Cnidaria are solitary, colonial, benthic or pelagic animals found in all seas and oceans of the world. There are also some brackish and freshwater forms. The Cnidaria appeared early in the geological record, fossils having been dated from the Precambrian era (-700 million years).

They are diploblast having a simple organization, a typically radial symmetry and without any defined respiratory, circulation or excretory system. This group of 11.000 species is divided into four classes: the Scyphozoa, the Cubozoa, the Hydrozoa and the Anthozoa (Figure 1). (Parker, 1982).

Figure 1. Simplified Taxonomy of the Cnidaria (adapted from Synopsis and classification of living organisms, 1982).