Phylum Cnidaria Coelenterata


 
 
Concept Explanation
 

Phylum Cnidaria Coelenterata

Phylum Cnidaria (Coelenterata): Cnidarians are mostly marine. A few live in freshwater. They are called coelenterates (which mean hollow gut) because they have a body cavity called gastrovascular cavity, or coelenteron. This cavity has a single opening for ingestion and egestion.

  • They have a radially symmetrical body, which means if you cut them longitudinally through any diameter, you will get two identical halves. The cells of cnidarians are organized into tissues. A characteristic organ is the tentacle.
  • Cnidarians use tentacles to catch prey and fight enemies. Nematoblasts, or cnidoblasts, are characteristic stinging cells present particularly in the tentacles to fight enemies and paralyse prey. These cells contain a coiled-up, barbed, threadlike structure that shoots out when stimulated.
  • They also release a poisonous substance. Cnidarians are called diploblastic because the body wall has two layers : ectoderm and endoderm.
  • Diploblastic also means possessing two germ layers. Germ layers are embryonic layers of cells from which the tissues of the organism develop. Cnidarians have a loose network of nerve cells. Cnidarians are of two types : polyps, which have a cylindrical body, and medusae, which look like umbrellas.
  • Many cnidarians are polymorphic (e.g., Physalia), or they change forms. They change from the polyp to the medusa form in the course of their life cycle. The polyp reproduces asexually to give rise to the medusa, which reproduces sexually in a cycle known as the alternation of generation. The polyp form is sedentary, while the medusa form swims freely. Polyps can lead a solitary life or can form colonies.
  • Some cnidarians, like those of the genus Hydra, exist only in one form. Hydra exists in the polyp form. It lives in freshwater and reproduces sexually and asexually. Jellyfish (Aurelia) are medusoid and marine. Coral organisms, sea anemones and sea pens are of the polyp form. Coral polyps have an external skeleton of large CaCO_3 . They live in vast colonies and when they die, their chalk like skeletons form coral reefs.
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    Sample Questions
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    Question : 1

    Hydra belong to

    Right Option : B
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    Question : 2

    Cnidarians are also known as the coelenterates because of the _________ .

    Right Option : B
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    Question : 3

    Cavity that helps circulation and digestion is found in _________________

    Right Option : B
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