4. Kingdom Animalia - part 09 - Phylum : Mollusca

 

4. Kingdom Animalia - part 09 - Phylum : Mollusca


Phylum : Mollusca (Mollis : soft)

  • e.g. Pila, Bivalve, Octopus (devil fish), Sepia (cuttle fish), Chaetopleura (Chiton), Pinctada (Pearl oyster), L oligo (Squid), Aplysia (Sea hare), Dentalium (Tusk shell).
  • This is second largest phylum. 
  • Molluscs are either free living or sedentary. 
  • They are aquatic or seen in marshy places. Few are terrestrial. 
  • These are soft bodied and show tube within tube body plan. 
  • These are bilaterally symmetrical, but few are asymmetrical due to torsion (twisting). 
  • Body is divisible into head, foot and visceral mass. 
  • Visceral mass is enclosed in thick muscular fold of body wall called mantle. 
  • Mantle secretes a hard calcareous shell, the shell may be external or internal or absent. 
  • Muscular foot is present on ventral side.
  • Digestive system is well developed, complete with anterior mouth and posterior anus. 
  • Buccal cavity has a rasping organ called radula which is provided with transverse rows of teeth. 
  • Aquatic forms show numerous feather like gills called ctenidia, useful for aquatic respiration. 
  • Gills are present in mantle cavity. (space between visceral mass and mantle)
  • Terrestrial forms may show presence of lungs. 
  • Circulatory system is of open type (except Sepia, which possesses closed type).
  • Blood contains a copper containing blue respiratory pigment called haemocyanin. 
  • Excretion occurs by kidneys, also called as organ of Bojanus. 
  • Nervous system is formed by three pairs of ganglia. 
  • Ganglia are interconnected by commissures and connectives. 
  • Sense organs such as
  1. eyes for vision
  2. tentacles for tactile sensation
  3. osphradia for testing purity of water are present. 
  • Sexes are separate, animals are  mostly oviparous, development is direct or indirect.

Economic importance - 

  • Pearl oyster gives precious pearls. 
  • Many molluscs are edible. 
  • Shells of molluscs are rich source of calcium.

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