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Bilateral Symmetry
• Most animals show bilateral symmetry, the
arrangement of body parts such that there is
only one way to cut the body to produce two
identical halves
• Bilaterally-symmetric animals have a front
(anterior), end (posterior),
back (dorsal), and belly
(ventral) surface
Bilateral Symmetry: the Flatworms
• Bilateral symmetry promotes actively moving
organisms, permits streamlining, favors the
formation of a central nerve center, and
contributes to the concentration of a head
region where nervous tissue and other
sensory systems are located
• The simplest bilaterally-symmetric animals are
the flatworms (phylum Platyhelminthes)
Platyhelminthes (Flatworms)
• Flatworms are the simplest animals in which
tissues are organized into real organs and
organ systems
• Marine flatworms include the turbellarians
Opening evolution’s can of worms
• Many advancements in organization and
structure occurred in marine worms
• The digestive tract and circulatory system
appeared in ribbon worms (Phylum Nemertea)
• Segmentation appeared in the annelids, or
segmented worms (Phylum Annelida)
– Series of repetitive segments
– Allows different regions of the body to develop
differently for different uses
Marine Annelids: the Polychaetes
• Almost all marine annelids are polychaetes
(class Polychaeta), which are also known as
“bristle worms” for their stiff and often sharp
bristles stemming from each of their repeated
– Bristles replaced by gills in many species
– Planktonic larvae stage
Marine Annelids: the Polychaetes
• Many polychaetes live in temporary or
permanent tubes
– Singular or aggregations
• Tube-dwelling polychaetes
are suspension feeders,
using feathery tentacles to
trap and capture suspended
How about this heat?!!?
• One of the most remarkable
polychates is the Pompeii worm
• Found only at hydrothermal vents in the
Pacific Ocean
– Temperatures as high as 80°C (176°F)!
– “Fleece-like” covering of bacteria on their backs;
the worms secrete mucus to feed the bacteria and
in return, are protected by a degree of insulation
on their backs
Polychaetes are pretty!
Christmas Tree Worm
Free-living polychaete
Marine Annalids: the Beard Worms
• Beard worms, or pogonophorans are highly
specialized annelids (class Pogonophora)
• Pogonophorans lack a mouth and a gut
• A tuft of long tentacles absorb nutrients in the
water and give these annalids their common
• Found at hydrothermal vents; symbiotic
associations with bacteria
Riftia pachyptila
Phylum Mollusca
• Molluscs (Phylum Mollusca) are soft-bodied
organisms enclosed in a CaCO3 shell
• Very successful; more species of Molluscs in
the ocean than of any other animal group
• Their body is covered by a mantle, a thin layer
of tissue that secretes the shell
Phylum Mollusca
• Molluscs have a ventral, muscular foot and a
scraping tongue, or radula
• Most Molluscs belong to one of three groups:
– Class Gastropoda (snails, limpets, abalone,
and nudibranchs)
– Class Bivalvia (clams, mussels, oysters, etc.)
