M N I S I N M L
PHYLUM: MOLLUSCA
STUDY OBJECTIVES
Student should be able to understand the following concepts: 1. Molluscan classification
2. Molluscan characteristics
3. Form and function of molluscan structures 4. Torsion
5. Coiling
CHARACTERISTICS
Bilaterally symmetrial; unsegmented
Ventral body wall speialized as musular foot
Dorsal body wall forms a pair of folds alled mantle, enolses mantle avity, modified
into gills or lungs; seretes the shell
Open irulatory system: 3 hember heart, blood vessels and blood sinuses, respiratory
pigments in blood
Gaseous exhange: gills, lungs, mantle or body surfae
1 or 2 kidneys (metanephridia) opening into periardial avity and emptying into
mantle avity
Nervous system: paired erebral, pleural, pedal and viseral ganglia, with nerve ords
and sub-epidermal nerve plexus; ganglia entralized in nerve ring in gastropods / ephalopods.
Sensory organs of touh, smell, taste, equilibrium and vision; eyes highly developed
in ephalopods
Reprodutive system: monoeious & dioeious, spiral leavage; trohophore & veliger
BODY PLAN
Molluscan body bears head-foot and visceral mass
portions
Head-foot: contains feeding, cephalic sensory and
locomotory organs
Visceral mass: contains digestive, respiratory and
reproductive organs
HEAD-FOOT
a) RADULA
Rasps off line particles of food and carries it towards digestive tract
Rasping organ used in scraping, tearing and rasping food. Radula is chitinous ribbon, bearing many rows of fine teeth,
b) FOOT
Used for locomotion, attachment to substratum, for food capture Waves of muscular contraction effects a creeping locomotion
Secrete mucus used as adhesive aid or slime track to glide on cilia Food is manipulated by combination of muscle and hydrostatic skeleton
VISCERAL MASS
a) MANTLE (PALLIUM) AND MANTLE CAVITY
Sheath of skin hanging down in two folds around soft body Encloses space between mantle and body wall
Called mantle cavity (pallial cavity)
Mantle cavity houses gills (ctenidia)/ lungs. Outer side of mantle secretes shell.
Inner side of mantle is ciliated.
Mantle, gills and lungs active in gaseous exchange
b) SHELL
Secreted by mantle and line by it. 3 layers:
1. Periostracum:
Outer horny layer composed of conchiolin (protein) Secreted by fold of mantle edge
2. Prismatic layer:
Middle layer composed of calcium carbonate. Secreted by glandular margin of mantle
3. Nacreous layer:
The Shell
• The mantle is responsible for secreting the shell. • The shell is comprised of three layers:
• The outside of the shell is covered by an organic layer - periostracum • The middle prismatic layer is characterized by densely packed prisms of
calcium carbonate laid down in a protein matrix
REPRODUTION
Most molluscs produce a
free-swimming ciliated larvae called the
trochophore larvae
In some molluscs the
trochophore develops into the adult, but in other
molluscs (e.g., gastropods) there is a second larval
Ctenidium (gill)
Interlamellar junctions
Ostium
Frontal cilia
Blood vessel Exhalent
CLASSIFICATION
CLASSES:
Caudofoveata Solenogastres
Monoplacophora – segmented worms Polyplacophora – chitons
Scaphopoda – tusk shells or tooth shells Gastropoda - snails
Bivalvia – bivalves
CLASS: CAUDOFOVEATA
Wormlike
Marine organisms
Mostly burrowers
Feed on microorganisms No shell
CLASS: SOLENOGASTRES
Marine
Wormlike
No shell
No radula
No gills
Haemaphroditic
CLASS: MONOPLACOPHORA
Bears low, round shell &
creeping foot
Serial repetition in
organs
Neopilina:
Has 5 pairs of gills 2 pairs of auricles 6 pairs of nephridia 1 or 2 pairs of gonads Nervous system with 10
pairs of pedal nerves
CLASS: POLYPLACOPHORA
Bears 8 plates
Mantle forms girdle around
margin of plates
On each side of foot is row of
gills
Water enters grooves (pallial
grooves) anteriorly, flows across gills and leaves posteriorly
3 chambered heart Pair of kidneys
(metanephridia)
2 pairs of longitudinal nerve
cords
Head and cephalic sensory
organs reduced, but
SUBCLASS: PROSOBRANCHIA
Mantle cavity anterior
One pair of tentacles
Sexes are separate
Operculum – horny
plates are present
Mostly marine snails,
SUBCLASS: OPISTOBRANCHIA
2 groups:
Tectibranchs: with gills
& shell present
Nudibranchs: no shell or
true gills
2 pairs of tentacles
2nd pair of tentacles
modified: rhinophores (chemoreceptive)
All marine
SUBCLASS: PULMONATA
Terrestrial & freshwater snails Anterior mantle cavity
developed into air-breathing lung
Aquatic spp:
1 pair of non-retractile
tentacles
Pair of eyes at base of
tentacle
Land forms:
2 pairs of retractile tentacles Eyes located on ends of
posterior
CLASS: BIVALVIA
Omnivores
They are the only type of molluscs that does not have a
radula.
