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Mollusks & Echinoderms
Learning Target Objectives (I can…)
• Compare and contrast each group of mollusks and
echinoderms and list examples of each.
• Identify: body systems and their components, body
structures (anatomy) and functions (physiology).
• Differentiate between open and closed circulatory systems.
Vocabulary:
Mollusk * ganglia * cephalization * mantle * visceral
mass * gastropoda * hemocoel * radula * bivalvia *
adductor muscles * incurrent/excurrent siphon * gills
*cephalopod * echinoderm * endoskeleton * external
fertilization * water vascular system * regeneration *
aboral surface * oral surface * skin gills * parthogenesis
Mollusks & Echinoderms
Mollusks – “soft” bodied organisms
- true coelom (fluid filled cavity within the mesoderm
- many have top-shaped larva called a trocophore. This can
swim using cilia. The cilia are also used to draw food into the
mouth.
- bodies usually have 3 distinct parts:
1) muscular foot
2) head (except bivalves)
3) visceral mass (contains the organs, or “guts”)
- have the following body systems: circulatory, respiration,
digestive, excretory, nervous, reproductive
The muscular foot of a mollusk is used for locomotion
(movement).
The head (when present) has a mouth, sense organs, and
cerebral ganglia (nerves) (cephalization)
The visceral mass contains:
heart – circulatory system
stomach, intestine, anus – digestive system
kidney – excretory system
a reproductive system
Above the visceral mass is the mantle (forms from epiderm)
that usually secrete a shell made of calcium carbonate.
Organisms with shells have less surface area for gas
exchange. Some have gills located in the space between the
mantle and the visceral mass. These are specialized for gas
exchange. Gills = respiratory system
Mollusk Body Plan
Section 27-4
Squid
Snail
Shell
Mantle cavity
Foot
Clam
Early
mollusk
Gills
Digestive tract
Classes of Mollusks:
1) Class Gastropoda
Gastropoda means “stomach foot”
- most are univalves (have a single shell, or “valve”), some
have no shell
- distinct head region
- body twists over so head is near anus in snails
This class includes: snails, slugs
a) Snails
- have gills (aquatic snails) or modified lungs (terrestrial
snails) which must be kept moist for gas exchange
(respiratory system). In dry times, snails pull into their shell &
seal the opening with a mucus plug.
- open circulatory system – blood does not remain in vessels
during circulation. It spills into a “blood cavity” called a
hemocoel.
- radula – “tongue” covered with chitinous teeth (used to
scrape off algae or cut off leaves)
- Land snails are hermaphroditic. Aquatic snails have
separate sexes.
- internal fertilization
- create mucus for foot to slide on
- have 2 eyes on retractable tentacles (these can be pulled
into the head area when in danger)
- eaten as “escargot”
b) Slugs
- no shells
- live in moist environment (land slugs come out at night)
- cause crop damage
2) Class Bivalvia (or Pelecypoda = “hatchet foot”)
Bivalvia – “2 valves” (2 part shells)
Ex: clams, oysters, scallops
- filter feeders
- most are sessile
- no distinct head but have ganglia at anterior end
- a muscular foot
- 3 shell layers:
a) thin outer layer protects from acidic water conditions
b) thick, middle layer of calcium carbonate for protection
c) smooth, inside layer protects soft body with mother of
pearl. Layers of this are laid around any irritant, such as sand,
that gets between the mantle & shell.
- All are aquatic
- 2 adductor muscles pull shell tightly closed
- 2 tubes:
a) incurrent siphon – carries water with food particles into
the mantle area (cavity area). Food particles are trapped in
mucus on gills.
b) excurrent siphon – water is pumped out of the mantle
cavity
- gills are used for gas exchange (respiratory system) as well
as food gathering (digestive system). Cilia on the gills fan
food into the mouth. Gills also hold eggs of female clam
(reproductive system).
- open circulatory system
- kidneys (excretory system)
- 3 pairs of ganglia (nervous system)
- seperates sexes (sperm leaves through excurrent siphon &
enters female’s incurrent siphon – eggs are located on gills!)
