The Phylum Cnidaria consists of sessile or free swimming animals, characterised by primary radial, often modified as biradial or quadriradial, symmetry; and provided with unique cnidae (stinging capsules). The basic structure is sac-like, with a single terminal opening, functioning both as mouth and anus. The internal space of the sac is called the coelenteron or gastrovascular cavity, which may be subdivided by radial partitions, the mesenteries or septa. The mouth is surrounded by a circle or circles of tentacles. The body wall consists of an ectoderm and an entoderm, separated by a (primarily) ectodermally derived non-cellular mesogloea or partly mesenchyme. The mesogloea forms a membrane in hydroids, a thick fibrous layer in anemones, and a jelly-like filling in medusae. All Cnidaria are characterised by the unique stinging capsules, called cnidae or nematocysts. A nematocyst is basically a capsule (especially but not only located on the tentacles), with an armed or naked thread coiled inside. When stimulated, the thread is fired by eversion.
The Cnidaria display two basic structural types: 1) the sessile polyp, that is more or less cylindrical and has the mouth at the free distal end, with a thin mesogloea; 2) the free swimming medusa, that is saucer- or bell-shaped, with the convex surface upward and with the mouth and surrounding tentacles at the under side. Cnidarian species may be exclusively a medusoid, exclusively a polypoid, or may go through both medusoid and polypoid phases (alternation of asexual polypoid and sexual medusoid generation).
The medusae are often the reproductive phase and produce gametes. After fertilisation, the zygote develops in the typically cnidarian planula larvae, which is ciliated, motile and gastrulate. Polyps may propagate by fission or by budding. Budding occurs rarely in medusae also.
The Phylum Cnidaria contains four superclasses: Hydrozoa, Anthozoa, Cubozoa, and Scyphozoa. As far as the North Sea plankton is concerned, only the Hydrozoa (in the pelagic phase) and Scyphozoa are of interest, together consisting of siphonophores (Class Siphonophora), leptolid medusae (Class Leptolida) and large medusae (Class Scyphomedusae). Anthozoa do occur in our area but are exclusively polypoid and sessile; the planktonic Cubozoa occur only in tropical seas. The key to the cnidarian species from the North Sea plankton starts at Page 14: Cnidaria.
[Descriptive part after Hayward and Ryland, 1995; Brusca and Brusca, 1990]