Phylum Coelenterata | Class 11 Biology

Animals are multicellular eukaryotic organisms. They have a heterotrophic mode of nutrition as they take nutrients from other organisms. Moreover, the central vacuole is absent in animals and they store food as glycogen or fat. Furthermore, Animals can transmit impulses due to the presence of nerve cells, Sponges do not have nerve cells. The broad classification of Animalia is based on common fundamental features such as Levels of organization (cellular level and tissue/organ/organ system), symmetry (Radial and Bilateral), Body cavity or coelom, Phylum( Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata). The cellular level of the organization includes sponges, tissue level includes coelenterate, and one The term Phylum was coined by Georges Leopold Cuvier(1829). Let us have an overview of Phylum Coelenterata.

Phylum Coelenterata

Distinguishing Features

1. All are aquatic and are mostly marine,  except a few like Hydra, which is fresh water. They are solitary or colonial. Both sessile and free swimming forms are found.
2. Body form varies considerably. Many colonial coelenterates like Obelia is a trimorphic colonies having three kinds of zooids. (i) Polyps- nutritive zooids. They provide food to the colony. (ii) Blastostyles- Budding zooids. They produce medusae by budding. (iii) Medusae- sexual zooids. They have testes or ovaries to produce gametes(sperms or ova) and take part in sexual reproduction. The occurrence of more than one type of individuals in their colonies performing different functions is called polymorphism.
3. They show radial symmetry.
4. Cnidarians are diploblastic animals i.e., derived only from two embryonic germ layers, viz., ectoderm and endoderm.
5. They are the first multicellular animals from an evolution point of view that show tissue level of organisation.
6. The body wall consists of two layers of cells; the outer epidermis and inner gastrodermis. Between the epidermis and the gastrodermis, there is a gelatinous layer, called mesogloea. 
7. The body cavity, known as the gastro-vascular cavity (coelenteron) contains only one opening, called the mouth on the hypostome which also acts as the anus. Thus there is present an incomplete digestive tract.
8. Both intra and extra-cellular digestion are present.
9. Respiration and excretion are carried out through the body surface by diffusion. Ammonia is the chief excretory waste.
10. “primitive Nervous System” consists of a network of nerve cells and their processes.
11. Some coelenterates, e.g., corals have a skeleton composed of calcium carbonate.
12. Both sexual and asexual(budding) reproduction are present. The power of regeneration is also developed.
13. The cleavage is holoblastic. Direct or indirect development is found. In Obelia planula larva is found. In the Aurelia planula, scyphistoma and ephyra larvae are present.
14. Life history often shows the phenomenon of alternation of generation (metagenesis). In Obelia an asexual polypoid generation alternates with a sexual medusoid generation.

Classification 

Chief on the basis of the dominance of the medusoid or polypoid phase in the life cycle, the phylum Cnidaria is divided into three classes.

• Hydrozoa
  • Both marine and freshwater forms are found in this class.
  • Either only polyps are found or polyps and medusae are present exhibiting polymorphism, e.g., Hydra, Obelia and Physalia, Porpita, Velella, Millepora (Hydroid coral).
• Scyphozoa (Gk. Skyphos= cup)
  • They are exclusively marine.
  • They are solitary and represented by the bell or umbrella-shaped medusae which are without velum, e.g., Aurelia, Rhizostoma.
• Anthozoa (= Actinozoa ; Gk. anthos= flower)
  • They are exclusively marine.
  • They are represented by polyp form. Medusa form is absent, e.g., Gorgonia(sea fan), Metridium(sea anemone), Adamsia(sea anemone), Alcyonium(deadman’s finger), Fungia(mushroom coral), etc.

Examples

Hydra

• Hydra is a freshwater, carnivorous coelenterate which is commonly found in lakes, ponds, slow moving streams and other standing waters where weeds and other vegetation are commonly present.
• Hydra moves by differenr methods such as somersaulting, looping, etc. 
• The animal reproduces both asexually and sexually. 
• Hydra has great power of regeneration which was first discovered by Trembley. 
• Hydra can digest proteins, fats, and some carbohydrates but not starch.

Obelia- The Sea Fur  

• Obelia is a marine form that lives in shallow coastal waters. 
• Obelia is a sedentary and trimorphic form consisting of three types of individuals in a colony, i.e., polyps, blastostyles and medusae. 
• The polyps are mainly nutritive in function, while the blastostyles give rise to reproductive individuals, the medusae by budding.
• The medusae contain gonads for sexual reproduction. Obelia is carnivorous. Both asexual and sexual reproduction occur in this animal. 
• Alternation of asexual and sexua stages( not diploid and haploid) occurs in the life cycle. The phenomenon is called metagenesis.

Physalia- The Portuguese man of war  

• It is a marine pelagic coelenterate. Physalia exhibits a remarkable example of polymorphism and division of labour. 
• The poison of dactylozooids is neurotoxic which is very harmful. It is also a nuisance for the bathers.

Aurelia-The jelly fish

• It occurs in coastal waters of most of the oceans of the world. In India they are quite common all along the Chennai coast.

Adamsia- The Sea anemone  

• Adamsia is a marine and found attached to the empty shell of gastropods(molluscs) occupied by hermit crab(genus Eupagarus). 
• It shows commensalism. The association between Adamsia palliata(sea anemone) and Eupagrus prideauxi(hermit crab) is a classical example of commensalism.

Facts about Cnidarians

• Scientific name: Cnidaria.
• Common name(s): Coelenterates, corals, jellyfish, hydrozoans.
• Basic animal group: invertebrate.
• Size: 3/5 of an inch to 6.5 feet in diameter; up to 250 feet long.
• Weight: up to 440 pounds.
• Lifespan: A few days to more than 4,000 years.
• Diet: Carnivore.

Special Characteristics 

• Animals of this phylum shows polymorphism.
• Nervous system is of diffuse type.
• Incomplete elementary canal.
• Tissue grade body organisation.
• Digestion is partially intracellular and partially intercellular.
• Exoskeleton of the animals is made up of calcium carbonate.
• In case of polymorphism, animals continuously change the shape of body, when they appear like umbrella form shape-(medusa form), when they appear in cylindrical form -(polyp).
• Asexual reproduction by budding.
• Cnidarians show alternation of generation(Metagenesis).

FAQs on Phylum Coelenterata

Question 1: Give two characteristics of Cnidarians.

Answer: 

All are aquatic and are mostly marine,  except a few like Hydra, which is fresh water. They are solitary or colonial. Both sessile and free swimming forms are founds. They show radial symmetry.

Question 2: Define polymorphism in Coelenterates.

Answer:

Polymorphism is the existence of multiple types of structurally and functionally diverse individuals within a population. The phylum Coelenterata’s class Hydrozoa contains a huge number of colonial species with several forms of individuals known as zooids.

Question 3: Give three examples of Coelenterates.

Answer:

Hydra, Aurelia, and Physalia are three examples of coelenterates.

Question 4: What is the common name for coelenterates?

Answer: 

Cnidarian, also called coelenterate, is any member of the phylum Cnidaria (Coelenterata), a group made up of more than 9,000 living species.

Question 5: What is the importance of Coelenterates?

Answer:

Many species of coelenterates are used as food which contributes to the economic growth of the nation.