To study the classification with reasons of the phylum mollusca

Title : To study the classification with reasons of the phylum Mollusca.

Phylum Mollusca :- Mollusca mean soft bodied animals. [Latin, molluscs - soft]. Aristotle first used this term for cuttle- fish. In Latin soft nut enclose in a thin shell is called mollusc. Thus, it is referring to the bivalve shell & soft bodied animal enclosed in the shell.

General characters of mollusca:

(1)They are aquatic, mostly marine, some are fresh water & Some are terrestrial. 

(2)They are bilaterally symmetrical; however, gastropods & cephalopods lose their bilateral symmetry & become asymmetrical due to torsion or spiral twisting.

(3)Body of the molluscs is soft, differentiated into four Parts anterior head, dorsal visceral mass, ventral foot and mantle. 

(4)Epidermis is single layered, generally ciliated with mucous glands.

(5)Muscular foot is present on the ventral side which is locomotory organ and modified for creeping, Swimming and burrowing.

(6)A thin muscular, fleshy fold covers the dorsal body wall called mantle or pallium & space enclosed by it is called mantle cavity.

(7)Shell is secreted by outer surface of mantle. shell is hard, calcareous, & it may be bivalved , univalved, spiral or cone like or even absent in some animals.

(8)Respiration by gills called ctenidia. Body surface, mantle or lungs are respiratory organs in terrestrial forms.

(9)Digestive system is complete. Buccal cavity contains a grasping organ, the radula with transverse row of teeth. 

(10)Circulatory system is of open type. Excretion is brought about by one or two pairs of sack-like kidney.

(11)Nervous System consists of Paired ganglia, cerebral, Pleural, pedal & visceral ganglia interconnected by commissures and connectives.

(12)Tentacles, eyes, statocysts & osphradia are sense organs. 

(13)Sexes are separate, some are hermaphrodite, fertilization is external or internal.

Phylum Mollusca is divided into six classes:-

1) Amphineura - e.g. Chiton

2) Gastropoda - e.g. Pila 

3) Pelecypoda - e.g. Unio 

4) Scaphopoda - e.g. Dentalium

5) Cephalopoda - e.g. Octopus

6)Monoplacophora - e.g. Neopilina

1]Class: Amphineura: Chiton 

Phylum: Mollusca - Body soft, divide into head, foot, mantle, visceral mass. 

Class : Amphineura - Head reduced, no eyes & tentacles, mouth & anus terminal.

Sub-class: Polyplacophora - Dorsal surface convex, ventral surface bears flat foot. 

Family: Chitonidae - Shell composed of 8 articulating plates or valves.

Genus: Chiton 

Species: barnesi 

2]Class: Gastropoda: Pila [Snail]

Phylum: Mollusca- Body soft , divide into head, foot, mantle, visceral mass. 

Class : Gastropoda: Body asymmetrical; spirally twisted may be dextral or sinistral.

Order: Mesogastropoda: Presence of operculum, Single auricle & monopectinate gill.

Family: Ampullariidae: Gills & lungs Present & are separated by mantle cavity.

 Genus: Pila

Species: globosa 

3]Class: Pelecypoda: Unio [Bivalves].

Phylum: Mollusca: Body soft, divide into head, foot, mantle, visceral mass.

Class : Pelecypoda: Bivalved shell, body laterally Compressed, head indistinct.

Order: Unionoida: Large fresh water mussels. 

Family: Unionidae: Head absent, bivalve usually with beak ie. elevated portion.

Genus: Unio

4]Class - Scaphopoda - Dentalium [Tusk shell].

Phylum: Mollusca Body soft, divide into head, foot, mantle, visceral mass.

Class : Scaphopoda -Elongated, tusk like shell, eyes and tentacles are absent.

Order: Dentaliida - straight or distinctly curved hardy shell.

Family: Dentaliidae - Radula tooth has cutting edge & is smooth & rather arched.

Genus: Dentalium

Species: vulgare

5]Class: Cephalopoda - Octopus [Indian Devil fish).

Phylum: Mollusca - Body soft, unsegmented.

Class : Cephalopoda - Mouth with horny jaws and Surrounded by arms or tentacles. 

Order: Octopoda - Long non-retractile 8 oral arms with sessile suckers. 

Family: Octopodidae - Suckers are in one or two series, internal shell reduced.

Genus : Octopus 

Species : macropus

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