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Classification of Animals

Classification of Animals Synopsis

Synopsis 

Basis of Classification 

  • The animal kingdom includes all metazoans.
  • Metazoans are multicellular animals with holozoic nutrition.
  • All the animals have certain features in common which form the basis of animal classification.
  • These different features are
     
 

Phylum Porifera

  • Phylum Porifera includes pore-bearing animals.
  • They are also called sponges.
  • They are marine, diploblastic, either radially symmetrical or asymmetrical animals.
  • They show cellular level of organisation.
  • Body has a number pores called ostia.
  • The central cavity called spongocoel open outside through the osculum. 
  • In the canal system, the water current enters through ostia, passes through the spongocoel and exits the body through the osculum.
  • Choanocytes or collar cells line the spongocoel.
  • The body skeleton is made up of spicules or sponging fibres.
  • Sponges are hermaphrodites.
  • They reproduce asexually by fragmentation and sexually by the formation of gametes.
  • Fertilisation is internal and the development in indirect.
  • Examples: Sycon, Euspongia
Phylum Cnidaria 
  • The old name of this phylum is Coelenterata.
  • Aquatic mostly marine. Only hydra is a freshwater form.
  • They are sessile or free swimming.
  • They are radially symmetrical and diploblastic.
  • They show the tissue level of organisation.
  • There is a gastro-vascularcavity with a single opening called hypostome.
  • Digestion is extracellular and intracellular.
  • Cnidoblasts are the stinging cells of cnidarians found around the mouth and on the tentacles. They are used for the defense and catching prey. The name ‘Cnidaria’ is derived from the cnidoblasts.
  • Cnidarians show two body forms, a polyp and a medusa. Cnidarians exhibiting both these body forms show alternation of generation.
  • Polyps reproduce medusa asexually, and medusa produce polyps sexually.
  • Examples: Hydra, Physalia
     

Phylum Ctenophora

  • These animals were earlier placed in the phylum Cnidaria but later separated in different phylum because of lack of cnidoblasts.
  • They are commonly known as seawalnuts or comb jellies.
  • They are exclusively marine,radially symmetrical and diploblastic animals.
  • They show tissue level organisation.
  • Ctenophores bear cilia which are arranged in eight external rows called comb plates which help in locomotion.
  • Digestion is both extracellular and intracellular.
  • Bioluminescence is commonly seen in ctenophores. It is the property of living organisms to emit light.
  • Examples: Pleurobranchia, Beroe