PHYLUM MOLLUSCA
AFIFA
UNIVERSTY OF GUJRAT
BS 3RD

 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.

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.

 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.

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

 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

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

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

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

 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

 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.

 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.

 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

 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.

 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

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.

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.

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.