Cnidaria: More on Morphology
Cnidarians are said to be the simplest organisms at the tissue grade of organization; their cells are organized into true tissues. Cnidarians are essentially bags made of two cell layers. The outer ectoderm, or epidermis, contains the cnidocysts, the stinging cells that are characteristic of the phylum. The inner endoderm, or gastrodermis, lines the gut, which in some cnidarians may be divided up by septa (as in the Anthozoa) or elaborated into branching canals (as in many Scyphozoa. In between epidermis and gastrodermis is the mesoglea, a layer of jellylike substance which contains scattered cells and collagen fibers. The mouth is often, but not always, surrounded by a ring of tentacles.
A diagram of a typical medusa shows these features: