Phylum : Mollusca

GENERAL CHARACTERS
CLASS OF PHYLUM MOLLUSCA

1. Phylum
Mollusca
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2. General Characteristics
50,000 or more species of molluscs.
They share 3 major sets of characteristics:
Body enclosed by a mantle that secretes a shell
made of calcium or some other stiff structure.
A mantle cavity between the mantle and the
internal organs; the anus, reproductive, and
excretory ducts open into the mantle cavity.
A ventral, muscular foot that is highly modified
among the various groups of molluscs.
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3. General Characteristics Cont’d
Free-living, multicellular animals
Invertebrates (lack a backbone)
Unsegmented
Have a true coelom
Bilateral symmetry
Simple to complex nervous system
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4. Habitats & Niches
Some mollusc habitats include:
Marine (mostly)
Freshwater (a few)
Land (a few)
Mollusc niches (life-style):
Most live on the ocean bottom and are mostly sedentary
Some are free-swimming (the cephalopods)
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5. The six major mollusc classes
Those 6 classes are:
Monoplacophora(mono=single; placo=shell; phora=bearing)
Ex. Neopilina
Aplacophora(a=without)
Ex. Neomenia
Polyplacophora(poly=many)
Ex. Chitons
Gastropoda(gastro=body cavity; poda=foot)
Ex. Common garden snail, abalone, turban snail
Scaphopoda
Ex. Dentalium
Cephalopoda(cephalo=dealing with head; poda=foot)
Ex. Squid, octopus, etc.
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6. Shell in Mollusca
The soft, unsegmented bodies of mollusk is
covered by hard and calcareous shell which
acts as protective covering.
Shell may be external or internal
Study of mollucscanshell called conchology.
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7. CLASS: Scaphopoda (Dentalium)
Shell opens on both ends
External ,cyndriccallytubular, curved and tapered.
Oral end is wider, shape like elephants tusk, white in
colour with grooves on entire shell.
In Cadulusis cucumber shaped.
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8. Class Aplacophora
Shell is absent, mantle is thick with calcareous spicules.
Acts as protective covering in Solenogaster.
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9. Class Monoplacophora
NeopilinaSingle shell dorsal large cup like shell is present.
shell is thin, subcircularwith raised and curved apex.
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10. Class Polyplacophora
Chitons are the only genus in the class Polyplacophora.
Their specific characteristics:
Posses a shell that consists of 8 overlapping plates.
Overlapping calcareous plates forms a solid armour covering the
dorsal surface.
The cephalic and anal shell plates are keeled middorsally .
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12. Class Gastropoda
Most popular class of molluscs.
Consists of snails and snail-like creatures.
Specific characteristics:
They have a dorsally located shell (often coiled).
Shell is composed of single piece that is univalved and spirally
coiled.
The shell has elongated form and hollow cone spirally coiled
around central axis called columella.
A single coil of shell round the axis is called whorl.
Dextral shell: aperture in right side and coling anticlockwise.
Sinistral: left aperture coilingclockwise.
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14. Class Bivalvia (Pelecypoda)
Specific Characteristics:
They have no head.
They have two shells held together by powerful muscles.
They have a ventrally located foot that sticks out between the two
valves.
Habitats include: marine and freshwater.
They tend to burrow into soft mud or sand or attach to rocks or
other shells.
Large gills are used for respiration and filter feeding.
Food is trapped by mucus on the gills and moved by cilia.
Water enters and exits through siphons.
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16. Class Cephalopoda
Contains the largest molluscs.
Specific Characteristics:
May lack a shell (like an octopus)
Shell may be reduced to a stiffening rod (like the squid or nautilus)
The foot is highly modified to form a group of tentacles around the
mouth.
They are found in deep and shallow waters along many coasts.
Squids & Nautilus are free-swimming and move very quickly.
Octopuses are found among rocks or crawling on the bottom of the
ocean.
Complex brain, two lateral eyes, excellent eyesight.
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