Mollusks are second only to arthropods in
numbers of living species
 Mollusca = “Soft Bodied” (Latin)
~555 Million Years Ago
 Range in size and
body shape
 Largest  1000
lbs, 18m long
(giant squid)
 80% less than 5
cm
 Body of three regions: head-foot, visceral mass,
and mantle
 Mantle cavity functions in excretion, gas
exchange and reproduction
 Bilateral symmetry
 Protostome development
 True coelomates (eucoelomates), reduced size
 Open circulatory system in all but cephalopods
 Radula present and used for scraping food
 Three main regions
 1. Head-foot
 Head (nervous structures) and foot
(locomotion)
 2. Visceral Mass
 Contain organs
 3. Mantle
 Mantle
 Usually attaches to the
visceral mass and
enfolds most of the
body. May secrete the
shell.
 Mantle Cavity
 Between the mantle and
foot. Opens to the
outside and functions in
gas exchange, excretion
and reproduction.
 Radula
 A rasping structure
found in the mouth.
“Toothed tongue”
 “Stomach foot”
 Largest and most varied
class of mollusks
 ~35,000 living species
 Examples: snails, slugs,
conch, and limpets
 Marine, freshwater, and
terrestrial habitats
 Most have shells –
calcium carbonate
 Torsion - 180° twisting of
the visceral mass,
mantle, and mantle
cavity. Used to bring
head into shell.
 Operculum closes shell
opening.
 Shell coiling: whorl, apex
 Flattened foot for
locomotion
 Cilia
 Muscular contractions
 Use radula for scraping
food
Maintenance Functions
 Gas exchange occurs in
mantle cavity
 Open circulatory system
 3 chambered heart
 Hydraulic skeleton –
fluid under pressure in
body
 Some dioecious, some
monoecious
 “Two valves”
 Second largest molluscan
class: ~30,000 species
 Includes clams, oysters,
mussels, and scallops
 Many edible, some form
pearls
 Marine and freshwater
 No head or radula
 Most are filter feeders
 Adductor muscles used for
defense
 Mantle attaches to shell
around adductor muscles
 Pearl forms when sand lands
between mantle & adductor
 Foot projects from front end
of animal, through the valves
 Foot used for burrowing
 Siphon – “neck” used for
intake of water and food and
release of waste.
 Complete digestive tract
 Reduced nervous system
 Two convex halves of the shell – valves
 Umbo – oldest part of shell near anterior end
Maintenance Function
Reproduction
 Cilia covered gills
 Most are dioecious, some
 Cillia move water in and
monoecious
 Gonads in visceral mass
 External fertilization (most)
 Trocophore larvae
out of mantle
 Incurrent siphon
 Excurrent siphon
 Gills in mantle cavity for
gas exchange
 Food trapping – cilia
move food toward
mouth
 “Head Foot”
 Includes octopuses, squid, cuttlefish, and nautiluses
 Most complex mollusks/invertebrates
 Foot modified into a circle of tentacles and a siphon
 Shell reduced or absent in most
 Head in line with elongate visceral mass
 Shell reduced or absent in all but nautilus
 Cuttlefish
 Shell is internal. Cuttlebone is used to make
polishing powder and bird treats (for calcium)
 Squid
 Pen: internal, chitinous structure
 Cartilaginous plates in mantle wall, head, neck
 Octopus
 Absent
Locomotion
 Cuttlefish and squid very
Maintenance Functions
 Adhesive cups used for
capture.
 Beaklike jaws and radula
 Complete digestive tract
 Closed circulatory system
 Complex nervous system
motile
 Octopus more sedentary
 Chromatophores for
 Predators
 Jet-propulsion system
 Muscle contractions
 Forces water out
through narrow funnel
 Brain and Eye
defense (color change)
 Discharge of ink
 Cephalopods are dioecious
 One tentacle of male is
modified for
spermatophore transfer
 Tentacles intertwine
during copulation.
Spermatophores burst
open and eggs are released
through the oviduct.
 Fertilized eggs attach to
substrate
 “many plates”
 Chitons
 Shallow marine waters
 Reduced head, flattened foot
 Shell divides into eight dorsal
valves
 Crawl similar to gastropods
 “Boat foot”
 Tooth shells or Tusk shells
 Burrowing marine animals
 Conical shell that is open
at both ends
 Head and foot project from
wider end
 “One plate”
 Undivided, arched shell
 Broad, flat foot
 Repeated pairs of gills and
foot-retractor muscles
 Thought to be extinct
until 1952
 “Without plate”
 Lack a shell and crawl on
ventral surface
 Body and nervous system
similar to flatworms
 Most are surface dwellers
on corals