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Molluscs

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Main Characteristics of Molluscs

"More than 50 000 mollusc species are known as well as 30 000 extinct."

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Characteristics of molluscs

  • unsegmented soft body
  • most have internal or external shell
  • have a mantle - a fold in the body wall that secretes the hard prtesctive shell  
  • muscular foot and/or tentacles
  • have a radula - a toothed structure used to grate food
  • two pairs of gills except in polmonate snails

Anatomy
  • almost all molluscs divided into two functional regions: head-foot and the visceral lump
  • head-foot is most visible part, easily seen in snails and slugs
  • also is mostly muscular and covered in cilia to help the mollosc move around
  • visceral mass is the rest of the body, which is entirely nonmuscular and contains the species organ (digestive, reproductive, etc)
  • most mollucs have a shell, a thick layer made of calcium carbonate
  • the mantle, the fold of tissue covers the visceral mass and shell (if present)
  • the mantle cavity is where the lungs and gills are housed

Methods of Obtaining Energy
  • all molluscs have different diets depending on class and surroundings
  • organisms like slugs and snails consume plants and sometimes other small creatures, containing a tougue that shreds all food
  • bivalves eat plankton and algae, bigger ones like squid and octupi eat fish, crustaceans and other molluscs

  • mollusks living in water use their gills as a means of obtaining energy, gills aborb oxygen and let out carbon dioxide
  • in this case, gills not only used for respiration but also feeding as cilia coats the walls of the gills as particles of food pushed out by gills and ingested by the organism
  • some bivalve mollucs feed on food particles in the water column, while others use a tubular siphon to vacuum food particles from the surface (called suspension-feeders)

Development & Evolution
  • It is beleived mollusks evolved from species similar to the one the right
  • contained its gill, reproductive and excretory organs within the mantle cavity, ressembles limpet-like fossils containing same structure which was first found in 1957

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  • during developement, molluscs underwent torsion, causing the mantle cavity and all organs to end up resting over their heads

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  • differentiates from other species because the classes of molloscs has always been debatable because they have evolved based on adaptations to environments and changing conditions
  • Most other phylum are not as divided, are not as diverse and have not evolved to the extent that the phylum molluca have
  • one example would be ferns, who did not evolve like that of the mollusc



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