Download Phylum Mollusca

Phylum
Mollusca
Introduction
Mollusk – slugs, snails,
octopus, squid, clam, oyster
50,000 living mollusk
species and about 35,000
fossil species.
The number of known
species is second only to
the Phylum Arthropoda.
Introduction
The majority of mollusks
are marine, but large
numbers of species occupy
freshwater and terrestrial
habitats.
Some mollusk swim while
others are sedentary.
Introduction
Oysters produce pearls.
Some mollusk shells are
highly valued by collectors.
In times past these shells
were used as money and
today are used
ornamentally for such items
as buttons and jewelry.
Common characteristics
shared by mollusk
Soft, flexible body
 Bilateral symmetry
 Eucoelomates (body cavity
containing internal organs)

-Digestive system with two
body openings – a mouth
in the head and the anus
emptying into the mantle
cavity.
Most have a radula (except
bivalves). The radula is
usually toothed (may be a
single or could be
thousands) and is adapted
for scraping, tearing,
stabbing, and cutting.
In some the teeth are
hollow and poisoncontaining and are used as
weapons.
Thick muscular foot,
primarily used for
locomotion
Mantle (thin membrane
surrounding coelom); also
secretes the shell in those
forms that possess one.
A well-developed head, which
may bear sensory tentacles
(except bivalves)
An open circulatory system
(except cephalopods, which is
closed) with a heart, blood
vessels, and hemocyanin.
When oxygenated such blood
is bluish in color; when
deoxygenated the blood is
colorless.
Respiration - Gas exchange
is via gills, lungs, or the
body surface.
Secrete mucus – uses
include locomotion, food
entrapment, and prevention
of water loss.
Classes of Mollusks (3 of 7)
1.Gastropoda –includes
snails, limpets, slugs,
conchs
2.Bivalvia – oysters, clams,
scallops; made of two
hinged shells,
3.Cephalopoda – squid,
octopus, nautilus
GASTROPODS - largest
and most diverse class
(about 40 000 or 80%);
Most have a single coiled
shell (made of calcium
carbonate) into which the
body can be withdrawn.
Shells vary in color, shape,
ornamentation, and size
according to the species.
Slugs are shell-less. Shells
range in size form
microscopic to a 24 inch
marine snail ( Pleuroploca
gigantean). A sea hare,
Aplysia, can reach 3 ft.
Feeding Habits – Many are
herbivores (snails), with
multi-toothed radulas for
scraping algae from various
substrates.

Some are carnivores –
feeding on small fish, worms,
plankton and on smaller
mollusks.
The conch feeds on fish
and annelid worms that they
first paralyze with poison
contained in their hollow
radula teeth. The poison is
also toxic to humans,
causing paralysis and
sometimes death.
Muscular foot is used for
movement in most species.
Reproduction
terrestrial – hermaphrodites
 Aquatic – external
fertilization

Nervous system – simple;
small brain; nerves
coordinate movement and
behavior.
BIVALVES
Two shells, of similar size,
held together by a hinge
ligament coming together at
the umbo; shell covers soft
body; muscles open and
close (adductor muscle)
Shells can range in size –
the freshwater seed shells
are among the smallest
know, being less than 0.1
inch in length, while the
shell of the giant clam may
exceed 4 ft in length (500
lbs).
No distinctive head or
radula
Feeding habits – most are
sedentary filter feeders.
Draw water in, water moves
over the gills, then exits
through; as waste moves
over gills, food and
sediment trapped in the
mucus is sorted and then
eliminated
Most have a single pair of
gills for respiration and for
trapping minute food
particles.
Separate sexes with
external fertilization.
Most are marine, but many
live in brackish water in
streams, ponds, and lakes.
CEPHALOPODS – “headfooted”
Most complex of the
mollusks. All are marine.
Have tentacles w/suckers
instead of a single foot.
They are used for
movement and capturing
food.
Have radula and sharp,
beak-like jaws to tear apart
food. They are carnivorous.
Eating fish, bivalves and
crabs.
Have a closed circulatory
system.
Only the nautilus has a
shell.
Move by jet-propelled
swimming (66 ft/second)
Reproduction –Internal
fertilization, then eggs are
laid. Tentacles transfer
sperm into female’s body.
Nervous system –
Complex brain w/ ability
to learn (capturing prey
or avoiding harmful
situations). Highly
developed sense of
vision
Respiration – gills
Most have chromatophores,
special pigment cells, in
their skins, which allow
them to change color
rapidly. May have a sac
that secretes an ink-like
substance called sepia; this
is expelled when the animal
is alarmed, hiding the
animal as it flees.
Cephalopods are worldwide
in distribution and are found
in all depths of the ocean
.