Phylum Coelenterata | phylum Cnidaria | General characteristics of phylum Cnidaria | class 11

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This lecture is about
GRADE RADIATA
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 specialized 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
hydrozoan polyp that may reach two metres in length. Cnidarians are found in two basic forms
the polyps and the medusae. Polyps are cylindrical animals, which in most cases are nutritive in
function, hence named as gastrozoids. The medusae are umbrella like in form. These are free
swimming. The medusae 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 ofense.
The coelentrates 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 difusion.
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
(CaC03
). 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, Obelia, sea-anemone and corals, while other are free
living and motile e.g. jelly ishes etc. Many live as solitary individuals e.g. Hryda jelly ishes 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 diferent functions for the colony.
In Coelenterates reproduction takes place by asexual 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
medusae by asexual method. The medusae 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 alternate 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 diferent 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