From a Greek word Cnidos(means thread), the term Cindaria has been derived. The body wall of these animals is made up of two definite cell layers. Hence they are known as diploblastic animals. In the cnidarian group the tissue grade of organization is observed for the first time in the animal kingdom. Animals that are sedentary or free swimming, diploblastic, tentacles with nematocysts, presence of tissue grade of organization but without any organ-systems and a central canal with a single aperture as mouth are regarded as the members of the cnidarian group.
CHARACTERISTIC FEATURES OF CNIDARIA
i) Living in water
Most of the Cnidarians are marine and few are sweet water (as Hydra).
ii) Solitary or colonial
Cnidarians are solitary or colonial in nature. Either they are attached to certain submerged material or free swimming. They are radially symmetrical (body may be divided into two equal halves by cutting into any of the radial plane).
iii) Diploblastic animals
Cnidarians are all diploblastic animals. Their body wall is composed of two definite cell-layers and so they are known as diploblastic. Outer layer of the body is calld ectoderm and the inner cell-layer is known as endoderm. Between the ectoderm and endoderm there is a non-cellular jelly like substance known as mesoglea. However they are acoelomate (there is no coelom).
The central cavity of the body which is lined by endoderm is known as coelenteron or gastrovascular cavity which may or may not be branched.
At the terminal part, there is a triangular structure that is regarded as hypostome; at the hypostome there is a mouth aperture which is surrounded by tentacles. There is no anal aperture; so the entrance and exit of water, both take place through the mouth aperture.
The body and tentacles are provided with special type of stinging cells on the ectoderm and they are known as cnidoblasts that contain whip like structure known as nematocysts. They are helping mainly in defense and food capturing by paralyzing the prey. Nematocysts are characteristic of cnidarians.
vii) Ill developed tissueCindaria possess ill developed tissue system but there is no development of organ-system.
viii) Absent of some organic system
There is no respiratory, excretory and blood vascular systems in Cnidarians.
ix) Primitive nervous system
There is primitive form of nervous system in Cindaria. Though there is lack of central nervous system yet the nerves are arranged in a net-like fashion, i.e., nerve net.
Digestion of Cindaria takes place both by extracellular and intracellular processes.
They reproduce both by asexual and sexual methods. Asexual takes place by budding and sexual occurs by sperms and ova. Generally, the animals are unisexual but bisexual may occur.
xii) Larval stage
In the life-cycle, there is ciliated planula larval stage. Generally two phases are present, such as sessile, elongated asexual phase of polyp and sexual free floating umbrella like medusa. Corals make colonies of polyp. Some forms polyp and medusa stages alternate during life-cycle. This is known as metagenesis.
Some example of Cindaria
i) Hydra (sweet/fresh water animal)
ii) Obelia (Sea fur),
iii) Aurelia (Jelly fish),
iv) Porpita, Physalia (Portuguese man-of-war),
v) Pilema, Metridium (Sea-anemone),
vi) Pennatula (Sea-pen),
vii) Gorgonia (Sea-fan),
viii) Corallium (Red coral),
ix) Tubipora (Pipe coral),
x) Meandrina (Brain coral),
xi) Madrepora (Staghorn coral).
What are the importance of Cindaria?
i) Some species of the phylum cnidaria are producing corals.
ii) Corals are deposited on the ocean to form island. Human beings are inhabiting on these coral islands,
iii) Corals of various types are used for decorating the house,
iv) Corals are also used as ornaments,
v) Coral zones are indicating about the accumulation of petroleum,
vi) Cnidarians are utilized as research materials.