Mollusk Notes

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First found May 22, 2018

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`Mollusks
Phylum Mollusca
Characteristics
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Soft-bodied invertebrate covered with protective mantle that
may or may not form a hard, calcium carbonate shell
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Includes chitons, snails, slugs, clams, oysters, squid, octopus, &
nautilus
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Second largest animal phylum
Have a muscular foot for movement which is modified into
tentacles for squid & octopus
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Complete, one-way digestive tract with a mouth & anus
Have a fully-lined coelom (eucoelomate)
Cephalization - have a distinct head with sense organs & brain
Have a scraping, mouth-like structure called the radula
Go through free-swimming larval stage called trochophore
Trochophore Larva
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Body organs called visceral mass lie below mantle
Have circulatory, respiratory, digestive, excretory,
nervous, & reproductive systems
Bilaterally symmetrical
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Most have separate sexes that cross-fertilize eggs
Gills between the mantle & visceral mass are used for gas
exchange
Includes 4 classes --- Polyplacophora (chitons), Gastropoda
(snails, slugs, nudibranchs, conchs & abalone), Pelecypoda
or Bivalvia (clams, oysters, & mussels), & Cephalopoda
(squid, octopus, & nautilus)
SNAIL, CLAM, CHITON, & SQUID
Class Polyplacophora
Characteristics
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All marine
Have a shell divided into 8 over-lapping plates
Live on rocks along seashore feeding on algae
CHITON
Class Gastropoda
Characteristics
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Head has a pair of retractable tentacles with eyes located at
the ends
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Have a single shell or valve (snails) or none (slugs)
Known as univalves
Snails
* May be marine, freshwater, or terrestrial
* Aquatic snails breathe through gills & use their radula to scrape
algae for food
* Terrestrial snails use their mantle cavity as a modified lung &
saw off leaves
* Have open circulatory system
* Land snails are hermaphrodites
* Aquatic snails have separate sexes
* Use internal fertilization
SLUG
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Pteropods
* Called "sea butterflies"
* Marine
* Have a wing-like flap for swimming
"SEA BUTTERFLY"
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Oyster Drills
* Radula modified to drill into oyster shells
OYSTER DRILL
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Nudibranch
* Marine slug
* Lacks shell
NUDIBRANCH
Class Bivalvia or Pelecypoda
Characteristics
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Sessile or sedentary
Includes marine clams, oysters, shipworms, & scallops and
freshwater mussels
Filter feeders
Have two-part, hinged shell (2 valves)
Have muscular foot that extends from shell for movement
Scallops clap valves together to move
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Shell secreted by mantle & made of 3 layers --- outer horny
layer protects against acids, middle prismatic layer made of
calcium carbonate for strength, & inner pearly layer next to soft
body
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Mantle secretes substance called "mother of pearl" to
surround irritants like grains of sand
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Oldest, raised part of shell called umbo
Powerful anterior & posterior adductor muscles open & close shell
Lack a distinct head
Have an incurrent & excurrent siphon that circulate water over
the gills to remove food & oxygen
INTERNAL CLAM ANATOMY
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Have heart & open circulatory system
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Nervous system made of 3 pairs of ganglia, nerve cords, &
sensory cells that detect light, chemicals, & touch
Separate sexes with external fertilization of eggs
Class Cephalopoda or Amphineura
Characteristics
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Includes octopus, squid, cuttlefish, & chambered nautilus
All marine
NAUTILUS
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OCTOPUS
Most intelligent mollusk
Well developed head
Active, free swimming predators
Foot divided into tentacles with suckers
Use their radula & beak to feed
Closed circulatory system
SQUID
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Lack an external shell
Highly developed nervous system with vertebrate-like eyes
Separate sexes with internal fertilization
Squid
* Largest invertebrate is the Giant Squid
* Large, complex brain
* Ten tentacles with longest pair to catch prey
* Use jet propulsion to move by forcing water out their excurrent
siphon
* Chromatophores in the skin can help change squid color for
camouflage
* Can squirt an inky substance into water to temporarily blind
predators
* Have internal shell called pen
* Female lays eggs in jellylike material & protects them until
hatching
GIANT SQUID
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Octopus
* Eight tentacles
* Similar to squid
* Crawls along bottom looking for prey
OCTOPUS
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Chambered Nautilus
* Has an exterior shell
* Lives in the outer chamber of the shell
* Secretes gas into the other chambers to adjust buoyancy
NAUTILUS
Economic Importance of Mollusks
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Used by humans for food
Pearls from oysters
Shells used for jewelry
Do crop & garden damage
Serve as intermediate hosts for some parasites such as flukes
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