SHELL
Bivalves always have two shells (valves) held together by hinges and
strong muscles.
Valves are drawn together by adductor muscles Function: protection
Umbo – oldest part of the shell
MANTLE
Hangs down on each side of visceral mass, shielding pair of gills on
each side
Posterior edges of mantle folds modified – forming dorsal
Most common bivalves include:
GILLS
Gaseous exchange: mantle & gills Gills are modified for filter feeding
Consists of filaments on either side of central axis,
forming lamellae.
Water route:
Through incurrent siphon, propelled by cilliary action, Enters tubes through pores at filaments into the
FEEDING AND DIGESTION
Filter Feeding
A physical process which
involves very small particles moving through a fluid at low velocities.
Respiratory currents bring oxygen & organic materials to gills
Gland cells on gills secrete
mucus to trap food
particles.
Style sac opens into the stomach & secrete a
gelatinous rod called
crystalline style.
Digestive enzymes are
freed in digestive gland for
intracellular digestion.
INTERNAL FEATURES
3 chambered heart in pericardial cavity
(2 auricles, 1 ventricle)
Pumps blood through gills & mantle Kidney is “U” shaped
Nervous system: 3 pairs of ganglia
Sense organs:
pair of statocysts in foot
Pair of osphradia in mantle cavity
REPRODUCTION and DEVELOPMENT
Sexes are separate
Marine embryos: 3 free-swimming larval stages:
Trochophore, veliger larva and spat Fertilization is external
In freshwater clamps:
Fertilization is internal
Development of a bivalved glochidium larva
Glochidium discharge to attach on gills or skin of fish and
CLASS: CEPHALOPODA
Most advanced of molluscs
All marine & active predators (carnivorous)
“head-footed” – modified foot concentrated in head region Edges of foot are drawn out into arms & tentacles that bear
sucker discs
LOCOMOTION
Swim by expelling water from mantle cavity through ventral
funnel / siphon
Some common cephalopods include: ○ Octopus
○ Squid
SHELL
Shell is made buoyant by series of gas chambers
Buoyancy allows the animal to swim
Shell of Nautilus – divided by transverse septa into internal
chambers. Chambers are connected by a tube called
siphuncle, and secretes gas into empty chambers.
Cuttlefish (Sepia sp.) – mantle encloses shell.
Squid (Loligo sp.) – shell has disappeared, leaving a thin
body strip (the pen) which is enclosed by the mantle.
EXTERNAL FEAUTURES
8 – 10 appendages around mouth. No external shell,
1 pair of gills
Octopus has 8 arms Cuttlefish has 10 arms
Nautilus has 60-80 tentacles
Heads of cephalopods have pair of large & complex
COLOUR CHANGES
CHROMATOPHORES – pigment cells in skin that produces
colour change
Pigment cells are controlled by nervous system to hormones Colour change: protection, behaviour, alarm, courtship
INK PRODUCTION
Cephalopods (excl. Nautilus) have an ink sac that empties
into rectum
Sac contains ink gland – that secretes into sac sepia, a dark
fluid containing pigment melanin.
FEEDING AND NUTRITION
Predacious – feed on small fish, molluscs, worms,
Arms used for food capture & handling.
Inner surface of arms bear powerful suction cups. Strong jaws – with tongue like radula
Octopus & cuttlefish have salivary glands that secrete
a poison for immobilizing prey
REPRODUCTION
Separate sexes
Male’s seminal vessicle sperm encased in
spermatophores & stored in the sac that opens into mantle cavity
1 arm of adult male modified as intromittent organ – hectocotylus,
which during copulation plucks a spermatophore from his own mantle cavity & inserts it into mantle cavity of female.
Eggs are fertilized as they leave oviduct & are
IMPORTANCE OF MOLLUSCS
Molluscs serve as food for humans as well as
for other animals.
Some are used as environmental indicators because their
bodies are filled with water.
There are about 85,000 species of molluscs. It is the largest marine phylum.
There is a risk of food poisoning from toxins that
accumulate in moluscs.