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Phylum Coelenterata: General Characteristics, Skeleton, Digestion, Metagenesis, Examples and Classification

Have 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.

Table of Contents:

General Characteristics of Phylum Coelenterata

Habitat

Coelenterates are aquatic, the majority of them being marine. They are either sessile or free-swimming.

Symmetry

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.

radial symmctry in hydra

Level of Organisation

Cnidarians have cells organised into tissues, hence, said to have tissue level of body organisation.

Germ layer 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.

diploblastic body organisation in cnidaria

Defence Mechanism in Coelenterates

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.

caidoblast in cnidarians

Following are the roles of cnidoblasts:

  • Defence
  • Anchorage
  • Capturing prey

Digestion in Coelenterates

Coelenterates possess a single gastro-vascular body cavity known as coelenteron with mouth or oral cavity called the hypostome

gastrovascular cavity ni hydra

Both intracellular and extracellular digestion takes place in them. Mechanism of digestion is summarised as follows:

Mechanism of digestion

Skeleton of Coelenterates

Skeletons made up of calcium carbonate can be found in a few cnidarians like corals.

corals

Body Forms and Alternation of Generation of Coelenterates

Cnidarians can exist in either or both the body forms which are mentioned below:

  • Polyp: Cylindrical and sessile without gonads. For example: Hydra, Adamsia.
  • Medusa: Free-swimming and having a shape resembling an umbrella. They have gonads. For example: Aurelia or jellyfish.

polyp and medusa

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 

metagenesis ni cnidaria

Examples of Coelenterates

  • Portuguese man-of-war (Physalia)
  • Brain coral (Meandrina)
  • Sea anemone (Adamsia)
  • Sea-fan (Gorgonia)
  • Sea-pen (Pennatula)

brain coral and sea-fan

Classification of Phylum Coelenterata

Phylum Cnidaria is classified into three different classes, on the basis of dominance of the medusoid or polypoid phase.

Class Hydrozoa

Body form is either in the form of only polyp or both polyp and medusa. Example: Obelia, Hydra etc.

hydra

Class Scyphozoa

Body form is of medusoid type. Example: Aurelia etc.

aurelia-gellyfish

Class Anthozoa

Body form is of only polyp type. Medusa stage is absent. Example: Gorgonia (Sea-fan), Adamsia (Sea anemone) etc.

adamsia[sea-anemone]

Practice Problems of Phylum Coelenterata

  1. Which of the following cells is not specialised for a particular function?
    1. Interstitial cells
    2. Gastrodermal cells
    3. Choanocytes
    4. Nematocysts

[metagenesis]

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.

  1. What is ‘a’ and ‘b’ in the figure given below?

Solution: The given figure is of metagenesis. 

  • Polyps reproduce asexually to form a large number of medusa. Hence, ‘a’ is asexual reproduction.
  • The medusae reproduce sexually to give rise to polyps. Hence, ‘b’ is sexual reproduction.
    1. If ‘1’ represents the intracellular digestion, ‘2’ represents the extracellular digestion, then coelenterates undergo
      1. only 1
      2. only 2
  • both 1 and 2
    1. Coelenterates don’t undergo digestion

Solution: Coelenterates show both extracellular and intracellular digestion. Extracellular digestion occurs in the gastro-vascular cavity.

Hence, the correct option is c.

    1. Which of the following is used by coelenterates to capture their prey?
  • Hypnotoxin
    1. Saliva
    2. Skeleton
    3. Both a and b are correct

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.

FAQs of Phylum Coelenterata

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:

  • Class Hydrozoa
  • Class Scyphozoa
  • Class Anthozoa

Question4.- Describe the process of alternation of generation in coelenterates.

Answer. Coelenterates can exist in either or both of the following forms:

  • Polyp: Sessile and cylindrical
  • Medusa: Free-swimming and umbrella-shaped

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.

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