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Chapter 35
Mollusks
Annelids
Mollusks
• Describe the key characteristics of mollusks.
• Describe the body plan of mollusks.
• Name the characteristics of three major classes
of mollusks.
• Compare the body plans of gastropods,
bivalves, and cephalopods.
• .
Characteristics of Mollusks
• Phylum Mollusca is a
diverse group of
invertebrates such as
clams, snails, slugs,
squids, and octopuses.
• They are called mollusks,
from the Latin molluscus,
which means “soft.” Many
mollusks have soft bodies
and some have a hard
shell.
Characteristics of Mollusks
• Some mollusks are sedentary filter
feeders, while others are fast-moving
predators.
• Mollusks are coelomates.
• Most aquatic mollusks and annelids have
a larval stage called a trochophore.
Body Plan of Mollusks
• The body of a mollusk is generally divided
into:
– The visceral mass, which contains the heart
and the organs of digestion, excretion, and
reproduction
– the head-foot, which consists of:
• the head, which contains the mouth and sensory
structures
• the foot, a large, muscular organ for locomotion
Body Plan of Mollusks
• The coelom is limited to a space around
the heart.
• A layer of epidermis called the mantle
covers the visceral mass.
Body Plan of Mollusks
• In most mollusks, the mantle secretes one
or more hard shells containing calcium
carbonate.
• This disadvantage of a shell is offset by
having gills.
• The gills are protected within the mantle
cavity.
Body Plan of Mollusks
• Most mollusks are bilaterally symmetrical.
• The nervous system consists of paired
clusters of nerve cells called ganglia .
• The main feeding adaptation of many
mollusks is the radula.
Mollusk Body Plan
Class Gastropoda
• The largest and most
diverse class of mollusks is
Gastropoda, whose
members are called
gastropods.
• Most gastropods, including
snails, abalones, and
conchs, have a single
shell. Others, such as
slugs and nudibranchs,
have no shell.
Class Gastropoda
• Gastropods have an open circulatory
system in which a heart pumps
hemolymph from gills or lungs into the
hemocoel.
abalones
Snails
• Snails live in moist and aquatic environment.
• Most snails locate food using eyes at the end of
tentacles.
• Snails survive dry periods by retreating into
their shells and sealing the opening with a
mucous plug.
Fresh Water Snail
Land (Common) Snail
Other Gastropods
• Slugs are terrestrial and lack shells.
• Pteropods, or “sea butterflies,” have a foot
that is modified into a winglike flap and
used for swimming rather than crawling.
Grey Slug
Sea Butterfly
Characteristics of Gastropods
Class Cephalopoda
Class Cephalopoda
• The class Cephalopoda includes
octopuses, squids, cuttlefishes (below),
and chambered nautiluses.
• They are called cephalopods, which
means “head-foot.”
Class Cephalopoda
• They are specialized as
free-swimming predators.
• They have tentacles and
beaklike jaws on the head.
• Their nervous system is
very advanced.
• They have a closed
circulatory system.
• Many use pigments to hide
and disguise themselves
chambered nautilus
Squids
• Squids are cephalopods with ten tentacles.
• They propels themselves by pumping jets of
water with the mantle through an excurrent
siphon.
Deep Sea Squid
Giant Squid
Octopuses
• Octopuses have
eight tentacles and
are similar to squids.
• They often crawl
along the ocean
bottom or lie in wait
for prey.
Class Bivalvia
• The class Bivalvia
includes aquatic mollusks
such as clams, oysters,
and scallops.
• They are called bivalves
because their shell is
divided into two valves
connected by a hinge.
• Each valve consists of
three layers.
Giant Clam
Class Bivalvia
• Most bivalves are sessile filter feeders.
• Bivalves lack a distinct head and have
three pairs of nerve ganglia.
Flame Scallop
Clams
• Clams are bivalves that live
buried in mud or sand.
• The mantle cavity of a clam
is sealed except for a pair of
hollow, fleshy tubes called
siphons.
– Water enters through the
incurrent siphon.
– Water leaves through the
excurrent siphon.
Characteristics of Bivalves
Annelids
• Identify the structures that provide the basis for
dividing annelids into three classes.
• List the advantages of body segmentation.
• Describe the structural adaptations of
earthworms.
• Compare the three classes of annelids.
Characteristics of Annelids
• The phylum Annelida is made up of
bilaterally symmetrical, segmented worms.
• This phylum includes common
earthworms, feather-duster worms, and
bloodsucking leechs (below).
Characteristics of Annelids
• They are called
annelids, which means
“little rings” and refers
to the many body
segments.
• Annelids have a true
coelom that is divided
into separate
compartments by
partitions.
Social Feather-duster Worms
Characteristics of Annelids
• Most annelids have external bristles called
setae (singular, seta), and some have
fleshy protrusions called parapodia
(singular, parapodium).
Class Oligochaeta
• Annelids of the class Oligochaeta
generally live in the soil or in fresh water
and have no parapodia.
• Oligochaeta means “few bristles”; these
annelids have a few setae on each
segment.
• The most familiar member is the
earthworm.
Class Oligochaeta
Structure and Movement
• An earthworm’s body has over 100 nearlyidentical segments.
• Circular and longitudinal muscles line the
interior body wall.
• Locomotion is made possible by
segmentation.
Class Oligochaeta
Feeding and Digestion
• Earthworms ingest soil as they burrow through it.
• Soil is moved through these structures:
–
–
–
–
–
–
mouth
pharynx
esophagus
crop
gizzard
intestine
• includes the typhlosole
– anus
Class Oligochaeta
Circulation
• Contractions of the aortic arches and the
dorsal blood vessel force blood through
the closed circulatory system.
Class Oligochaeta
Respiration and Excretion
Respiration and Excretion
• Oxygen and carbon dioxide diffuse
through the skin, which contains many
small blood vessels and must be kept
moist.
• Cellular wastes and excess water are
excreted through nephridia.
Class Oligochaeta
Neural Control
• The nervous system of an earthworm
consists of a chain of ganglia connected
by a ventral nerve cord.
• Sensory structures are found in all
segments but are concentrated at the
anterior end.
Class Oligochaeta
Reproduction
• Earthworms are hermaphrodites, but an
individual worm cannot fertilize its own
eggs.
• During mating, earthworms press their
ventral surfaces together.
• They are held together by their setae and
by a film of mucus secreted by each
worm’s clitellum.
Class Oligochaeta
Reproduction
• The sperm from each worm move
through the mucus to the seminal
receptacle of the other.
• The clitellum secretes a tube of mucus
and chitin.
• Fertilization occurs inside the tube, which
forms a protective case for the young
worms.
Earthworm Dissection Hearts and
Reproductive Organs
Anatomy of the Earthworm