Phylum Mollusca2
Molluscs
One of the largest of all phyla Have adapted to a wide variety
of habitats– Terrestrial, marine, benthic,
and accomplished swimmers.
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The Molluscan Body Plan
Despite the external differences between snails, clams, and squids the body plan is similar and distinct from all other phyla
The Mollusca body plan includes:– A large muscular foot– A radula– Mantle and mantle cavity– Usually a small head– Soft unsegmented body– A hard non-living calcareous shell
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The Mantle Cavity
Houses the visceral mass and comb-like gills– Gills are respiratory in function and can collect food particles
Mantle cavity also is the site for reproductive, excretory, and digestive systems
The molluscan coelom is very small; being restricted to the area surrounding the heart and gonads
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Taxonomic Summary
Phylum Mollusca– Class Polyplacophora– Class Gastropoda – Class Bivalvia– Class Scaphopoda – Class Cephalopoda
Systems
SkeletalSkeletal- Mantle may secretes a shell. Use hydrostatic pressure for ventral muscular foot.
MusclesMuscles -Ventral muscular foot and other muscles present.
DigestiveDigestive- complete complex with salivary glands, digestive gland and Rasping tongue (Radula).
Systems
CirculatoryCirculatory - Open except for Cephalopoda. Dorsal heart, usually in a pericardial cavity.
RespiratoryRespiratory - Ctenidia (gills) in mantle cavity, respiratory pigment is copper.
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Systems
ExcretoryExcretory- by nephridia usually connecting to the pericardial cavity,
– the coelom is usually reduced to the cavities of the nephridia, gonads and pericardium.
NervousNervous - Nerve ring with various pairs of ganglia—two pairs of nerve cords, one innervating the foot, the other the visceral mass (modified ventral ladder-like system)
Systems
IntegumentaryIntegumentary – Mantle
EndocrineEndocrine - nervous systems produces hormones.
ReproductiveReproductive - varied- monoecious, protandric, or dioecious. Larva in marine = trochophore and veliger, in freshwater clam is glochidium.
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Chiton Lifestyles
Found close to shore mainly in the intertidal where they live on hard substrates
– Strong foot and low profile help from being swept away
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Ingestion and Digestion
Radula is used to scrape algae from the rocks
Mouth is anterior and anus is posterior; linear digestive tract
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Class Gastropoda
Defining characteristics– Visceral mass and nervous system become twisted
90-180° during embryonic development
Largest and most varied group of the phylum Mollusca
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Gastropod Morphology and Movement
The typical snail consists of a visceral mass, which sits atop a muscular foot
– The visceral mass is protected by a univalve shell that is coiled
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Prosobranchia Anatomy
Molluscan gill – Consists of a series of
flattened, triangular sheets
– Water is drawn into the animal by gill cilia
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Class Bivalvia
Defining characteristics– Two-valved shell– Body flattened laterally
This class contains clams, oysters, mussels, scallops, and shipworms
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Bivalves and You
Many species are edible– In today’s waters this can cause certain problems
since bivalves are filter feeders
Also, an important food item for our commercially important fish
Important to humans in their ability to strain harmful bacteria out of polluted waters– Some can pump up to 40 liters per hour
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Bivalve Locomotion?
Most bivalves move by expansions and contractions of the foot
Mussels usually are sessile and attach to substrate with threads
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Bivalve shells
Protein hinge ligament stores energy required to open the shell
Adductor muscles is responsible for closing the valves
The umbo is dorsal and the valves are addressed as left and right
Siphons are posterior and the excurrent siphons are always dorsal
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Bivalve Feeding
Bivalves are filter feeders– Ctenidia, gills, are
used for feeding & respiration and are enlarged and folded to increase surface area
– Cilia on the gills create a current to bring food-laden water into the mantle cavity
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Bivalve Reproduction
Dioecious The male sheds sperm into
the water column– Sperm can enter the
female or fertilization can occur in the water column
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Class Scaphopoda
Defining characteristics– Tusk shaped conical
shells, open at both ends– Development of anterior
thread-like adhesive feeding tentacles
Elongated tapering tubular shells that burrow in sandy bottoms
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Class Cephalopoda
Defining characteristics– Closed circulatory
system– Foot modified to form
flexible arms and siphons
– Ganglia fused to form a large brain encased in a cartilaginous cranium
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Cephalopod Characteristics
The most highly organized of all the molluscs, the foot is closely associated with the head
Nautilus, octopus, cuttlefish, and squids Shell size varies from a full shell (nautilus) to a
vestige of a shell in the squid and octopus Can be as large as 1,000kg. and 18m.
Squids 10 tentacles. Propels by pumping jets of water with the mantle through an excurrent siphon.
Octopuses 8 tentacles They often crawl along the ocean bottom or lie in wait for prey.
Chambered Nautiluses The only existing cephalopod with external shell. Shell is coiled & divided into chambers. The body is confined to the outermost chamber.
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Defensive Mechanisms
Since they are susceptible to predation they have evolved chromatophores
– May posses hundreds or thousands, all controlled by the brain
An ink sac that is associated with the digestive system and discharged out the anus
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Sensory & Nervous System
Eyes– All but the nautilus have
image forming eyes that are like mammals
Convergent evolution
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Reproduction
Sexes are separate– Males have one modified
arm (hectocotylus) for sperm transfer
Sperm are enclosed in packets called spermatophores
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Squid
10 sucker bearing arms surround the mouth– 2 longer tentacular arms can shoot forward and capture prey
Large bites of food are removed and swallowed quickly Form large schools and feed on crustaceans, and fishes The shell is reduced and lies under the mantle, pen Large folded gills and a circulatory system distribute
oxygen quickly High oxygen demands are meet by the contraction of the
mantle cavity– This method also removes waste products
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Squid Locomotion
The foot forms the funnel The upper end of the mantle is
extended into a pair of triangular fins
– Water enters the mantle cavity at the free end
– When the mantle contracts the edge is tightly sealed and water is forced out the funnel
– Can move very quickly 5-10 m/s
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Octopods
Have 8 arms all equal in length– Rows of suckers from base to tip
Compact body with a greatly reduced shell The body is extremely flexible and assumes a streamlined shape
when swimming with squid-like jets through the funnel