Mollusks and Segmented Worms
Ch 27 Turboblast Version!!!
Mollusks
• Phylum: Mollusca (includes slugs, snails, squids, octopus,
clams, oysters, cuttlefish)
• Slow-moving or jet-propelled
• Most mollusks are aquatic, most live in ocean
• Some live in fresh water, moist terrestrial habitats
• Some sessile (oysters), some swim freely
• Some have shells (hard covering), others are adapted to live
without hard covering
• ALL are bilaterally symmetrical; coelomates, digestive tract
with mouth and anus, muscular foot, mantle
• Mantle—membrane that surrounds internal organs
• In shelled mollusks, mantle secretes the shell
How Mollusks feed
• Use rasping structure called radula
http://www.youtube.com/watch?v=Ipzi9nn5Tf
g
• Radula—located in the mouth; tonguelike
organ with rows of teeth; used to drill, cut,
grate, or scrape food
• Bivalves lack radulas, they are filter feeders
How Mollusks Make Babies
• Sexual reproduction, mostly
have separate sexes
• In most species, release eggs
and sperm into water
simultaneously, external
fertilization
• Gastropods on land and few
bivalves—hermaphroditic
(each individual makes male &
female gametes) with internal
fertilization http://youtu.be/h0YH4YnS1bo
• Developmental stages are
similar for all mollusks
Nervous Control in Mollusks
• Simple nervous systems that coordinate
movement and behavior
• Some more advanced mollusks have brains
(cephalopods demonstrate left/right hemispheres)
• Paired eyes ranging in complexity from simple
cups to complex eyes
– Octopuses’ eyes have irises, pupils and retinas
similar to human eyes
• http://www.youtube.com/watch?v=GQwJXvlTWDw&feature=related
Octopuses are very intelligent!
Circulation in Mollusks
• Well-developed circulation systems with
chambered heart (2 or 3 chambers)
• Open circulatory system—blood moves through
vessels and into open spaces around the body
organs
• Exposes body organs directly to blood with
oxygen and nutrients and removes metabolic
wastes
• Closed circulatory system—blood moves through
the body enclosed entirely in a series of blood
vessels; efficient means of gas exchange
Respiration in Mollusks
• Gills—specialized parts of the
mantle consisting of filamentous
projections containing rich blood
supply for gas transport
• Increase surface area through
which gases can diffuse
• In land snails and slugs, mantle
cavity has evolved into primitive
lung
• Many mollusks have a siphon which
expels water and wastes
•
http://youtu.be/EpUDW1hoBmo
How Do Mollusks Excrete (remove
metabolic wastes)?
• Mollusks use nephridia (similar to kidneys)
• Nephridia are organs that remove metabolic
wastes, mollusks have one or two which
collect wastes from coelom which is located
around the heart only
• Wastes are discharged into mantle cavity, and
expelled from the body by pumping gills
Diversity in Mollusca
• Phylum Mollusca is divided into 3 classes
• Gastropoda (one-shelled mollusks)—
stomach-footed mollusks, foot is
positioned under the rest of the body
• Most have shells (thick or thin), some
have none (slugs)
• Shelled gastropods: snails, abalones,
conches, periwinkles, whelks, limpets,
cowries, cones
• Occupy freshwater, saltwater, or moist
terrestrial habitats
• If without shell (slug), body protected by
mucus layer
• Sea slugs are covered in nematocysts
(colorful sea slugs called “nudibranchs”)
http://rainsoftottawa.files.wordpress.com/2012/05/nudibranchs
2.jpg
Diversity of Mollusca
• Bivalva (two-shelled or bivalved
mollusks)—clams, oysters, scallops
• Mostly marine, few in freshwater
habitats
• No distinct head or radula, use large
muscular foot for burrowing into mud
or sand
• Two shells connected by ligament at
hinge
• Filter feeders, gill cilia that beat to
draw water current through siphon
• http://www.youtube.com/watch?v=tF
mMS_a7Q9I
Diversity in Mollusks
• Cephalopoda (Head-footed mollusks)—octopus, squid, cuttlefish,
chambered nautilus http://www.youtube.com/watch?v=L8qwRCs_7x0
– Chambered nautilus only one with shell, cuttlefish have reduced internal shell
• Most complex structures in cephalopod class
• Foot has evolved into tentacles with suckers, hooks, other adhesive
structures
• Swim/walk over ocean floor in pursuit of prey or in escape, capture prey
with tentacles
• Radula is beak-like jaw, food is torn and pulled into mouth by radula
• Siphons to expel water, can expel forcefully in any direction, helps
movement by jet propulsion
• FAST! Up to 20 meters per second
• Can also use dark ink to evade predators which clouds water
• http://www.youtube.com/watch?v=SCgtYWUybIE&feature=fvst
• http://www.youtube.com/watch?v=__XA6B41SQQ&feature=related
Annelids (Segmented Worms)
• Phylum Annelidae (earthworms,
leeches, bristleworms)
• Bilaterally Symmetrical,
coelomates, mouth + anus
• Some have larval stages similar
larval stages of mollusks (suggesting
a common ancestor)
• Basic body plan = tube within a
tube
• Most have tiny bristles called setae
(SEE-tee) on each segment (assist in
movement by anchoring body to
surface that provides leverage)
• Found in moist environments (not
in polar regions, deserts)
Segmentation!
