PHYLUM MOLLUSCA

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1) PHYLUM MOLLUSCA

1.1) HISTORY

1.2) DIAGNOSTIC CHARACTERS OF PHYLUM MOLLUSCA

1.3) CLASSIFICATION OF PHYLUM MOLLUSCA

1.3.1) CLASS I AMPHINEURA

1.3.2) CLASS II SCAPHOPODA

1.3.3) CLASS III GASTROPODA

1.4) SUB-CLASS (A) PROSOBRANCHIATA (= STREPTONEURA)

1.5) SUB-CLASS (B) EUTHYNEURA

1.5.1) CLASS IV BIVALVIA OR PELECYPODA

1.5.2) CLASS V CEPHALOPODA

1.6) SUB-CLASS A DIBRANCHIATA

1.7) SUB-CLASS B TETRABRANCHIATA

1.8) SUB-CLASS C AMMONODIA

HISTORY

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

1. Worm like without shell and foot.
2. Integument thick with minute spicules.
  Example :- Chaetoderma

Order 2 Polyplacophora

1. Body oval with large flat foot.
2. Shell of a midorsal row of 8 broad plates.
  Example :- Chiton, lschnochiton

CLASS II SCAPHOPODA

Tooth shells or tusk shells.
Shell and mantle tubular, curved and open at both ends.
Example :- Dentalium

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

1. Two rows of mamentous gills.
2. 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

1. Nervous system not concentrated.
2. Single ctenidium.

Sub-order 1 Taenioglossa

- Gill filaments in single row.
- Shell spirally coiled or boat shaped.
  Example :- Cyprea, Triton, Pila

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

1. Viscera and nervous system secondarily unwound.
2. 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

- Sea slugs. No external shell.
- Dorsal surface with extensive projections called cerrata.
  Example :- Doris, Aeolis

Order 2 Pulmonata

1. Fresh-water and land snails and slugs.
2. Shell a simple spire or absent.

Sub-order 1 Basommatophora

- Fresh-water small limpets.
- One pair of tentacles and eyes near the tentacle base.
  Example :- Limnea, Planorbis.

Sub–order 2 Stylommatophora

- Land snails.
- Tentacles 2 pairs. Eyes at the tip of hind pair.
  Example :- Helix, limax.

CLASS IV BIVALVIA OR PELECYPODA

Bivalve shell made of 2 lateral valves.
Mantle flattened gills 1 or 2 pairs.

Order 1 Protobranchiata

1. Gill with central axis bearing 2 divergent rows of short filaments.
2. Foot flat ventrally. Two adductor muscles.
  Example :- Solenomya, Leda.

Order 2 Fillibranchiata or Lamellibranchiata

1. Gills enlarged. filliform.
2. Often with byssus.
  Example :- Mytilus. Arca.

Order 3 Pseudolamellibranchiata

1. . Gills from vertical folds.
2. Anterior adductor muscle large. Siphon absent.
  Example :- Pecten. Oyster. Pearl oyster.

Order 4 Eulamellibranchiata

1. Gills filaments united by interfilamental and inter lamellar junctions.
2. Equalor sub-equal adductor muscles.
  Example :- Lamellidens. Unio. Anodonta. Teredo, Venus.
  Ensis. Sanguinolaria. Solenocurtus. Mya

CLASS V CEPHALOPODA

Head large, eyes conspicuous and complex.
Shell external or internal or absent

SUB-CLASS A DIBRANCHIATA

Gills 2, Nephridia 2
Shell internal or reduced.

Order 1 Decapoda

1. Ten arms.
2. Internal chambered shell.
  Example :- Sepia, Loligo, Rossia.

Order 2 Octopoda

1. Eight arms.
2. Shell absent.
  Example :- Octopus

SUB-CLASS B TETRABRANCHIATA

Gills, kidneys and auricles each 4 in number.
Shell spirally coiled.
Example :- Nautilus, Argonauta.

SUB-CLASS C AMMONODIA

Shell external
Suturelines irregular
Example :- Ammonites.

REFERENCES :-

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