General Characteristics and Classification of Cnidaria

By Nitul Kalita

A brief introduction to phylum Cnidaria


Phylum Cnidaria (also called as coelenterata) includes about 9000 species. Some are found in fresh water, but the greater numbers are in marines. Coelenterates (Greek word. koilos- hollow + enterongut) are the simplest of eumetazoans. .They include the hydra, jellyfish, sea anemones and corals.





GENERAL CHARACTERS-


1. Cnidarians are multicellular animals with tissue grade of organisation. 

2.Body radial or bi-radially symmetrical with oral-aboral axis.  

3. They are diploblastic animals. 

4. Coelenterate animals are represented by two morphologically different types of individuals, polyps and medusa. 

5. Polyps are sessile/stationary (attached to the substratum) with a tubular body (eg. Hydra). Whereas medusa are free-swimming with an umbrella or bell-shaped body, eg. Aurelia, Metridium. 

6. They are all aquatic, mostly marine and some freshwater (eg. Hydra). 

7. They may exhibit solitary or colonial habitat. 

8. One of the most characteristic structures of all coelenterates is the stinging cells, called nematocysts. 

 9. Nematocysts serve for adhesion, food capture and offence and defence.  

10. They have a single gastro-vascular cavity, into which mouth opens.

11. The Planula larva present in the life history of Coelenterata.






CLASSIFICATION-



Phylum Cnidaria has been divided into four Classes-

Class1- Hydrozoa

Class2- Scyphozoa

Class3- Anthozoa

Class4- Cubozoa


Class 1.Hydrozoa (Gr., hydra- water + zoon- animal) 

1. Hydrozoa mostly colonial and marine.

2. Some hydrozoa are solitary and freshwater. 

3.They exhibit tetramerous or polymerous radial symmetry. 

4. Gastro-vascular cavity without stomodaeum, septa or nematocysts bearing gastric filament. 

5.They exhibit either only polyps or both asexual polyps and sexual medusa form. 

6.Gonads are epidermal in origin.

7. Examples are- Hydra,Obelia etc.


Class 2.Scyphozoa (Gr.Skyphos-cup + zoios-animals) 

1. They are exclusively marine and solitary forms. 

2.The medusa stage is dominant but the polyp stage reduced or absent. 

3.The gonads are gastro-dermal and sex cells are shed or released in digestive cavity.

4. Examples are- Aurelia,Haliclystus,etc.

Class 3 Anthozoa (Actinozoa): (Gr.Anthos- flower + zoios- animal) 

1.They are exclusively marine, may be solitary or colonial.

2.All are polyp forms, nomedusae.

3.They have gastro-dermal gonads. 

4. Gasto-vascular cavity is divided into completed and incomplete septa. 

5. Skeleton either external or internal. 

6. Fertilisation is external. 

7. Examples are- Telesto,Tubipora, etc.


Class 4 Cubozoa (Cubomedusa)

1.All are marine,found in tropical and subtropical oceans.

2. The medusa phase is long and dominant.

3. The medusa are squarish in cross section.

4. Radial canals are replaced by four radial pouches.

5. Examples are- Charybdaea, Chiropsalmus,Chironex.


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