Mollusca

 

General Information

  • Mollusca means “soft-bodied”

  • Second largest phylum with approximately 85,000 recognised living species.

  • Largest marine phylum, 23% of all named marine organisms

  • Include chitons, clams, mussels, snails, sea slugs, tusk shells, octopus, and squid

  • Gastropods (snails and slugs) are the most numerous and account for 80% of the total

  • Highly diverse in size, anatomical structure, behaviour and habitat

  • The phylum is typically divided into 7 or 8 taxonomic class, two are entirely extinct.

  • Includes Cephalopod molluscs, such as squid, cuttlefish and octopuses are amongst the most neurologically advanced of all invertebrate
  • The giant squid or colossial squid is the largest known invertebrate species.

 

Defining Characteristics

- The body is soft and segmented, into three region, head, a vesceral mass and ventral foot

- Body is covered with mantle and shell

    - a mantle with a significant cavity used for breathing and excretion

    - a protective calcareous external skeleton, the shell, is secreted from the back.

- Two types of basic symmetry are involved in their body structure:

    - Active animals in pursuit of food have bilateral symmetry

    - Sessile animals have radial symmetry (Heller, J. 2015)

- The presence of a rasping tongue (radula) in the mouth.

    - This is drawn out by sliding it over a cartilage cushion, gathers food and carries it back in. It consists of a narrow elastic ribbon beset with rows of teeth. Bivalves have no radula.

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