Molluscs are soft-bodied organisms that typically have a head and foot region. Their bodies are often covered by a hard shell. They have a tissue-based body organization and are bilaterally symmetrical. The body is divided into regions including the head, mantle, foot, and visceral mass. The shell, when present, is usually univalve or bivalve. Molluscs have a complete digestive system with a digestive gland, a rasping organ called a radula, and circulatory, respiratory, excretory and nervous systems. They are classified into groups including bivalves, monoplacophorans, gastropods, cephalopods, scaphopods, a

2. MOLLUSCA

3. WHAT IS MOLLUSCA
Molluscs are a clade
of organisms that all have
soft bodies which typically
have a "head“ and a "foot“
region.Often their bodies
are covered by a hard
exoskeleton, as in the
shells of snails and clams
or the plates of chitons.

4. GENERAL CHARACTARISTICS
 Tissue-system grade
of body organisation.
 Tripoblastic,
ceolomate,
unsegmented (except
monoplacophora)
and bilaterally
symmetrical.

5.  Body divisible into
head, mantle, foot
and visceral mass.
 Shell, when present,
usually univalve or
bivalve, constituing
in exoskeleton,
internal in some.

6.  Ceolom reduced and represented mainly by
pericardial cavity, gonadial cavity and kidney.
 Digestive system complete with a digestive gland
or liver (hepatopancrease); a rasping organ, the
redula, usually present.

7.  Circulatory system
mainly of closed type,
but some emptying into
sinuses; heart with one
or two auricles and one
ventricle; blood with
amoebocytes and
haemocyanin.
 Respiration direct or by
gills or lungs or both.
 Excretion by paired
metanephridia(kidneys)

8.  Nervous system of
paired ganglia,
connectives and
nerves . Ganglia
usually forms a
circumenteric ring.
 Sense organs include
eyes, statocystsand
receptors for touch,
smell or taste.

9.  Diococcus or
monococcus; one or two
gonads with gonoducts,
opening into renal ducts
or to exterior.
 Fertilization external or
internal; development
direct or through free
larval forms.
 Terrestial or
acquatic(freshwater or
marine).

10. CLASSIFICATION

11. BIVALVIA
 Body enclosed with
bivalve shell and
laterally compressed.
 No head,tentacles,eyes,
jaws and redula.
Example:
 Scallop
 Whelk
 Fileclamp

12. MONOPLACOPHORA
 Body bilaterally
symmetrical, with a dome
shaped mentle.
 Internal segmentation.
Example:
 Neopilina

13. GASTROPODA
 Well developed head
with eyes and tentacles;
redula present.
 Foot large and flat.
Example:
 Pila(apple snail)
 Patella(limpet)
 Slugs

14. CEPHALOPODA
 Body elongated
dorsoventrally.
 Head distinct and large
with well developed
eyes; foot as tentacles
and siphon.
Example:
 Nautilus
 Octopus

15. SCAPHOPODA
 Body within a tubular
shell, open at both
end.
 No head, mouth with
tentacles,no eyes.
Example:
 Dentalium
 Cadulus

16. APLACOPHORA
 Body worm-like
with a mantle.
 No shell and foot.
Example:
 Neomania
 Cheatoderma

17. POLYPLACOPHORA
 Dorso-ventrslly flattend
body.
 Shell as 8 calcareous
dorsal plates.
Example:
 Chaetoplura
 Chiton

18. PEARL CULTURE
A pearl is a hard
object produced within the
soft tissue (specifically the
mantle) of a living shelled
mollusc. Just like the shell
of a clam, a pearl is made
up of calcium carbonate in
minute crystalline form,
which has been deposited
in concentric layers

19. DEVELOPMENT OF A PEARL
A pearl is formed when the mantle tissue is
injured by a parasite, an attack of a fish or another
event that damages the external fragile rim of the shell
of a mollusk shell bivalve or gastropod. In response,
the mantle tissue of the mollusk secretes nacre into the
pearl sac, a cyst that forms during the healing process.

20. Chemically speaking,
this is calcium carbonate
and a fibrous protein called
conchiolin. As the nacre
builds up in layers of
minute aragonite tablets, it
fills the growing pearl sac
and eventually forms a
pearl. It is a myth that a
grain of sand or grit can
cause a pearl to form, as
nacre does not adhere to
inorganic substances.