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Mollusks
Kingdom Animalia,
Phylum Mollusca
(means “soft”)
What is a Mollusk?
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Soft body with internal or external shell
Ex: snails, slugs, clams, squid, and octopi
Trochophore larva
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Body plan:
● Foot – takes on many forms
● Mantle – covers the body &
secretes shell
● Shell – (present in most)
● Visceral mass – internal organs
General Characteristics
• Bilaterally symmetrical
• Triploblastic
General Characteristics Con’t
• Open circulatory system
–Exception is cephalopods have
closed circularoty system
• Radula usually present (tongue)
• Protostomes
Groups of Mollusks
Three major classes of mollusks
Gastropoda
Bivalvia
Cephalopoda
Gastropods
• “Stomach Foot”
• Snails, Slugs, Nudibranchs, Conchs,
Whelks, etc.
• 35,000+ species
• Freshwater, Marine, or Terrestrial
• Shell-less or single-shelled
• Move by secreting mucous with cilia
or use muscular foot
Snail
Slug
Nudibranchs
(sea slugs)
Check out the nudibranch gallery at nationalgeographic
Gastropod Anatomy
Torsion
• “twisted” body
–180 degree rotation of visceral
mass
–Significance: allows the snail to
retract it’s head into the shell first
and it’s foot last.
Interesting Facts!
• Land snails can lift ten times their own
weight up a vertical surface (like a
wall).
• Largest Snail = Giant African Land Snail
–Can weigh 2 pounds!
Defense from Predators
• Shells!
• But what about poor land slugs and
nudibranchs?
–Land slugs are usually nocturnal
–Some nudibranchs prey on cnidarians
and recycle their nematocysts
–Some sea hares can squirt ink to hide
themselves
Gastropod Feeding
• Most are predators or scavengers
• Radula: tongue-like organ that
scrapes algae or other plant-like
material
Radula
• Some predatory gastropods have
radula modified to pierce prey
Gastropod Respiration
• Gas exchange occurs in mantle
cavity – gills or diffusion
• Siphon- inhalent tube
–Where water enters body
Gastropod Circulation
Have open circulatory system
Blood not contained w/in
vessels; instead it washes over
the body tissues
Blood acts as a hydrostatic
skeleton
Nervous System
• Nerves concentrated into large
ganglia
• Most ganglia located in head
region
• Simple or complex eyes
• Osphradia- chemoreceptors that
help to detect prey
Excretion
Nephridium- kidneys
Ammonia = primary nitrogenous
waste produced in aquatic
species
Uric acid = primary nitrogenous
waste produced in terrestrial
species
Reproduction
Can be monoecious or dioecious
Usually external fertilization
where sperm and eggs released
into water
Some internal fertilization in
snails
Snail Reproduction
Economic Importance
• Delicious-Escargot
• Intermediate host for different
parasites
• Snails and slugs
can be serious
agricultural pests
Class Bivalvia
General Characteristics
• Includes clams, oysters, mussels,
scallops
• Two shells
–Hence “Bi-valvia”
• 30,000+ species
• Marine and Freshwater
• Mostly filter feeders
Interesting Facts!
• Largest bivalve -734 pounds and 4
long
• Ocean Quahog can live to be 220
years old!
Bivalve Respiration
• Incurrent and Excurrent Siphons
–Water enters and exits here
• Gills greatly expanded and cilliated
Circulation
• Open Circulatory System
–Blood not contained w/in vessels
–Blood “washes” over body tissues
by action of the beating heart
Bivalve Feeding and Digestion
• Filter feeders
–Labial palps filter out food
particles
–Non-edible particles flushed out
through the excurrent siphon
Nervous System Con’t
• Most sensory organs are located
in the margin of the mantle
–Have ganglia
–Statocysts and Chemoreceptors
Reproduction
• Mostly Dioecious
• Gonads located in visceral mass
• External fertlization
Economic Importance
• Mmm Tasty!
• Pearl production
–Multi-billion dollar
industry
Class Cephalopod
Cephalopods
octopi, squids, cuttlefish, and nautilus
● soft-bodied , head is attached to foot
● foot is divided into tentacles with
sucking disks
• Use jet propulsion
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Cephalopod Shell
• Nautilius only one with
external shell
• Internal in squid (pen) and
cuttlefish
(cuttlebone)
• Absent in
octopi
Movement
• Use siphon for jet propulsion
–Squeeze mantle cavity forcefully
• Sometimes have external “wings”
used to help steer
Feeding
• Active predators
–Many hunt at night
• Food captured by tentacles and
brought to mouth
• Jaws and radula used
Respiration & Circulation
• Closed circulatory system
–3 Hearts
–Blood is contained w/in vessels
• Respiration through gills
• High metabolic rate
Nervous System
• Very large brain
• Advanced, large eyes
• Chemoreceptors
• Chromatophores-pigment cells
• Some of the smartest animals on
the planet
• Some display bioluminescence:
use ATP to light up
• squid: brown or black ink
Octopus
(creeps on tentacles)
Squid
Reproduction
• Dioecious
• Males have testes and packed
sperm in spermatophores
• All larval development occurs in the
egg
Economic Importance
• Yummy food source- calamari
• Bait
Octopus versus Shark