile polyp. Both include a mouth/anus surrounded by stinging tentacles. There are two layers, the epidermis (shown in red), which is derived from the ectoderm, and the gastrodermis (shown in blue), which is derived from the endoderm.

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.