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

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