Class Gastropoda
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Snails - largest class of molluscs
~ 85,000 living spp.
~ 15,000 fossil spp.
Habitats: marine benthos, ocean
plankton, freshwater, land
Gastropod characteristics
• 1. Torsion - primitive bilateral symmetry
lost during development
• Twisting of visceral mass, mantle, and
mantle cavity
After torsion
Gastropod characteristics
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2. Definite head: eyes, tentacles
3. Flat ventral foot
4. Mantle cavity w/gills or lungs
5. Buccal cavity w/ radula
Characters cont.
• 6. Coiled shell - one continuous
piece
– operculum on posterior foot of
some
plano-spiral
helico-spiral
Characters, cont.
• 7. Centralized complex nervous system
• 8. Hermaphroditic or dioecious
• 9. Oviparous or ovoviparous
10. Larval form
• Archaeogastropoda have trochophore
larva
• Many marine snails have veliger larva
• Freshwater + terrestrial species usually
hatch as young snails
Gastropod classification
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3 subclasses:
Prosobranchia
Opisthobranchi - reduced shell
Pulmonata - lungs
Subclass Prosobranchia
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Mantle cavity anterior
1 or 2 gills
shell and operculum usually present
most are dioecious
Prosobranchs, Order
Archaeogastropoda
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Primitive snails
External fertilization
Trochophore larva
2 bipectinate gills
2 auricles
2 coelomoducts
Prosobranchs, Order
Archaeogastropoda
• A. slit shells - deep water species
– Abalones (Haliotis)
– 9 spp on our Pacific coast
– Commercially harvested
Prosobranchs, Order
Archaeogastropoda
• B. keyhole limpets - Fissurella
– conical shells
• C. Limpets - Acmaea
Prosobranchs, Order
Archaeogastropoda
• D. topshells, turban shells, star shells
– Astraea
Prosobranchs, Order
Archaeogastropoda
• E. Nerites (Nerita) intertidal in Caribbean
– some freshwater + terrestrial
Prosobranchs, Order
Mesogastropoda
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Male with penis
Fertilization internal
1 monopectinate gill
1 auricle
1 coeloduct
Prosobranchs, Order
Mesogastropoda
• A. Freshwater apple snails – Viviparus, Pomacea
Prosobranchs, Order
Mesogastropoda
• B. Turret shells - worm shells; caecums,
ceriths, some freshwater genera, all
have high shells
Prosobranchs, Order
Mesogastropoda
• C. Violet snails - pelagic grazers on
man o’war
– Vellela, Porpita
– Janthina: secretes bubble mass for
floatation
Prosobranchs, Order
Mesogastropoda
• D. Cap shells, slipper shells
– Crepidula change sex w/age
• see slides of veliger larva
Prosobranchs, Order
Mesogastropoda
• E. Conchs - Strombus
– Feed on algae, turtlegrass
– Commercial harvest in Caribbean
Prosobranchs, Order
Mesogastropoda
• F. Cowries; Cypraea - graze on algae,
sponges, gorgonians, and tunicates
• Mantle covers most of shell when
extended
• Favorites of shell-collectors
• No periostracum
• Italians call them “porcellanos” = little pigs
– Porcelain got its name
Cowries
Prosobranchs, Order
Mesogastropoda
• G. Heteropods - pelagic, swimming
snails w/reduced shell
• Carinaria (see Fig. 10-30, p. 399)
Prosobranchs, Order
Mesogastropoda
• H. Moon shells - predators on molluscs:
– bore into shells
– Lunatia
– Polinices
Moon shells:
• Muscular suction disk holds snail on
clam shell
• Drill through shell with radula:
• Wiggle proboscis into flesh
• Remove most of flesh from clam
Prosobranchs, Order
Mesogastropoda
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I. Helmet shells, tritons, tuns
Predators on molluscs and echinoderms
Long proboscis
Large shells
Prosobranchs, Order
Mesogastropoda
• J. Periwinkles - intertidal
– Littorina
Prosobranchs, Order
Neogastropoda
• Mostly same characteristics as
Mesogastropoda
• Radula has only 3 teeth in a transverse
row
• Most are marine
• Most are carnivores
Prosobranchs, Order
Neogastropoda
• A. Drills - Murex, Urosalpinx
• Drill holes in shells of bivalves and
barnacles
• Oyster drill and others cause economic
losses
Prosobranchs,
Order Neogastropoda
• B. Whelks - Busycon
• Tulip shells - Fasciolaria
• Predators and carrion feeders
Prosobranchs,
Order Neogastropoda
• C. Olives, miters, and volutes
• Colorful, mostly tropical snails
• prey on invertebrates
Prosobranchs,
Order Neogastropoda
• D. Cones - Conus
• Most prey on inverts
• Those that prey on fish can be
dangerous to humans
• Fig 10-42, p. 409
Conus
• Long proboscis with harpoon-like
radular tooth
• venom gland
“Glory of the sea” cone shell
Subclass Opisthobranchia
• Detorsion - brings mantle cavity to right
side
• 1 gill, 1 auricle, 1 coelomoduct
• Shell often reduced or absent
• Mantle cavity often reduced or absent
• Many are secondarily bilateral symmetric
• Hermaphroditic
Subclass Opisthobranchia
• A. Bubble shells - Bulla
• Predators with thin, reduced shell
Subclass Opisthobranchia
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B. Sea hares - Aplysia
Thin, caplike shell overgrown by mantle
Vegetarians (herbivores)
Can eject milky fluid from mantle cavity
A. californica has largest body of all
gastropods
Subclass Opisthobranchia
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C. Sea slugs
With or w/o shell
Berthelinia is a “bivalve” gastropod
Fig. 10-23, p. 392
Subclass Opisthobranchia
• D. Pteropods or sea butterflies
• 2 orders with and w/o (naked) shells
• Planktonic with foot modified into winglike flaps for swimming
• Often occur in enormous numbers
More
Pteropods
Subclass Opisthobranchia
• E. Nudibranchs
• Often with secondary gills and cerata
(hornlike dorsal projections)
• Many endemics
Subclass Opisthobranchia
• F. Parasitic Opisthobranchs
• 2 orders
• One ectoparisitic on bivalves and
annelids
• One endoparasitic in sea cucumbers
Subclass Pulmonata
• Charactistics like Opisthobranchia, but
w/o gill
• Mantle cavity converted to lung
• Shell usually present
• No operculum
• Hermaphroditic
Subclass Pulmonata
• A. freshwater snails:
– Lymnaea, Physa, Planorbis
– Freshwater limpets
• Most come to surface for air
• Some have developed secondary gills
• Descended from terrestrial ancestors
Subclass Pulmonata
• B. Land snails and slugs
– Helix
– Limax
• C. Intertidal slugs w/posterior anus
Class Scaphopoda
tusk shells
• ~ 300 spp.
• Sedentary, marine burrowing, in 6 1800 m depth
• Shells resemble elephant tusks
• Most burrow in sand, few in mud
• Feed on microscopic organisms;
foraminiferans
• Considered offshoot of early bivalve
ancestors
Scaphopod characteristics
• 1. Tusk-like shell open at both ends
• 2. Bilateral symmetry (like bivalves)
• 3. Rudimentary head
– no eyes
– Head has threadlike, food gathering
tentacles (captacula)
Foot
Scaphopod characteristics
• 4. Radula present
• 5. Circulatory system reduced - sinuses
– Gills absent
• 6. Dioecious, trochophore and veliger
larval stages
Scaphopod classification
• Two families (representative species):
• Dentalium - conical shell
• Cadulus + Siphonodentalium - globular
shell, enlarged foot
