Phylum: Mollusca, its classification and characteristics

  1. multicellular, triploblastic, system grade of body organization.
  2. mostly bilaterally symmetrical except gastropods.
  3. mostly marine, some are found in fresh water whereas some are terrestrial.
  4. body is soft and unsegmented.
  5. body is divided into head, foot and visceral mass.
  6. body enclosed in shell formed of calcium carbonate.
  7. shell may be external, internal, reduced or absent.
  8. locomotion by muscular foot which is modified for creeping, swimming and burrowing.
  9. complete alimentary canal and grasping organ radula is present in buccal cavity except pelecypoda.
  10. haemocoelomate, coelom is reduced to pericardial cavity and cavities of gonads and kidneys.
  11. respiration by ctenidia (gills), pulmonary sac in terrestrial forms.
  12. excretion by metanephridia.
  13. tentacles, eyes, statocyst (balancing organ), osphradium (water testing organ), etc are present as well developed sense organs.
  14. mostly unisexual, few are bisexual.
  15. development is direct or indirect.

Phylum Mollusca (General characteristics and classification) - Online  Science Notes

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Molluscs are divided into six classes on the basis of nature of foot and shell. They are:

Class 1: Monoplacophora

  1. mostly marine.
  2. body is segmented enclosed in sub circular shell with spirally coiled protoconch.
  3. head without eyes and tentacles.
  4. locomotion by flat ventral muscular foot.

examples: Neopilina (connecting link between annelida and mollusca)

Mollusca (Mollusks) | Mollusca, Snail, Animal science

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Class 2: Polyplacophora (Amphineura)

  1. exclusively marine.
  2. body is enclosed by shell consisting of eight transverse rows of oval or longitudinal plates (valves).
  3. head reduced without eyes and tentacles.
  4. locomotion by thick, flat, sole like ventral foot.

examples: Chiton, Chitoderma

Class 3: Gastropoda

  1. freshwater or marine or terrestrial, also found in moist soil.
  2. body is covered by spirally coiled shell.
  3. distinct head with eyes and tentacles.
  4. adult gastropods are asymmetrical due to torsion.

Examples: Pila globosa (apple snail), Limax (grey slug), Helix (garden snail)

Class 4: Scaphopoda

  1. mostly marine.
  2. body is enclosed in tubular shell open at both ends.
  3. head and eyes are absent.
  4. conical foot used for burrowing.

examples: Dentalium (tusk shell), Cadutus

Class 5: Pelecypoda (bivalvia)

  1. mostly marine, few are found in fresh water.
  2. body is enclosed in bivalve shell which are joined mid dorsally.
  3. head, eyes, radula and tentacles are absent.
  4. hatchet shaped foot (claw like foot).

examples: Unio (fresh water mussel), Mytilus, Pinctada (pearl oyster)

Class 6: Cephalopoda

  1. mostly marine.
  2. distinct head with large eyes and tentacles.
  3. foot is modified into arms and tentacles with sucker.
  4. shell is external, internal, reduced or absent.

examples; Octopus (devil fish), Loligo (squid), Sepia (cuttle fish)

Phylum Mollusca #2 (Class: Bivalvia) Flashcards | Quizlet

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Phylum: Mollusca, its classification and characteristics