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Molluscs
Zoology Chapter 10
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Phylum: Mollusca
• Classes include:
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2)
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Gastropoda (snails, slugs)
Bivalvia (clams, oysters, scallops)
Cephalopoda (squid, octopus, chambered nautilus)
Polyplacophora (chitons)
Monoplacophora
Scaphopoda
Caudofoveata
Solenogastres
• We will focus on the first 3 listed
• Read Chapter 10; Do: Q 3-15 page 201; Due:
11/8
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Phylum: Mollusca
Characteristics
• 1. Body bilaterally symmetrical, unsegmented,
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definite head
2. Muscular foot (ventral body wall), adapted for
locomotion
3. Dorsal body wall forms mantle enclosing
mantle cavity; may contain gills or lungs,
secretes shell
4. Epithelium ciliated with mucous glands &
sensory nerve endings
5. Coelom limited to small cavity surrounding
heart (pericardial cavity)
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Mollusc Characteristics, cont.
• 6. Digestive system complex, radula (rasping organ)
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present, anus usually empties into mantle cavity
7. Circulatory system – open, heart is 3-chambered
(2-chambered in gastropods), blood vessels &
sinuses present, respiratory pigments in blood
8. Gas exchange – gills, lung, mantle, or body
surface
9. Metanephridia (kidneys, 1 or 2) open into
pericardial cavity & empty metabolic waste into
mantle cavity
10. Nervous system – paired cerebral, pleural, pedal
& visceral ganglia; ganglia centralized in nerve ring
11. Sensory organs of touch, smell, taste,
equilibrium, & vision (some have eyes) are highly
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developed
Mollusc FYI
• 2nd largest phyla (Arthropoda largest)
• 75,000 named living species; 35,000 fossil species
• Diversity vast
– Size:
• Giant squid 18m long & 454kg
• Giant clam 1.5m long & 225kg
• 80% 5cm or less
– Niche/lifestyle:
• Aquatic & terrestrial
• Bottom feeders, borers, pelagic forms, burrowers
• Herbivorous grazers, predaceous carnivores, ciliary filter feeders
• Economic Importance
– Food (to humans & other organisms), jewelry (pearls),
shipworms & some snails (burrowers), crop/garden pest,
intermediate host to many trematodes
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Mollusc Body Plan
• Consists of head-foot portion & visceral
mass portion
– Head-foot – contains feeding, cephalic,
sensory, & locomotor organs
– Visceral mass – contains digestive, circulatory,
respiratory, & reproductive organs
• Mantle surrounds head-foot & visceral
mass, a mantle cavity between mantle and
body mass, mantle cavity contains gills or
lung, mantle secretes shell
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Head-Foot
• Mouth w/ sense organs
– radula – rasping tongue may contain up to 250,000 teeth, not in
bivalves or some gastropods
– http://www.youtube.com/watch?v=mLVDwlrSq5U
• Photosensory receptors, include eyes
– http://www.pond5.com/stock-footage/351946/snail-emerges-and-it-seyes-pop-out-on-stalks-realtime.html
• Tentacles often present
– http://www.youtube.com/watch?v=T-52LjG8mO4
• Foot portion for locomotion – muscular contraction for creeping
movements (have mucous glands for gliding)
– Some modified for attachment, compressed as in bivalves, or modified
into funnel-shape for jet propulsion
– http://www.youtube.com/watch?v=mLVDwlrSq5U
– http://www.youtube.com/watch?v=mLVDwlrSq5U
– http://www.google.com/search?q=snail+radula&tbo=p&tbs=vid%3A1&
source=vgc&hl=en&aq=f#q=jet+propulsion+in+squid&hl=en&tbs=vid:
1&ei=vFrPTLC2IYSclgf8_cjnCA&start=10&sa=N&fp=35970b3192b2a18
– http://www.youtube.com/watch?v=_2iXHBuSIJY
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Mantle & Mantle Cavity
• Covering visceral mass is the mantle & mantle
cavity
– Mantle is sheath of skin surrounding visceral mass
with a space between (mantle cavity)
– Mantle outer surface secretes shell
• Houses respiratory organs (lungs or gills);
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surface also serves in gas exhange
Products of digestive, excretory, reproductive
systems empty into mantle cavity
Shell – composed of 3 layers
– Periostracum is outer layer (very resistant)
– Prismatic layer is middle layer (dense CaCO3 layers)
– Nacreous layer is inner layer (protein & CaCO3,
continuously laid and very smooth, “mother-of-pearl”)
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Visceral Mass
• Open circulatory system contains a
pumping heart
• Digestive system specialized to feeding
habits of various molluscs
• Paired kidneys, ducts do double duty as
discharge duct of sperm/ova
• Nerve ganglia paired, connected by nerve
cords, sensory organs
• Reproduction – most are dioecious, some
gastropods are hermaphroditic
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Class: Gastropoda
• These include snails, slugs, nudibranchs, conchs, whelks,
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periwinkles, sea slugs, sea hares, & sea butterflies
40,000 living species & 15,000 fossil species
SNAIL
Univalve
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Torsion –
a rotation of the mantle cavity
to the front of the body
• Occurs during veliger larval stage; 2 stages of 90
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rotation
After torsion, anus & mantle cavity are located anterior
and open above mouth; the left gill, kidney, & heart
auricle now located on right side but right gill, kidney, &
heart auricle now on left; nerve cords twisted in figure
“8”.
