The phylum Mollusca includes several classes of soft-bodied animals that often have a calcium carbonate shell secreted by a mantle. The body consists of a head, foot, and visceral mass. Molluscs exhibit a range of habitats from terrestrial to marine. Notable classes include Gastropoda (snails and slugs), Bivalvia (clams and oysters), and Cephalopoda (octopuses, squid, and nautiluses), which have complex nervous systems. Molluscs play important ecological roles and some species are economically significant as food sources or decorations.

1. PHYLUM MOLLUSCA
AFIFA
UNIVERSTY OF GUJRAT
BS 3RD

2.  Def:-
 The coelomate animal phylum whose members possess a head –
foot, visceral mass, mantle cavity .
Most molluscs also possess a radula
and a calcareous shell.

3. CHARACTERISTICS
 Body of two parts: head foot and visceral mass.
 Mantle that secretes a calcareous shell and covers the
visceral mass.
 Shell: Monovalvular or bivalvular or absent or internal
 Bilateral symmetry.
 Mantle cavity function in excreation,gas
exchange, elimination wastes products.
 Open circulatory system in all except in one class
cephalopod.
 Protostome characteristics.
 Sense organs: Eyes, statocyst, sense or touch, smell and
gustatory.

4.  Reproductive system: Dioecious or monoecious, gonads
with one or two ducts.
 Fertilization: External or internal.
 Excretion: By nephridia or kidney.
 Habit and habitat: Terrestrial, aquatic, marine or fresh water.
the body of molluscs has three main reagion
1) head-foot:- is elongted with an anterior head containing the
mouth ,nervous nd sensory stucture and elongated foot used
for locomotion.

5.  Visceral mass contains the organs of
digestion,cirulation,reproduction and excretion.

6.  Mental: attatches to viscral mass,may secreate a shell that
overlies the mantle.
 The shell of mollusc is secreated in three layers
 Outer layer called periostracum .it is secreated from
mantale,s outer margin nd protein layer.
 Prismatic layer is middle layer and contains CaCO3 and
organic materials. Thickest layer……
 Nacreous layer is inner that is form from thin sheet of
calcium carbonate.its secreation thickns the shell

8.  Mantle cavity: between the mantle and foot is space called
mantle cavity.
 Functions:
 gas exchange
 excretion
 elimination of digestive wastes
 release of reproductive system

9.  Radula : the mouth of molluscs possess a resping structure
called radula.
 Consists of rows of teeth.

10. 
 Classification of phylum
Mollusca
 Class Aplacophora
 Class Polyplacophora
 Class Monoplacophora
 Class Scaphopoda
 Class Bivalvia
 Class Gastropoda
 Class Cephalopoda
 Class Caudofoveata

11.  Cephalopoda: there are approximately 650- 700 species
 includes octopus,squid, cuttlefish.
 Most complex form of molluscs
 Anterior end modified to forms tentacle's or armed used for
capture prey, attachment,locomotion.
 Shell:
 external shell is present in nautilus
 In some shell is absent as in octopuses
 In cuttlefish shell called cuttlebone

12.  Digestive system
The digestive tract consists of three parts:
 esophagus, which may contain a crop; stomach, which
mashes food; and caecum , where most digestion and
absorption occur.
 Locomotion in cephalopods :
 is accomplished mainly by jet propulsion
 Octopus can use their arms to "walk,“or crawl
 Reproduction:
 dioecious
 A female possess single ovidut.
one tantacles of male cephalopods called hectocotylus modify to
form spermatophor transfer.

13.  Nervous system:
 Cephalopods possess well-developed nervous systems and
complex sensory organs.
 Economic importance; Many species of squid and octopus are eaten.
 Nautilus shells are often used decoratively.
 The internal shell of a cuttlefish, or cuttle bone, is sold in the pet trade as
a calcium source for birds.
 Giant cephalopods such as squid and octopuses are also a great source
of sea-monster folkore.
. Class Gastropoda:
includes the snails and slugs.

14.  Shell:
Gastropods are characterized by "torsion," a process that
results in the rotation of the visceral mass and mantle on the
foot.
 Mostly have a single, usually spirally coiled shell .
 Body form: Gastropods have a muscular foot which is
used for “creeping" locomotion in most species modify for
swimming.
 Most gastropods have a well-developed head that includes
eyes, 1-2 pairs of tentacles, and a concentration of nervous
tissue

15.  Reproduction:-Gastropods are dioecious, and some forms
are hermaphroditic.
 Marine species have veliger larva. torsion occure at that
stage.
 Monoplacophora: Living representatives of this Class were not
discovered until 1952,
 Paleozoic fossil monoplacophorans had been known for
some time
 11 species are known.
 Most live at great depths and all are marine
 . Monoplacophorans are regarded as ancestral to
bivalves, gastropods, and cephalopods.

16.  Class aplacophora:
 250 species approxemetely
 Lack shell and crawl on ventral surface.
 Ladderlike nervous system
 Gonochoric/gonochoristic/dioecious (sexes separate)
 • Simultaneous hermaphrodite
 • Sexual
 • Fertilization
 o external
 o internal
 Aplacophorans move via cilia through or on substrate.
 • Motile
 feed on microorganisms and detritus
 E.g andrew

17. CLASS SCAPHOPODA
 There are approximately 900 species of Scaphopoda
 Tooth shells or tusk shells
 Scaphopods are all marine species
 Conical shell at both ends.
 Mostly buried in substrate with head and foot at downward apex
of shell upward
 Incurrent and excurrent water enters
nd leaves mantle cavity at the apex.
 Sexes are sparate trochophore and viliger larvae produced.

18. CLASS POLYPLACOPHORA
 Polyplacophorans include about 600 extant species
 Nervous system resembled with apolacophora.
 A chiton has calcareous spicules in the mantle.
 The head is reduced, and lacks eyes and tentacles.
 Sexes are separate.
 Feed on attached algae.

19. CLASS BIVALVIA
 There are approximetely 1500 species of bivalvia.
 characterized by possessing two shells secreted by a
mantle .
 The oldest part umbo…
 Pearl formation.
 Most marine bivalves go through a
trochophore stage before turning into a
free-swimming veliger larva .
 a glochidium attaches to fish gills.
 The glochidia larvae of some freshwater mussels can be
serious parasites of fish.