PHYLLUM

MOLLUSCA
MOLLUSCA

Wahyu Prihatini -
2010
Wahyu Prihatini - 2010
 Some Useful Terms
• Mantle - a single or paired outgrowth of the body wall
that lines the inner surface of the valves of the shell in
mollusks
• Trochophore - small, free-swimming, ciliated aquatic
larva of mollusks and annelids
• Setae - stiff bristle-like hair used for movement in
earthworms
• Radula - a band in the mouth of most mollusks that is
set with numerous, minute teeth and is drawn backward
and forward over the floor of the mouth to break up
food
• Crop - a pouch in the foregut of arthropods and annelids
for holding food
 More Useful Terms
• Torsion - the rotation of the visceral mass,
mantle and shell 180˚ with respect to the head
and foot of the gastropod. This brings the
mantle cavity and anus to an anterior position
above the head
• Septa - a division of the coelum (a partition
between segments of it) in annelida
• Visceral mass – area which contains most
internal organs
 General Information
• Mollusks and Annelids have three germ layers
• Both are also protostomes
• Mollusks move by using a foot that is extremely
muscular
• Mollusks have one coelum while Annelids have
multiple coelums
• Mollusks reproduce sexually while Annelids
reproduce both sexually and asexually
• Both have bilateral symmetry
Phylum Mollusca
1. Class Aplacophora
2. Class Polyplacophora
3. Class Monoplacophora
4. Class Gastropoda
5. Class Cephalopoda
6. Class Bivalvia
7. Class Scaphopoda
 Mollusca
1. Aplacophora including solenogasters, and the subclass
Caudofoveata which are deep-sea worm-like creatures
2. Polyplacophora including chitons that live on rocky
marine shorelines
3. Monoplacophora deep-sea limpet-like creature
4. Gastropoda including sea snails with shells and marine
snails without a shell or with a reduced shell; land
snails and slugs, freshwater snails
5. Cephalopoda including squid, octopuses, nautilus,
cuttlefish; all marine
6. Bivalvia including clams, oysters, scallops, mussels
7. Scaphopoda including tusk shells; all marine
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– Shell
– Mantle
– Mantle cavity, contains
• Ctenidia
• Anus & renal/genital pores
– Foot: wide, flat and muscular
• Note epipodial tentacles in some
– Head
• cephalic tentacles
• radula
– Visceral mass enclosed by mantle
• Contains main body systems
 Feeding/digestion

• Radula or filter feeding

Structure used for filter-feeding in bivalves?
• Salivary glands with toxins, digestive enzymes
• Complete digestive system
• Anus empties into mantle cavity
 Circulation/gas exchange
• Ctenidium (sketch)
– Ctenidial axis & ctenidial filaments
– Hemolymph flow, water flow & countercurrent
exchange
• Circulatory system
– Open (except Cephalopoda)
– Heart with two atria and a muscular ventricle
– Pericardial cavity is part of reduced coelom
Circulation/gas exchange
 Osmoregulation/excretion
Metanephridia (=kidneys)
– Also associated with reduced coelom
– Nephridiopore (renal pore) empties into?
 Nervous system
“Primitive” forms: i.e. Polyplacophora
– Ganglia at head
– Four longitudinal cords
– Cross-connections
 Nervous system
Gastropoda
– Ganglia at head
– Two longitudinal cords
– Additional ganglia

Pre-torsion Post-torsion
 Reproduction
• Gonochoristic or hermaphroditic
• Gonads associated with reduced coelom
– Ancestral condition: spawning via genital pores
(empty into mantle cavity)
• Sometimes linked with nephridiopore
– Some with elaborate structures & behaviors for
internal fertilization
1. Class Aplacophora
No shell, worm-like

Photo: Woods Hole Oceanographic Institute

 Aplacophora

The foot of this animal is contained in the groove above

Solenogastres
2. Class Polyplacophora
“many plates”
 Polyplacophora
Foot

Chiton
 3. Class Monoplacophora
Mouth Nephridium

Nerve cord Ctenidium

Gonads
Heart atria
Pedal retractor
muscle
Anus
 Monoplacophora
• Extant spp discovered in 1952
• Only 20 spp, all marine, deep water zones (1800-7000m).
• Poorly studied
• Likely ancestor of gastropods, cephalopods and bivalves,
and the bivalvia and scaphopods
• Monoplacophorans and Polyplacophorans evolved shells
independently from a shell-less ancestor. Evidence: shells
differ in internal layer structure
• Superficially similar to gastropod limpets
 Monoplacophora
• Limpet-like shell
• Chiton-like segmentation
• Mostly deep sea

www.mcz.harvard.edu
 Monoplacophora

Monoplacophora were thought ho be extinct until

someone found one in the fifties, as a result we
don’t have many pictures of them.
 4. Class Gastropoda
• More active than mono and polyplacophorans.
– Highly cephalized: tentacles, eyes.
• Gonochoristic (dioecious).
• Veliger larva (an advanced version of the trochophore
larva).
• Mostly bottom feeders, some filter feeders.
• Most inhabit relatively large shells that serve as shelter,
sea slugs(gastropods) don’t.
• Most marine gastropods wander on rocky parts of the
bottom because their foot doesn’t attach to sediments like
sand and mud.
 Gastropoda

• Most diverse taxon of mollusca

• Estimates range from 40,000-100,000 spp
(probably 60,000 extant, 15,000 extinct spp)
• Three major groups:
– Order Prosobranchs : benthic marine spp
– Order Opisthobranchs : secondary loss of the shell
– Order Pulmonates : air breathers
 Gastropoda
• Snails, slugs, nudibranchs et al.
Torsion is unique to gastropods.
Most gastropods are dextral.

