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; 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.