Grade Radiate Cells

Phylum Coelenterata/Cnidaria – Diploblastic animals: The name Cnidaria has been given to this group of animals due to the presence of special cells called

cnidocytes. These cells give rise to nematocysts-the stinging cells, characteristic of this group.

Cnidarians have double layer organization and are therefore diploblastic having tissue grade organization and have organs. During the development two germinal layers are formed the outer ectoderm and inner endoderm from which their bodies are constructed. The ectoderm forms outer covering and some cells of this layer in most animals give rise to nematocysts while the endoderm cells become speciaialized for digestion of food.

Between the two layers is a jelly-like mesoglea. In these animals there is only one cavity which serves as digestive as well as body cavity which is called gastrovascular cavity or enteron and opens to the outside by only one opening the mouth. So the animals of this group have sac like digestive cavity

In coelenterates the arrangement of body parts is in relation to centralized axis (symmetrical).

An object is symmetrical where there is a correspondence in form and arrangement of parts so that a plane passing through the center divides it into similar halves. The coelenterates have radial symmetry and are aquatic, found both in marine and freshwater habitats.

The coelenterates range in size from microscopic Hydra to macroscopic, branchioceranthus, a hydrozoans polyp that may reach two meters in length. Cnidarians are found in two basic forms the polyps and the medusa. Polyps are cylindrical animals, which in most cases are nutritive in function, hence named as gastrozoids. The medusa are umbrella like in form. These are free swimming. The medusa are involved in sexual reproduction as they have gonads.

The mouth is surrounded by a series of tentacles. These bear stinging cells or nematocysts, which are organs of defense and offense.

The coelenterates are carnivores and feed upon small organisms which come into contact with them. These organisms are immobilized by nematocysts and taken into the digestive cavity as food where it is digested and then distributed by diffusion.

The nervous system is in the form of a network of neuron cells forming an irregular net or plexus in the body-wall. There is no central nervous system.

Many colonial coelenterates such as corals produce a hard exoskeleton formed of calcium carbonate (CaCO3). It is secreted by epidermal cells that take lime from sea water. The skeleton of coral is responsible for formation of small coral islands or large coral reefs.

Most species are sessile, for example Hydra, and Obelia, sea-anemone and corals, while other are free living and motile e.g. jelly fishes etc. many live as solitary individuals e.g. Hydra jelly fishes and sea-anemones and quite a large number are colonial e.g. physalia, vellela etc. a colony is an aggregation of individuals or zooids that perform different functions for the colony.

In coelenterates reproduction takes place byasexual as well as sexual means e.g. Hydra reproduces asexually by the formation of buds on its surface. The bud after some time separate from the parent and develops into a new individual. In Obelia for example there is asexual as well as sexual reproduction.

It has a kind of zooid known as blastostyle which gives rise to individual zooids called medusa by asexual method. The medusa when released in water develop reproductive organs which produce gametes that unite to form zygote from which Obelia colony is again formed.

The life cycle of coelenterates is characterized by the presence of alternation of generations. There are two generations; one reproduces by sexual means and the other by asexual means. Both generations are diploid. Often the two generations consist of one free-living and one attached stage. Therefore asexual generation and sexual generation alternation with one another. This is known as alternation of generations e.g., Obelia.

Polymorphism – A Characteristic Feature of Coelenterates (Cnidaria)

The occurrence of structurally and functionally more than two different types of individuals, called the zooids within the same organism is called polymorphism.

For example, in Obelia there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusa…..

The common examples of coelenterates are:

1. Hydara:  A freshwater – coelenterate. It exists only in polyp form; therefore alternation of generations is absent.
2. Obelia: Amarince colonial that exhibits alternation of generations.
3. Aurelia (jelly fish): The polyp is reduced and medusa is dominant in jellyfish.
4. Actinia (sea anemone): The body consists of polyp only enteron is divided by large partitions called mesenteries.
5. Madrepora: The body is covered with hard calcareous skeleton formed of calcium carbonate. They are commonly called corsal. The skeleton forms large coral reefs and even small islands.

Fig. 10.5coelenterates (Cnidarians)

Coral reefs

Corals are formed from the secretions produced by specialized polyps that are present in certain coelenterates. These polyps become covered by stony cups due to hardening of their secretions. From the mouth of the stony cup a polyp can pass out its tentacle for the purpose of feeding and withdraw itself where not feeding. Most such coelenterates are colonial. They stony net-work or mass of such coelenterates are called corals. Living polyps are found on the surface layer of corals whereas underneath the mass are dead stony structures only and there are no polyps inside. The stony masses that are formed in this way are called coral reefs. These are mostly formed of calcium carbonates (lime-stone).

Coral reefs are found in the coastal waters of Florida. West Indies, east coast of Africa, Australia and Island of Coral sea.