Pretorsion Post torsion

Prosobranch Opisthobranch Pulmonata
 a. Order Prosobranchs
• Mantle cavity anterior, due to torsion
• Most common, typical “snail”
• Mostly marine, some freshwater, terrestrial
• Most primitive group of gastropods
 b. Order Opisthobranchs
• Mantle cavity lateral or posterior, due to
detorsion or loss of shell
• About 2000 spp. e.g. nudibranchs (sea hares,
sea slugs)
• Ctenidia often lost. Gas exchange via cerata
 c. Order Pulmonata
• Highly vascularized mantle for
gas exchange (lung)
• 17.000 spp: slugs, pond snails
 Veliger larva
Metanephridium
Shell Velum

Stomach

Digestive
cecum

Foot

Esophagus
Veliger larva
Veliger larva
 5. Class Cephalopoda
Nautilus, cuttlefish, squid, octopus

• Swift, agile carnivores

• Closed circulatory system, 2 hearts
• Separate sexes
• Foot modified to form arms, tentacles, siphon
• Brain, cranium, complex image-forming eye
• 700 extant spp, 10,000 extinct spp
• Arose from limpet-like monoplacophorans
• Ergo, ventral became functional anterior, etc
Cephalopoda
 Octopus vs Squid
• No hard shells • Hard shells
• Short life-span, commit • Eggs are released into
programmed suicide after water in cases by most,
reproduction some tend their eggs and
• Eggs are watched over by then die
mothers until hatched • Also feed in plankton cloud
• Feed in plankton cloud until • Mostly swim and use
mature tentacles for hunting
• Crawl with some swimming • Eight arms, two tentacles
• Eight arms • Can be much larger than
• Extremely good sight octopuses, one has been
• Thought to be as intelligent as measured at 59 feet in
a dog length
 Cephalopoda
Posterior surface

Right

Ventral
Dorsal

Left
 Cephalopod eye

Iris
Retina Lens

Optic nerves Cornea

 eye

Optic lobe

Cerebral ganglion Buccal ganglia

statocyst
esophagus

Brachial nerves

Brain is surrounded by a cranium

 Cephalopoda
a. Ectocochleate cephalopods
– Have external shell with internally subdivisions
used for buoyancy control
– This ancestral group is almost completely extinct
– E.g. Nautilus
 Nautilus is the only cephalopod with an
external shell and lacking chromatophores
 Chromatophores (color cells)
 Iridocytes (reflective cells)
- Millions of these allow rapid changes in color,
polarized signals
- Also have photophores for bioluminescence

Cephalopods except Nautilus have ink sac

b. Endocochleate cephalopds
• Reduced internal shell, or shell absent
• Squids, cuttlefish, octopi
Tentacle
Arm
Funnel (siphon)

Collar Fin
Eye
 Shell (Pen)

Systemic
heart
Branchial heart
Ctenidium
Funnel

Hectocotylus (sperm-bearing arm in males)

Reproduction:
trochophore and veliger are bypassed,
and hatch into planktonic juveniles
6. Class Bivalvia (Pelecypoda)
 Bivalvia / Pelecypoda

• 8000 extant spp (1300 fresh water, 6700 marine)

• Specialized for infaunal habitat
• Sessile, little cephalization
• Can live in sedimentary.
• Some bivalves dig with a strong muscular foot and
others use their foot to create strong threads that
attach them to rocks.
 Bivalvia
(bivalves)

• Filter feeders anchored to the bottom.

• Filter feeders, using gills.
• 3 major groups of bivalves based on gill shape:
a. Protobranchs (deposit feeders, most primitive)
b. Lammelibranchs (suspension feeders, most
common)
c. Septibranchs (carnivores, most derived)
a. Order Protobranchs

• Gills for gas exchange only

• Tend to live in deeper waters
(>1000m)
 b. Order Lamellibranchs
• Gills: gas exchange + filter feeding
• Incurrent siphon, excurrent siphon

Cut-away of
gill structure
Hinge
Blood vessel
mouth
Ctenidium

Excurrent
siphon

Foot Incurrent
siphon
Locomotion
 Glochidia
glochidium

Glochidia on gills

Freshwater mussels
c. Order Septibranch
• Ctenidia lack filaments
• Feed on polychaetes, crustaceans
• Weird side group
7.Class Scaphopoda
• Shared (extinct)
common ancestor
with bivalves.
• 300-400 spp
• Lack ctenidia, heart.
• Burrowers.
• Have 100-200
captacula (tentacles)
with which to catch food
 Scaphopoda

Tusk shells