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).

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.