Anatomy
of a Clam
Section 27-4
Stomach
Coelom
Shell
Heart
Nephridium
Adductor muscle
Mouth
Anus
Excurrent
siphon
Adductor
muscle
Incurrent
siphon
Gills
Mantle cavity
Intestine
Mantle cavity
Foot
3) Class Cephalopoda
Cephalopod – “head foot”
- well-developed head with an eye structure like ours
- foot divided into tentacles with suckers
(squids = 10 tentacles, octopus = 8 nautilus = many, but shorter)
- free-swimming
- radula & a strong beak for eating
- closed circulatory system (blood stays within vessels)
- most have no shell or have an internal shell
- well developed nervous system, large brain
- marine predators
- may release “ink” which dulls a predators sense of smell &
distracts them, allowing escape
Exs: squid, octopus, nautilus, cuttlefish
Compare/Contrast Table
Section 27-4
Comparing the Three Major Groups of Mollusks
MOLLUSK GROUP
SHELL
FOOT
EXAMPLES
Gastropods
Shell-less or
single-shelled
Muscular foot
located on ventral
side and used for
movement
Snail, slug, sea
hare, nudibranch
Bivalves
Two shells held
together by one
or two muscles
Burrowing species
have muscular foot.
Surface-dwelling
species have either
no foot or a
“reduced” foot.
Clam, oyster,
mussel, scallop
Cephalopods
Internal shell or
no shell
Head is attached to
a single foot. The
foot is divided into
tentacles or arms.
Octopus, squid,
cuttlefish, nautilus
Echinoderms
Echinoderm – “spiny skin”
- larva are bilaterally symmetrical
- adults are radially symmetrical
- have endoskeleton – internal skeleton with spines poking
outward
- tube feet help movement, respiration, feeding, & excretion
- No true circulatory, respiratory, or excretory systems
- Do have a nervous system but no brain
- separate sexes
- external fertilization (each arm has 2 gonads which release
egg or sperm)
- sexual and asexual reproduction is possible
- water vascular system (found ONLY in echinoderms) – used
in movement & food getting. It includes:
a) sieve plate – opening on dorsal surface of starfish, etc.
where water enters
b) canals – which form a ring around the mouth area & then
run down each arm
c) tube feet – these are connected to the canals. Water
pressure changes allow these to grab & release objects
- regeneration – as long as the cut off piece includes part of
the central disk (where the mouth & stomach are found)
- ALL are marine
- aboral surface – top of body
- oral surface – underside of body
Section 28-4
Anatomy of a
Starfish
Eyespot
Endoskeletal plates
Anus
Stomach
Digestive glands
Ring canal
Radial canal
Madreporite
Reproductive glands
Tube foot
Sucker
- skin gills (hollow tubes) along with the thin surface of the
tube feet are used for gas exchange
- sea stars (starfish) have an eyespot (on each arm) that
responds to light
- sea stars eat clams, etc. by turning their stomachs inside
out through their mouths & into a clam’s shell. This way it
digests the clam right in the clam’s shell.
- sea cucumbers may defend themselves by evisceration,
ejecting their internal organ lining. This regenerates later.
- parthogenesis – ability of eggs to develop without
fertilization. This is common in sea urchins.
Exs: sea stars, sea urchins, sand dollars, sea cucumbers
Compare/Contrast Table
Section 28-4
Comparing Groups and Major Characteristics of Echinoderms
Characteristic
Sea urchins
and sand
dollars
Brittle
stars
Sea
cucumbers
Sea
stars
Sea lilies and
feather stars
Feeding
Detritivores
Detritivores
Detritivores
Most
carnivores
Herbivores
Shape
Disc- or globeshaped, no
arms
Star-shaped,
arms
Cucumbershaped, no
arms
Star-shaped,
arms
Stalk with
feathery arms
Movement
Burrow in
sandy ocean
bottom or
wedge in rock
crevices using
moveable
spines
attached to
endoskeleton
Move rapidly
along ocean
floor using
arms
Move slowly
along ocean
floor using
muscular body
wall to crawl
Creep slowly
along ocean
floor using
arms
Cannot move;
attached to
ocean bottom
Venn Diagram
Mollusks
Echinoderms
Venn Diagram
Compare Any 2 Mollusks:
Venn Diagram
Compare Any 2 Echinoderms: