PHYLUM CNIDARIA (= COELENTERATA )
Definition and Introduction
Tissue grade eumetazoans with a radial symmetry .The term coelenterate signifies the presence of a single internal cavity called coelenterons, or gastrovascular, cavity combining function of both digestive and body cavities, The term cnidaria indicates the presence of cells (Gr. Knide= nettle or stinging cells) About 9000 species known.
Aristotle knew cnidarians due to their stinging nature and called these cnide He regarded these intermediate between animals and plants .Peyssonel (1723)and Trembley( 1744) provd these to be animals .Hence Linnaeus (1758) cuvier (1796) and Lamard (1801) included sponges and cnidarians under his phylum Coelenterata. Finally ,Hatschek (1888) divided Coelenterata into three phyla spongiaria (= porifera ) cnidaria and ctenophore.
1.All aquatic, mostly marine. Solitary or colonial.
2. Body typically dimorphic (of 2 types ) sessile , cylindrical and hydra like polyp form and free swimming saucer, cup bell or umbrella shaped medusa form many species unclude both forms in life cycle .In some ,the two basic types are further modified into polymorphic individuals .
3. Anterior end either swollen into a hypostome, bearing terminal mouth or flattened into an oral disc with central mouth.
4. Mouth typically surrounded by highly contractile , thread like sensory tentacles,
5.Basic radial symmetry is a diagnostic character.
6. Next to sponges advancement of body organization .Physiological division of labour and consequent cell differentiation more elaborate and permanent. Also similar cells form permanent epithelial tissues, definite organs not formed .Thus body organization is of tissue grade.
7. The single body cavity, called coelenterons gastro vascular cavity is simple or branched, or divided into compartments by longitudinal plate like partitions called mesenteries. It opens out by a single mouth .No anus.
8. Body wall diploblastic, i.e. formed from two embryonic germinal layers ecto derm and endoderm , respectively forming its outer epidermis and inner gastrodermis. In between is a noncellular jelly like layer called mesogloea. In some advanced members , the mesogloea contains some cell derived from epidermis and gastrodermis.
9. Both epidermis and gastroderimis or only epidermis contain certain highly specialized and unique stinging cells or nematoblasts found nowhere else in the animals kingdom ,These cells are used for food capture, defense and attachment .
10. Most cnidarians are carnivorous and mainly feed upon small crustaceans of zooplankton. Digestion partly extracellular in gastrovascular cavity and partly intracellular in food vacuoles of some gastrodermal cells.
11. Gaseous exchanges and excretion (ammonia) through general body surface .
12. Reproduction asexual (building ) or sexual .Life cycle usually exhibits an alternation of generations ( metagenesis) between asexual polypoid and sexual medusoid phase .A free swimming ciliated stereoastrula called planula larva, occurs in life cycle of most cnidarians.
13. A primitive type of nervous system ,without brain is found in the form of an in traepidermal nerve net. Eye spots (= ocelli) and statocysts for equilibrium occur in some.