Phylum Mollusca: General Characteristics and Classification

Phylum Mollusca: General Characteristics and Classification

General Characteristics of Phylum Mollusca

  1. (Mollis or molluscs: soft)
  2. Kingdom: Animalia
  3. Habitat: Mostly aquatic and few are terrestrial
  4. Body is soft without segmentation
  5. Grade of organization: Organ system grade
  6. Germ layer: Triploblastic
  7. Symmetry: BIlaterally symmetry
  8. The body is divided into head, muscular foot and visceral mass.
  9. The body is covered by a mantle and a shell
  10. Respiration: by gills (ctinidia) in the mantle cavity. lungs in terrestrial forms
  11. Digestive system: Complete and developed
  12. Circulatory system: Closed type
  13. Heart consists of one or two auricle and one ventricle
  14. Excretion: pair of Metanephridia (Kidney)
  15. Nervous system: consists of three pair of ganglia (cerebral, visceral and pedal)
  16. Sexes: Dioecious and few are hermaphrodite
  17. Reproduction: gamatic fusion
  18. Fertilization: external or internal
  19. Development: direct or indirect by trochophore larvae.


Classification of Phylum Mollusca

There are six classes of which three are more prominent.

Class 1 Pelecypoda or Bivalvia

  • Habitat: mostly marine
  • They burrow in mud and sand.
  • Symmetry: bilateral and the body is laterally compressed.
  • No distinct head
  • Examples: Mussels, Unio, Mytilus,


Class 2 Gastropoda

  • Habitat: These are either aquatic or terrestrial
  • They possess a spiral shell.
  • The foot is large and flat.
  • Head is well developed with tentacles and eyes.
  • Examples: Hilex (Snail), Limax (Slug), Pila (Apple snail)


Class 3 Cephalopoda

  • Habitat: mostly marine.
  • They are adapted for swimming.
  • The foot is modified into eight to ten long tentacles in the head region.
  • The shell is either external, internal or absent.
  • Examples: Octopus, Loligo, Sepia, spirula, Nautilus


Class 4 Monoplacophora

  • Habitat: exclusively marine
  • Head is present without eyes and tentacles
  • Gills are external
  • Excretion: by serially arranged nephridia
  • Examples:


Class 5 Amphineura

  • Habitat: mostly marine
  • Body is elongated
  • Eye and tentacles are absent
  • Radula is present
  • Examples; Chiton, Neomenia


Class 6 Scaphopoda

  • Habitat: marine
  • Body is bilateral symmetry elongated and enclosed in tusk shell
  • Foot is reduced
  • Excretion: a pair of Kidney
  • Examples: Cadulus, Dentalium

Phylum Mollusca: General Characteristics and Classification