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  • The true animals are considered to have originated only about 600 million years ago.
  • The first clear-cut enumeration of all animals was made by Linnaeus in 1758 who included 4,236 different kinds of animals.
  • Symmetry of animals may be defined as the arrangement of parts in relation to points, planes and straight lines.
  • This kind of segmentation is called metameric segmentation and the phenomenon is known as metamerism.
  • The Animal kingdom is divided into two sub-kingdoms: Protozoa and Metazoa.
  • Protozoans are single-celled animals and metazoa include multicellular animals.
  • Porifera includes sponges. Their body is perforated by pores.
  • In the phylum Coelenterata about 9,000 species have been described so far.
  • Platyhelminths are free living or parasitic.
  • Nemathelminths/ aschelminths are round worms which are triploblastic with a false body cavity called pseudocoel.
  • Annelids are triploblastic, eucoelomate worms whose body is divided into segments.
  • In Arthropoda, sexes are generally separate and sexual dimorphism is often exhibited.
  • In Mollusca, between the exoskeleton and the visceral mass there is a membrane called mantle which secretes a calcareous shell.
  • In Echinodermata, there are special structures called tube feet used for locomotion and a water-vascular system.

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