– Class Cephalopoda (squid, octopus,
cuttlefish, and nautilus)
Phylum Mollusca, Class Gastropoda
• The Gastropods (Class Gastropoda) are the
largest and most varied group of Molluscs
• Gastropod means “stomach footed”; a typical
gastropod is a coiled mass of organs enclosed
by a dorsal shell, which rests on the foot
– Have head and sensory
structures on head
– Operculum: trap door,
pulls in snail to shell
Phylum Mollusca, Class Gastropoda
Phylum Mollusca, Class Gastropoda
• Most gastropods use their radula to scrape
algae from rocks (e.g., periwinkles, limpets,
and abalone)
• Others are carnivorous and use their radula to
drill into the shells of bivalves and other
• Nudibranchs, or sea slugs,
are gastropods that have lost
their shell altogether
Phylum Mollusca, Class Bivalvia
• Bivalves (Class Bivalvia) include clams,
mussels, oysters, and scallops
• The body of bivalves is laterally compressed
(flattened sideways) and consists of 2 hinged
valves that are mirror images of one another
Phylum Mollusca, Class Bivalvia
• In bivalves, the mantle forms a thin membrane
that lines the inside surface of the shell
• This creates a mantle cavity, within which the
entire body of the bivalve lies
Phylum Mollusca, Class Bivalvia
• Strong muscles, the adductor muscles, are
used to close the valves
• Two siphons, an incurrent and an excurrent
siphon, draw water into and out of the mantle
cavity, respectively
• Since many clams burrow into the sediment,
these siphons allow the clam to feed and
Phylum Mollusca, Class Bivalvia
• Not all bivalves are burrowers; mussels
secrete strong byssal threads to attach to
rocks and other surfaces
• Oysters cement themselves to
hard substances including other
• Scallops are unattached and can
swim for short distances by rapidly
ejecting water from the mantle
cavity and flapping their valves
Phylum Mollusca, Class Cephalopods
• Cephalopods (Class Cepahlopoda) are a group
of molluscs that include squid, nautilus,
cuttlefish and octopus
• They are the most highly evolved of all
molluscs; nearly all are agile swimmers with a
complex nervous system
• Cephalopods have a reduction in or loss
altogether of the shell
Phylum Mollusca, Class Cephalopods
• Cephalopods (which means “head-footed”)
have heads surrounded by a foot modified
into arms and tentacles, usually equipped
with suckers that are used to capture prey
Phylum Mollusca, Class Cephalopoda
• Largest of all Molluscs
• All have a radula, beak-like structure
• Cephalopods have large eyes, with acute
vision, extremely sensitive to slight details and
Phylum Mollusca, Class Cephalopods
• Cephalopods pump water through the siphon,
or funnel into their mantle cavity
• By forcing water out of the mantle cavity
through the siphon, cephalopods swim by
means of “jet propulsion”
• By changing the direction of the siphon,
cephalopods can move backward or forward,
or can remain motionless in one place
Jet Propulsion
Water enters
mantle cavity
Direction of water out of siphon
Water exits from
funnel or siphon
Direction of cephalopod motion
Nautilus: Order Nautilida
• Nautilus are called “living fossils”; only 6
species remain of an extinct superfamily
• Survived relatively unchanged for millions of
• Gas-filled chambers aid in buoyancy
• Paper nautiluses
are actually pelagic
octopus; named for
the paper-thin
eggcase in females
Squids: Order Teuthoidea
• Squids are better adapted for swimming than
Long & retractable
• Squids have two triangular fins
modified for swimming
• Elongated body
• Squids have eight arms &
2 tentacles, all with suckers
which circle the mouth
Squids: Order Teuthoidea
• Suckers only at broadened tips on tenacle
• The shell is reduced to a stiff pen embedded
in the upper surface of the mantle
• Cephalopods control their skin pattern and
color by way of chromatophores
• Chromatophores are pigment-containing and
light-reflective cells used for camouflage
• Inside the chromatophore, pigment granules
are enclosed in an elastic sac
– To change color, the sac is distorted by way of
muscular contraction
Cuttlefish: Order Sepiida
• Cuttlefish are among the most intelligent
• Cuttlefish have a calcified internal shell called
a cuttlebone, which aids in buoyancy
– Chambered, gas-filled
• Called “chameleons of
the sea” for their
astonishing ability to
change skin color at will
Octopus: Order Octopoda
• One of the most iconic cephalopods is the
• Octopuses have 8 long arms (“octo” means 8)
and no shell
• Common bottom dwellers, they are efficient
– Bite their prey (mostly shellfish)
with a pair of beak-like jaws
– Ink sac emits a dark cloud of fluid
to deter predators
You will never be as cool as a cephalopod
• Mimic octopus
• Vampire squid from hell
…or as smart???
Octopuses and cuttlefishes have
a remarkable capacity for
Giant nerve fibers
rapidly conduct
impulses allowing
cephalopods to
capture prey or
escape at amazing
Most cephalopods
display color
changes correlated
with particular
behaviors and
Octopus are
known to use
tools, and predict
World Cup
Paul the Psychic Octopus