Dissection of the
Clam
Venus mercenaria
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Taxonomy of Bivalve Mollusks
Kingdom
Phylum
Class
Genus
Species
•
•
•
•
•
Animalia
Mollusca
Bivalvia
Venus
mercenaria
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Bivalve Mollusks
• Soft bodies invertebrates
• Have a muscular foot that can
be extended for movement
• Two part, hinged shell
• Complete, one-way digestive
tract with a mouth & anus
• Separate sexes
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Bivalve Mollusks
• No eyes or distinct
head
• Have siphons to
circulate water
through their bodies
• Filter feed on plankton
• Mussels are large and
have a flat, more
oblong shell shape,
while freshwater clams
are smaller (3/4 inch)
and typically more
round
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Indirect Development
• Goes through
a freeswimming,
larval stage
called the
trochophore
• Adults move
slowly by
their
extendable
foot
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Bivalve Mollusks
• Protostomes – blastopore becomes
the mouth
• Have a fully lined body or mantle
cavity
• Body organs called visceral mass
protected by mantle which secretes
the shell
• Open circulation
• Most are sessile or sedentary
• Includes clams, oysters, scallops, &
mussels
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Bivalves
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External Anatomy
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Bivalve Shell
• Umbo is the raised, oldest part of the
shell and is used to find surfaces
Dorsal
UMBO
Posterior
Anterior
Ventral
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Bivalve Shell
• Shell show regular spacing of external
growth rings on shells and their
progressive crowding as the animals grow
older
Growth
Rings
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Bivalve Shell
• Bivalves have an incurrent and excurrent
siphon to bring in water containing food
& O2 and for excess water & wastes to
leave
Siphons
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Bivalve Shell
•The
hinge
ligament
holds
the two
halves of
the shell
together
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Internal Anatomy
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Scissor tips or a screwdriver & can be used
to open the shell
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Adductor Muscles
Strong adductor muscles help open & close
the valves and must be cut to examine
the interior of the clam
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Inside Shell Lining
• The inside of the
shell is pearly and
smooth to protect
the body
• Oysters coat
irritants with a
substance called
“mother of pearl”
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Teeth located on the dorsal edge of
the shell lock to keep shells from
sliding
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The mantle makes a CaCO3 (limestone)
shell
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Mantle cavity
Gills
Gills hang outside visceral mass (body) in
the mantle cavity & remove O2 as H2O
flows over them
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Water Flow
• Incurrent siphons aid in bringing in water
with food and oxygen
• Cilia direct water into the gills, while labial
palps direct water into the mouth
move
food
toward
the
mouth
Cilia on the gills pulls in water
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VISCERAL MASS -
contains respiratory, digestive, & reproductive systems
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HEART & PERICARDIAL CAVITY
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Nervous System
• Consists of 3
pairs of ganglia
(brain)
• Has 2 pairs of
nerve cords
extending from
this
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Released into Mantle Cavity to be removed
by excurrent Siphon:
•Nitrogen waste
from kidney
• Digestive waste
from anus
• Sperm or egg (if
external
fertilization)
• Larva (if internal
fertilization)
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