Phylum Mollusca are successfully living since Cambrian time. Presently 500,000 living species besides fossils. For the first time molluscs have acquired hard shell for protection. They are all free-living in fresh water, brackish water and sea water. Clams, oysters, squids serve as human food. Pearl oysters produce pearls for human ornaments. Certain snails serve as intermediate host for helminthic diseases of man and his animals. Gastropod molluscs reveal torsion and detortion. Phylum mollusca have medical importance. Pila globosa is the slowest moving animal.
DIAGNOSTIC CHARACTERS OF PHYLUM MOLLUSCA
- Unsegmented (except in Monoplacophora).
- Presence of shell mantle, visceral mass and foot.
- Circulatory system open.
- Excretion by paired metanephridia.
CLASSIFICATION OF PHYLUM MOLLUSCA
CLASS I AMPHINEURA
- Elongated or oval body.
- Head reduced without eyes and tentacles.
Order 1 Aplacophora
- Worm like without shell and foot.
- Integument thick with minute spicules.
Example :- Chaetoderma
Order 2 Polyplacophora
- Body oval with large flat foot.
- Shell of a midorsal row of 8 broad plates.
Example :- Chiton, lschnochiton
CLASS III GASTROPODA
- Univalve shell usually spirally coiled due to torsion.
- Tentacles and eyes present.
SUB-CLASS (A) PROSOBRANCHIATA (= STREPTONEURA)
- Nervous system in the form of 8.
- Gills (Ctenidia) anterior to heart.
Order 1 Aspidobranchiata
- Two rows of mamentous gills.
- Two auricles, two nephridia.
Sub-order 1 Docoglassa
- Nervous system less concentrated.
- Ctenidium replaced by secondary gills.
Example :- Patella, Acmea, Lottia.
Sub–order 2 Rhipidoglossa
- Shell valves without insertion plates.
- Few Ctenidia
Example :- Lepidioplurus, Haliotes Trochus, Turbo.
Order 2 Pectinibranchiata
- Nervous system not concentrated.
- Single ctenidium.
Sub-order 1 Taenioglossa
Sub–order 2 Rachiglossa
- Nervous system concentrated.
- Edge of shell with siphonal notch or canal.
Example :- Buccinium, Murex, Conus
SUB-CLASS (B) EUTHYNEURA
- Shell reduced.
- Nerves not twisted in the form of 8.
Order 1 Opisthobranchia
- Viscera and nervous system secondarily unwound.
- Single nephridium, auricle and gill.
Sub-order 1 Tectibranchia
- Shell internal, single gill.
- Parapodial lobes large.
Example :- Aplysia, Bulla.
Sub–order 2 pteropoda
- Shell present.
- Foot modified into wing like parapodia for swimming.
Example :- Cavolina.
Sub–order 3 Nudibranchia
Order 2 Pulmonata
- Fresh-water and land snails and slugs.
- Shell a simple spire or absent.
Sub-order 1 Basommatophora
Sub–order 2 Stylommatophora
CLASS IV BIVALVIA OR PELECYPODA
- Bivalve shell made of 2 lateral valves.
- Mantle flattened gills 1 or 2 pairs.
Order 1 Protobranchiata
- Gill with central axis bearing 2 divergent rows of short filaments.
- Foot flat ventrally. Two adductor muscles.
Example :- Solenomya, Leda.
Order 2 Fillibranchiata or Lamellibranchiata
- Gills enlarged. filliform.
- Often with byssus.
Example :- Mytilus. Arca.
Order 3 Pseudolamellibranchiata
Order 4 Eulamellibranchiata
CLASS V CEPHALOPODA
- Head large, eyes conspicuous and complex.
- Shell external or internal or absent
SUB-CLASS A DIBRANCHIATA
SUB-CLASS B TETRABRANCHIATA
SUB-CLASS C AMMONODIA
- Shell external
- Suturelines irregular
Example :- Ammonites.