Grade Radiate Cells Part 2
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:- Hydara: A freshwater – coelenterate. It exists only in polyp form; therefore alternation of generations is absent.
- Obelia: Amarince colonial that exhibits alternation of generations.
- Aurelia (jelly fish): The polyp is reduced and medusa is dominant in jellyfish.
- Actinia (sea anemone): The body consists of polyp only enteron is divided by large partitions called mesenteries.
- 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. 10.5 coelenterates (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).
The corals because of their massive structure serve as living place for a variety of sea life. |
Coral reefs are found in the coastal waters of Florida. West Indies, east coast of Africa, Australia and Island of Coral sea.
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