PHYLUM MOLLUSCA

Published in Zoology
Saturday, 03 June 2017 03:05
  • "Aristotle" recognised many Cephalopods and Gastropods.
  • In 1650 "Johnson" coined the word "Mollusca". "Linnaeus" included "Mollusca" in the phylum "Verms".
  • In 1790 Cuvier laid the foundation for modern classification of Mollusca.
  • In 1818 "de Blainville" proposed "Bivahlia" name.
  • In 1821 "Gold fuss" proposed the name "Pelecypoda".
  • In 1862 "Bronn" proposed the name "Scaphopoda".
  • Finally in 1883 "Lankester" gave final position to Mollusca.
  • Molluscs are soft bodied animals.
  • These are distributed all over the world.
  • About 90,000 species of Molluscs are known today.
  • Molluscans are bilaterally symmetrical animals.
  • They show soft, short bodies without segmentation.
  • The body has an anterior head, a dorsal visceral mass and ventral muscular foot.
  • Foot is modified for crawling, burrowing or swimming.
  • The body is covered by a fleshy mantle which secretes a calcareous shell.
  • The shell is usually external, or it may be internal, reduced or absent.
  • The shell may be of one piece and called univalve or of two parts and known as bivalve.
  • Between the mantle and the body is a mantle cavity into which the anus and kidneys open, and in which lie a pair of ciliated gills or ctenidia.
  • The coelom is reduced to cavities of pericardium, gonads and kidneys.
  • The main body cavity is a haemocoel.
  • There is a dorsal heart with one or two auricles and a single ventricle.
  • The respiratory pigment is haemocyanin.
  • Sexes are usually separate, development is either direct or there is a modified trochophore called a veliger larva.
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