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Mollusks
Squishy Squad
Categories
• Gastropoda (ex: snail)
• Bivalvia (ex: clam)
• Cephalopoda (ex: squid)
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Schaphopoda
Monoplacophora
Polyplacophora
Aplacophora
Caudofoveta
Evolution
• There are fewer cephalopods now.
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Only 550 species
Vertebrate competition?
• Overall, Mollusks are very successful.
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Over 100,000 species
Gastropoda
Bivalvia
• Triploblastic, coelomic
Characteristics 1
• 2 part body: head-foot, visceral mass
• Mantle secretes shell, covers visceral mass
• Mantle cavity – excretion, gas exchange,
elimination, release of gametes
• Bilateral symmetry
Characteristics 2
• Trochophore larvae, spiral cleavage,
schizocoelous coelom formation
• Coelom surrounds heart, nephridia, and
gonads
• Mostly open circulatory system (not in
cephalopoda)
• Radula used in scraping food
Body plan – head-foot
• Anterior head
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Mouth
Nervous structures
Sensory structures
• Foot
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Attachment
Locomotion
Body plan – visceral mass
• Contains organs for
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Digestion
Circulation
Reproduction
Excretion
• Dorsal to head-foot
Body plan – mantle
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Outer layer
Attaches to visceral mass
May secrete a shell (3 layers)
Mantle cavity (excretion, gas exchange,
elimination, release of gametes)
Body plan - radula
• Row of teeth on a chitinous belt
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Sits on odontophore (tongue)
Moves back and forth to bring food in
Gastropods
Limpets and Snails and Slugs
Oh my!
A large class
• 35,000 living species
• Marine, freshwater, and
terrestrial
• Helix pomatia
• Garden snails
• Int. host of human parasites
Torsion
• Occurs in early development
• 180° twisting of VM, mantle, and mantle
cavity
• Moves gills, anus, & openings from
excretory and rep. systems just behind the
head
Torsion
• Plausible advantages
1.Head enters shell 1st
 Some have operculum
2.Clean water enters mantle cavity
3.Sensory organs in front
Waste exits above head?
• Evolutionary adaptations
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Waste exits through notches behind head
Detorsion
 Twist 180°
 Untwist 90°
 Waste exits to side, behind head
Shell coiling
• Earliest fossils coiled in
one plane
• Most modern snails like
the one at the bottom
right
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Less room at tight end
Paired organs single
organs
Locomotion
• Crawling
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Cilia
Muscular waves (bonus snail)
• Modified foot for clinging
• Swimming (sea butterfly & sea hare)
Feeding
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Scrape algae using radula
Herbivores
Scavengers
Parasites
Predators (whelk proboscis)
Digestion
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Tract is ciliated
Food trapped in mucus strings
Protostyle - mucus mass in stomach
Digestive enzymes
Wastes as fecal pellets
Respiration
• Always involves mantle cavity
• Modern orgs have one gill
• Some have a siphon
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Inhalant tube
Burrowers extend it up
• Gills lost or reduced in land orgs
Open Circulatory System
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Leaves the vessels
Enters sinuses to bathe cells
Heart w/single auricle and ventricle
Blood acts as hydraulic skeleton
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Move blood to move body
Retract quickly, expand slowly
Nervous System
• Eyes – base on top or bottom of tentacle
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Simple pit of photoreceptors
Lens and cornea
• Statocysts on foot
• Ophradia – chemoreceptors
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Detect sediment
Detect prey
The other classes
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Caudofoveta
Solenogastres
Monoplacophora
Polyplacophora
Schaphopoda
Caudofoveata
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120 worm-like species
Marine
2 - 140 mm in length
Orient vertically in sand
Gills pointed up
Caudofoveata
• Feeding
Feed on microorganisms and
detritus (dead stuff)
o Oral shield and radula for food
intake
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• One pair of gills
Caudofoveata
• Dioecious
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Sexual reproduction
Fertilized eggs are brooded
Larvae swim freely
Solenogastres
• 250 species of marine animals
• Similar to caudofoveata
• BUT!!!
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No radula
No gills
• Bottom dwellers
• Feed on cnidarians
Monoplacophora
• Once thought extinct
• 1952 – Neopilina found
• 25 species now known
• Small w/ low, rounded shell
• Mouth has radula
• Look like limpets
Monoplacophora
• Unlike other mollusks –
repeating organs
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3 to 6 pairs of gills
2 pairs of auricles
3 to 7 pairs of metanephridia
1 to 2 pairs of gonads
10 pairs of pedal nerves
Polyplacophora
• Chitons
• (many plate bearers, Coat
of mail)
• 1000 species
• Most 2-5 cm, largest 30 cm
Polyplacophora
• Dorsoventrally flattened
• Convex dorsal surface w/7-8 limy plates or
valves
• Esthetes – photosensitive
Polyplacophora
• Blend in w/rocks
• Homebodies
• Can roll up like an armadillo
Polyplacophora
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Mantle cavity is tube-like
Closes at low tide
Some can breathe air
3 chambered heart
Pair of metanephridia
2 pairs of longitudinal
nerve cords
• Dioecious
Scaphopoda
• Tusk shells or tooth shells
• From subtidal zone to 6000 m
deep
• 900 species
• Most are 2.5-5 cm
• range from 4-25 mm
Scaphopoda
• Mantle wrapped around VM
• Shell open at both ends
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Foot at wider end
Burrows into sand or mud
Leaves small end exposed
Scaphopoda
• Water circulates via foot and cilia
movements
• No gills (gas exchange occurs in
mantle)
Scaphopoda
• Food
Detritus & protozoa from substratum
Caught on cilia on foot
OR
o Ciliated adhesive knobs on tentacles
(captacula)
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 Extend from head
 Food carried to head
 From radula to gizzard
Scaphopoda
• No tentacles, eyes, or osphradia
• Dioecious