– This allows space inside shell, so sensitive head can be
withdrawn into shell for protection
– Problem: “fouling”, which can occur since anus is above mouth;
still do not understand what evolutionary pressure would have
favored such a strange adaptation
• Coiling is the twisting or spiral winding of the shell; not
same as torsion
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Shell-less Gastropods
Marine Nudibranch
Terrestrial Slug
Marine Nudibranchs
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Website for nudibranchs
• http://ngm.nationalgeographic.com/2008/
06/nudibranchs/doubilet-photography
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Class: Bivalvia
Class includes: scallops,
oysters, clams, mussels,
shipworms
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• http://www.youtube.com/watch?v=Z4EFROfVyAA&featur
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e=related
Most bivalves are sedentary suspension feeders; no head
or radula & little cephalization (but some have eyes –
scallops)
Umbo – olders part of the shell
3-chambered heart
Crystalline style – gelatinous rod in stomach which
rotates and releases digestive enzymes to digest food
Dioecious, fertilization external, development usually
from trochophore to veliger larvae to spat to adult in
marine forms
– Some freshwater clams have a larval form called
glochidia which brood in gill tubes of adult and when
a fish passes by the clam disperses them at fish; the
glochidia attach to gills and live parasitically for 20-70
days.
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Cephalopoda
• These include squid, octopus, chambered
nautilus, & cuttlefish
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All are marine, all are active predators
Feed on small invertebrates, fish, crustaceans, worms
Arms adapted to quickly snatch prey and delicately manipulate it
Salivary glands of cuttlefish/octopus have poison glands which
secrete venom
– All have a strong beak with radula capable of tearing food
• During embryonic development, the head & foot
become indistinguishable; ring around mouth
bearing tentacles & arms derived from head but
actual tentacles & arms derived from foot; part
of foot tissue develops into funnels within
mantle cavity
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Class: Cephalopoda
Cuttlefish shell - internal
Chambered nautilus shell
external and unique; gas
filled chambers increase
as animal grows
Chambered Nautilus 
Cuttlefish
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Squid
Squid shell much
reduced; thin
flexible strip
(pen)
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Octopus
Octopus – shell is absent
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Cool Cephalopod Stuff
• Eyes
– Good example of convergent evolution
(eye of human & eye of cephalopod)
– Eye contains cornea, lens, chambers,
retina
Human Eye
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• Ink Production
– Cephalopod ink is a dark pigment released into water
by most species of cephalopod, usually as an escape
mechanism. All cephalopods, with the exception of
the Nautilus and the species of octopus belonging to
the suborder Cirrina are able to release ink.
– Consists mainly of melanin & mucous & assorted
amino acids; manufactured in ink gland; stored in ink
sac and emptied into rectum
– Some other molluscs, like sea hares, are able to make
and eject ink; example of convergent evolution.
– Ink used in foods for coloring; it is tasteless; some
new studies indicate squid ink may be toxic to some
cells esp cancer cells
– Also used as ink; sepia is name of ink mainly from
cuttlefish
– http://www.instructables.com/id/How-to-HarvestSquid-Ink/
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• Color Change
• Color change in cephalopods due to cells called
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chromatophores
These cells are located in skin and are quite
complex; under nervous system & hormonal
control
Cause skin color and texture to change quickly
Some deep sea squids are bioluminescent
http://video.google.com/videoplay?docid=125655308012313047#
http://www.oceanfootage.com/video_clips/GMO
01_041
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• Locomotion
• Mantle cavity modified into funnel shaped siphon
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which is used to aid in jet propulsion; siphon
direction can be controlled so that direction of
movement is controlled
Squid & cuttlefish have lateral fins serving as
stabilizers; squid built for speed while cuttlefish
are a bit slower swimmers
Octopus better suited for slower swimming or
crawling across substrate
http://www.youtube.com/watch?v=rOvhs5icTfg
http://www.youtube.com/watch?v=3NtPcUR31N
Q
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Gee Whiz……
• http://www.youtube.com/watch?v=FFOEZ
h1Lbbg
• http://video.google.com/videoplay?docid=
4007016107763801953#
• http://www.youtube.com/watch?v=SCAIe
dFgdY0
• http://www.youtube.com/watch?v=l3PvvT
_Ktx8
• http://www.youtube.com/watch?v=q5ZQH
2Uzpew
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