• Segmentation provides diversified
functions
• Bodies divided into ringed segments,
segmentation continues internally as
each segment is separated from others
by partition
• Segmentation is important adaptation
for movement because each segment
has its own muscles that can lengthen
and contract individually
• Allows for specialization of body tissues
and modifications for functions such as
sensing and reproduction
Nervous System
• Simple nervous system, organs and anterior
segments modified to sense environment
• Some sensory organs sense light
– Eyes and retinas are evolved in certain species
• Brain in anterior segment in some species
• Nerve cords connect brain to nerve centers
called ganglia
Circulation/Respiration
• Closed circulatory system (blood in vessels)
• Must live in water or wet areas for gas
exchange directly across moist skin
Digestion/Excretion
• Complete internal digestive tract that runs length
of the body
• Food/soil taken into mouth and passes across the
gizzard
• Gizzard—muscular sac and hard particles that
helps grind food before it passes into intestine
• Undigested material and solid wastes exit
through the anus
• Two nephridia in almost every segment that
collect waste and transport through coelom and
out the body
Worm Babies
• Earthworms and leeches are hermaphrodites
• During mating, two worms exchange sperm
• Each worm forms capsule for eggs and sperm,
eggs are fertilized in capsule, capsule slips off
worm and left behind in soil
• In 2-3 weeks, young worms emerge from capsule
• Bristleworms have separate sexes, reproduce
sexually (release eggs, sperm into seawater)
• Hatched larvae become part of plankton, once
segmentation begins, bristleworms settle to
bottom
Diversity in Annelids
• 3 classes: Oligochaeta (earthworms); Polychaeta
(bristleworms, etc.); Hirudinea (leeches)
• Chaeta = Bristles
Diversity in Annelids
• Earthworms—most well-known and easily
seen annelids
• Only a few setae on each segment, no distinct
head
• Eat their way through soil, which creates
spaces for air and water to pass through soil
• Soil passes through digestive system and
undigested materials pass out as castings
(waste)
• Castings of earthworms help fertilize soil
Diversity in Annelids
• Polychaeta includes bristleworms, fanworms,
lug worms, plumed worms, sea mice
• Primarily marine animals
• Most body segments have many setae
(bristles) and a pair of appendages called
parapodia used for crawling/swimming and
gas exchange
• Has distinct head with well-developed sense
organs and eyes
Diversity in Annelids
• Leeches—segmented worms with flattened bodies,
usually no setae
• Most live in freshwater streams or rivers
• Many species are parasites that suck blood or other
body fluids from hosts’ bodies
• Attach to host via front and rear suckers
• Bites aren’t painful because chemicals in saliva act as
anesthetic and anticoagulants
• Ingests 2x-5x its body weight per meal, then drops off
host
• http://www.youtube.com/watch?v=vpv9P1KOVMQ
Origins of Mollusks and Segmented
Worms
• Fossil record shows mollusks were plentiful and
diverse as early as 500 mya
• Gastropod, bivalve, cephalopod fossils found in
Precambrian deposits
• Chambered nautilus appears to have changed
very little
• Annelids likely evolved in the sea, possibly from
ancient flatworm larvae
• Fossil record of segmented worms is limited
because of their soft bodies
• Tubes made from polychaetes are most common
fossils from Annelidae, dating back to 540 mya