Animal Kingdom

Phylum Mollusca

Molluscans are very diverse animals and form an important part of the ecosystem in the world. Some animals that you are familiar with like snails, octopuses, squids, oysters, clams etc. all belong to Phylum Mollusca. Did you know that Molluscs with shells can produce pearls? Some of these are also eaten by humans in different regions of the world. Let us educate ourselves about this phylum of the Animal Kingdom.

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Phylum Mollusca

Snail

Many organisms belonging to this phylum have a calciferous shell. The bodies are generally very soft and are covered by the hard exoskeleton. They can be found in terrestrial regions as well as in the depths of the seas. You will be amazed to know that the size of the molluscans can range from 20 metres to one millimetre, with a few microscopic animals too. These animals continue to have an important role in the lives of humans.

They are a source of food as well as jewellery. The hard shells are used to make beautiful jewellery pieces. In some regions of the world, these are also raised as pets, even though it sounds bizarre. Pearls that are obtained from bivalves and gastropods are valuable as these pearls are lined with nacre. Natural pearls are formed when a small foreign object gets stuck in between the mantle and shell of the mollusc.

The bivalve molluscs are used as bioindicators of the freshwater and marine environments. But not all molluscans are good for humans.  Some are pests like the snails and slugs.

Learn more about Phylum Arthropoda here.

Characteristic Features of Phylum Mollusca

  • They are bilaterally symmetrical.
  • They are triploblastic, which three layers.
  • They show organ system grade of organisation.
  • The body is soft and unsegmented.
  • Body is divisible into three regions – head, a visceral mass, and ventral foot.
  • Body is covered by a mantle and shell.
  • They can be found in different habitats, both aquatic and terrestrial.
  • The body cavity is a haemocoel, through which blood circulates.
  • The digestive system is well developed and complex. It has a radula, which is a rasping structure that has chitinous teeth. It is used in feeding.
  • The nervous system is made up of paired ganglia, connectives, and nerves.
  • The circulatory system is open, with heart and aorta.
  • Respiration occurs through gills called ctenidia.
  • The ventral muscular foot helps in locomotion.
  • They have a pair of kidneys (metanephridia)
  • Sexes are separate and reproduction is through sexual reproduction.
  • Fertilisation can be internal or external.

Learn more about Phylum Annelida here.

Classification of Phylum Mollusca

The different classes under phylum Mollusca include:

  • Bivalvia
  • Gastropoda
  • Cephalopoda
  • Monoplacophora
  • Amphineura
  • Scaphopoda

Examples

Octopus

  • Clams
  • Mussels
  • Octopus
  • Pila ( Snail)
  • Oyster
  • Cuttlefish
  • Squid

Learn more about Phylum Porifera here.

Solved Questions For You

Q: Write a few sentences about Octopus.

Ans: An Octopus is sometimes known as the Devilfish. It belongs to the class Cephalopoda.  It is a soft-bodied mollusc with eight arms. Octopuses can camouflage very well in their surroundings. They have ink glands that they use to defend themselves or escape from predators.  They show all the characteristic features of Phylum Mollusca and are triploblastic, bilaterally symmetrical, and found in various regions of the ocean.

Q: Which is the second largest phylum after Arthropoda?

Ans: Phylum Mollusca.

Q: Mantle, foot and restricted coelom are the characteristic features of which phylum?

Ans: Phylum Mollusca.

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