Introduction to Coelenterata

Aristotle, the father of biology first called the coelenterates "cnidae" because of their stinging qualities. He considered them as intermediate organisms between plants and animals. In 1723, Peyssonel established their animal nature.

In 1774, Trembly described "Hydras". Linnaeus, Lamarck, and Cuvier placed coelenterates in the animal group Radiata. In 1847, Leuckart Coined the word coelenterata. He included sponges and ctenophores also in it. In 1888, Hatshek divided this coelenterate into three phyla:

1. Spongiaria
2. Cnidaria
3. Ctenophora

Nowadays cnidaria is considered the correct name for this phylum. In 1833, Ehrenberg coined the name "Anthroza". In 1856, Huxley coined (Haeckel's Scyphozoa including present-day's Scyphozoans and Anthozoans).

Zoologists who retain cnidaria and ctenophores in the same group called it phylum Coelenterata. They divided the phylum into two subphyla called i) Cnidaria for coelenterates and ii) Acnidaria for ctenophores.

The cnidaria is metazoans that bear tentacles. They show radial and biradial symmetry. They developed tissue-grade organization. Hence this phylum begins the Eumetazoa group. They do not show definite heads. They show a primitive diffused nervous system. The important external feature of the cnidaria is radial symmetry, but in Anthozoa, it is modified into biracial symmetry.

Polymorphism is an important aspect of these animals. All these forms are divisible into two main types, i) Polyp forms and ii) Medusa forms.

Polyp is sendentray form. It takes up asexual reproduction only. Medusa is a free-swimming form. It takes up sexual reproduction.

In class Hydrozoa, the organisms show both polyp and medusoid forms. In class Scyphozoa, the organisms show only medusoid forms. In class Anthozoa, the organisms show only polyp forms.

The life cycle of these animals shows an alternation of the asexual polyp form with the sexual medusoid form. This is called "metagenesis".

The cnidaria is mostly marine and few are freshwater forms. They are well-known hydra etc.

In the present-day world, 9000 species of coelenterates are present. Many coelenterates are marine and few are freshwater animals. Corals and coral reefs are more important contributions of this phylum.