Phylum Mollusca
General Characteristics:
Molluscus, which means “soft”
General Characteristics
 2nd largest animal phylum, next to Arthropoda
 Are bilateral, have 3 tissue layers, and a coelom
 All organ systems
 Have a soft body – protected by a shell in some
species
General Characteristics
 Have a mantle – fold of skin that surrounds the
organs. The space created inside of the fold is
called the mantle cavity
 Have a large ventral foot – used for location or
attachment. The foot is divided into tentacles in
squids and octopuses.
General Characteristics
 Most – but not all – mollusks have an open
circulatory system. A heart pumps blood into
vessels that are open-ended. The blood flows
out of the vessels and bathes the body organs.
General Characteristics
 Gas exchange occurs across the gills in
aquatic mollusks and across simple lungs in
terrestrial mollusks.
General Characteristics
 Like roundworms and segmented worms, all
mollusks have a digestive tract with 2 openings
and specialized regions.
 Have evolved many methods of obtaining food.
Most sessile and slow-moving mollusks are filterfeeders. Some mollusks, such as snails, are
herbivores with special mouthparts adapted to
gather vegetation or algae. Predatory mollusks,
such as squids and octopuses, use their beak-like
jaws to capture prey.
General Characteristics
 Simple nervous system with the exception of
squid and octopuses, which are capable of
learning behavior and adapting their actions
to locate and capture prey.
General Characteristics
 Most mollusks have separate sexes, and all
mollusks reproduce sexually. Reproduction
can involve external fertilization, as in clams,
or internal fertilization, as in squids and some
snails.
General Characteristics
 On the basis of both internal and external
features, it would seem that mollusks and
segmented worms are not at all closely related.
However, comparison of larvae indicates that
they probably evolved from a common ancestor.
Class Cephalopoda
 Octopuses, squids and nautiluses are
examples of cephalopods
 “Cephalo –” meaning “head”, and “poda”
meaning “footed”.
Octopus
Class Cephalopoda
 Zoologist think this group is the most recently evolved class
of mollusks.
 Cephalopods have a well-developed nervous system. Their
large brain and extensive network of nerves enable them to
respond quickly and control their body motions efficiently.
Octopuses are considered to be among the most intelligent
invertebrates.
 Unlike other mollusks, cephalopods have a closed
circulatory system. This means their blood travels inside a
continuous network of vessels. A closed circulatory system
provides tissues with a more steady supply of oxygen and
nutrients than does an open circulatory system. A closed
circulatory system is associated with advanced kinds of
animals.
Class Cephalopoda
 Cephalopods can emit a cloud of ink as a
decoy when danger threatens, and they can
change color to blend in with their
surroundings (cryptic coloration).
Cephalopods seize their prey (other mollusks
and crustaceans) with their tentacles and
paralyze it by venom produced in the salivary
glands
Class Cephalopoda
One way to distinguish squids from octopuses is
by the number of tentacles. Octopuses have 8,
squids have 10.
 Some species of squids and octopuses grow very
large.
 The giant squid is the world’s largest living
invertebrate. The largest one measured was just
over 60 feet long. Giant squids live in cold waters
of the North Atlantic, where their only predators
are thought to be sperm whales.
Octopus Intelligence
Class Cephalopoda
 The only cephalopod that can be found in the
Chesapeake Bay is the Brief squid. These agile,
quick swimmers grow to about 5 inches and
come into the Bay from the summer to fall.
Monsterquest: Giant Squid
Class Bivalvia
 Clams, mussels, scallops, and oysters are
examples of bivalues.
 The word bivalvia comes from “bi” meaning 2
and “valvia” meaning halves.
 A hinge attaches the two shells of a bivalve.
Clams
Mussels
Scallops
Oysters
Class Bivalvia
 Strong muscles called adductor muscles control
the opening and closing of the hinged shells,
allowing a bivalve to control the flow of water
over its body.
 Calcium carbonate, which is used to form the
shell is secreted from the mantle.
Class Bivalvia
 Most bivalves inhabit marine environments.
 Most bivalves are filter-feeders. Food particles in
the water flowing into the body are trapped by
mucus on the gills. Cilia on the gill cells pass the
food along to the bivalve’s mouth.
Gill
Mouth
Class Bivalvia
 Various species of clams, mussels, scallops and oysters
inhabit the Chesapeake Bay.
 Aside from the Blue crab, if there is one Chesapeake
Bay animal that everyone recognizes immediately, it is
the oyster.
 Two species of oyster make their home in the Bay: the
Fossil oyster and the American oyster. American
oysters are the more tasty species. They live in beds
(bars) mostly in mid-Bay regions in water between 8 to
25 feed deep. The locations of major oyster bars are
well-know to commercial oystermen.
Class Bivalvia
 Zoologists identify mollusks by their shell type
(if any) and by the kind of food they eat. Of
the 6 mollusk classes currently recognized, 3
contain the most familiar and numerous
mollusks. They are: Class Gastropoda, Class
Bivalvia and Class Cephalopoda.
Class Gastropoda
 Largest class of mollusks
 Gastropods include snails, slugs, nudibranchs,
abalones, and conches. The word gastropoda
comes from “gastro” meaning stomach and
“poda” meaning footed. Their foot appears to
be attached to their stomach.
Class Gastropoda
 Nudibranchs are a large family of sea slugs.
Many of them have feathery, external gills, are
poisonous, and employ warning coloration. A
few species of nudibranchs and other kids of
sea slugs can be found in the Chesapeake Bay.
Class Gastropoda
 Abalones are a family of large sea snails that
live mainly in cool to warm coastal waters.
They are a favorite food of sea otters and are a
delicacy among people.
Class Gastropoda
 Conches are another family of large sea snails.
They live mainly in tropical coastal waters, where
they feed on seaweeds. Their shells have been
used as money from the earliest times. They are
often made into soups and other dishes.