User:Pooja srivastava/E-Museum/Cnidaria

General characters of Cnidaria

• Coelenterata includes two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies).
  
• The term coelenterate is no longer recognized as scientifically valid, as the Cnidaria and Ctenophora are placed at equal rank under the Metazoa with the other phyla of animals.
  
• Phylum cnidaria contains over 10,000 species of animals found exclusively in aquatic and mostly marine environments.
  
• The name Cnidaria comes from the Greek word "cnidos," which means stinging nettle.  Cnidarians  got this name because their nematocysts eject barbed threads tipped with poison.
  
• Cnidarians  are multicellular animals.
  
• They contain  hollow body cavity  known as coelom
  
• They are aquatic and mostly marine in nature.
  
• They may be sedentary or free swimming.
  
• Body wall consists of 3 layers of cells-Ectoderm , Endoderm and Mesoglea.
  
• They show the phenomena of polymorphism. They exist in two forms-Polyp and Medusa.
  

Polyps-

1. It is ssesile and asexual zooid.
   
2. They are approximately cylindrical in shape and elongated at the axis of the body.
   
3. In solitary polyps, the aboral end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly.
   
4. The oral end contains the mouth, and is surrounded by a circlet of tentacles.
   

Medusae-

1. It is free swimming and sexual zooid.
   
2. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below.
3. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella.
4. The mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum).

• Cnidarians possess nematocysts which serves the function of paralysing the prey by injecting prey .
• Reproduction is both asexual and sexual. The eggs develops into a cilliated larva known as planula.

Some Common Examples