Coelenterata

Coelenterata is made up of two greek words i.e. coelon and enterata where coelon means hollow and enterata means cavity. Peyssonesrecognized it as animal and Leucard coined coelenterate.

General characteristics of phylum Coelenterata:

• Most of them are marine while a few are freshwater, solitary or colonial, sedentary or free swimming.
• The body is acoelomate i.e. without coelom, radially or biradially symmetrical.
• Tissue grade of body organization.
• Diploblastic
• Mouth is surrounded by hollow cavity called tentacles arrange in one or more whorls.Tentacles help in food capture, ingestion and for defense.
• They show polymorphism i.e. they show two types of individuals in their life cycle, the polyps, and the medusal. The polyps are sessile concerned with feeding, protection, and asexual reproduction and the medusal are free swimming concerned with sexual reproduction.
• Coelenterates are characterized by the presence of nematocysts. The nematocysts are stinging cells present on one or both layers of the body wall and are more abundant on the tentacles.
• Sense organs are simple or complicated with cell and statocysts.
• Digestion is both extracellular as well as intracellular.
• Respiratory, circulatory and excretory systems are absent.
• Nervous system and sense organs are present.
• Asexual reproduction(Budding) and sexual(fusion of gametes).
• Lifecycle is an alternation of generation.

Classification:

Coelenterata consists of about 9000 species.They are divided into three classes:

Hydrozoa:

• They are solitary as well as colonial, sessile and free swimming. Mostly marine, a few are freshwater.
• They exhibit tetramerous or polymerous radial symmetry.
• The gastrovascular cavity is without stomodaeum, septa or nematocysts bearing gastric filaments.
• Asexual polyps are dominant.
• Example; Hydra, obelia.

Scyphozoa:

• They are solitary and exclusively marine.
• The gonads are endodermal in origin.
• Gastrovascular cavity with gastric filament, may or may not be divided into four interradial packets by septa.
• Example: Aurelia, Rhizostoma.

Anthozoa:

• They are solitary or colonial, exclusively marine.
• Body cylindrical with a bilateral or biradial symmetry.
• The gastrovascular cavity is divided into the compartment by complete or incomplete septa.
• Fertilization is external.
• Example: Xenia, Corallium.