Phylum Cnidaria

Abstract

The Cnidaria is a well-defined phylum consisting of approximately 10,000 living species that are aquatic and primarily marine in nature. The monophyly of the cnidarians is supported in all newer molecular studies. The freshwater polyp Hydra was for a long time commonly used as an experimental organism, but has since been supplanted by the marine starlet anemone Nematostella. Both morphological and molecular studies have established the radiation of the Cnidaria, which is a sister group of anthozoans and medusozoans. The phylum is characterised by highly complicated structures known as nematocysts (cnidae), which are formed inside special cells called cnidocytes (nematocytes). The epithelial cells are sometimes anchored to the mesogloea and to the perisarc by hemidesmosomes and tonofilaments, respectively. Eggs and sperm are often shed freely in the water and differentiate from ectodermal cells in hydrozoans, and from endodermal cells in anthozoans, scyphozoans, and cubozoans. The ancestral cnidarian could be a holopelagic, advanced gastrula, and therefore one of the very first metazoan carnivores.