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1800-102-2727Have you ever played with ants? Definitely not! As they sting us and yes, that’s really a burning sensation. Can you guess some other animals which can sting their potential threats? Yes, there are such animals in the world.
One of the groups of lower invertebrates called phylum Cnidaria, also called Coelenterata, includes such animals. Cnidaria is derived from the word ‘knide’ meaning sting cells. Let’s learn more about coelenterates.
Coelenterates are aquatic, the majority of them being marine. They are either sessile or free-swimming.
Coelenterates have their bodies divisible into left and right halves through any plane passing through the centre of the body, hence, they are Radially Symmetrical.
Cnidarians have cells organised into tissues, hence, said to have tissue level of body organisation.
Cnidarians are diploblastic as the constituent body cells are arranged in two embryonic layers, outer ectoderm (epidermis) and inner endoderm (gastrodermis). An undifferentiated layer is sandwiched between them called mesoglea.
Cnidarians possess cnidoblasts, also known as cnidocytes, on the tentacles and body. Inside the cnidoblasts are present the stinging capsules or nematocysts which contain hypnotoxin. Injection of hypnotoxin into the prey paralyses the latter which is then fed upon by the cnidarian.
Following are the roles of cnidoblasts:
Coelenterates possess a single gastro-vascular body cavity known as coelenteron with mouth or oral cavity called the hypostome.
Both intracellular and extracellular digestion takes place in them. Mechanism of digestion is summarised as follows:
Skeletons made up of calcium carbonate can be found in a few cnidarians like corals.
Cnidarians can exist in either or both the body forms which are mentioned below:
Some Cnidarians remain in one form, either polyp or medusa, throughout their entire life cycle. On the other hand, certain cnidarians show metagenesis, i.e., alternation of generation between polyp and medusa forms. During metagenesis, polyps reproduce asexually by budding to form a large number of medusa. The medusae reproduce sexually to give rise to polyps. For example: Obelia
Phylum Cnidaria is classified into three different classes, on the basis of dominance of the medusoid or polypoid phase.
Body form is either in the form of only polyp or both polyp and medusa. Example: Obelia, Hydra etc.
Body form is of medusoid type. Example: Aurelia etc.
Body form is of only polyp type. Medusa stage is absent. Example: Gorgonia (Sea-fan), Adamsia (Sea anemone) etc.
Solution: Interstitial cells of cnidarians are undifferentiated cells. They have the ability to undergo differentiation, thereby forming specific cells. These are not specialised as they can form different cells.
Hence, the correct option is a.
Solution: The given figure is of metagenesis.
Solution: Coelenterates show both extracellular and intracellular digestion. Extracellular digestion occurs in the gastro-vascular cavity.
Hence, the correct option is c.
Solution: Cnidarians possess certain specialised cells called cnidocytes, on the tentacles and body. Inside them are present stinging capsules (nematocysts) which contain hypnotoxin. Injection of hypnotoxin into the prey paralyses the latter which is then fed upon by the cnidarian.
Question1.- Is Cnidaria same as Coelenterata?
Answer. Yes, Cnidaria and Coelenterata are the same. They are a group of lower invertebrates with tissue level of body organisation.
Question2.- Match the following:
Column I |
Column II |
||
a. |
Gorgonia |
1. |
Jelly fish |
b. |
Meandrina |
2. |
Sea-fan |
c. |
Adamsia |
3. |
Portuguese man-of-war |
d. |
Pennatula |
4. |
Sea anemone |
e. |
Physalia |
5. |
Brain coral |
f. |
Aurelia |
6. |
Sea-pen |
Answer.
Column I |
Column II |
||
a. |
Gorgonia |
2. |
Sea-fan |
b. |
Meandrina |
5. |
Brain coral |
c. |
Adamsia |
4. |
Sea anemone |
d. |
Pennatula |
6. |
Sea-pen |
e. |
Physalia |
3. |
Portuguese man-of-war |
f. |
Aurelia |
1. |
Jelly fish |
Question3.- Name the three classes of Coelenterata.
Answer. The three classes of phylum Coelenterata are mentioned below:
Question4.- Describe the process of alternation of generation in coelenterates.
Answer. Coelenterates can exist in either or both of the following forms:
Certain coelenterates like Obelia show alternation of generation between polyp and medusa forms. This is called metagenesis. During this, polyps reproduce asexually by budding to form a large number of medusa. The medusae undergo sexual reproduction to give rise to polyps.