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Mollusca and Annelida
By Daniel Smith, Mackenzie
Neighbors and Margaret Gaines
Some Useful Terms
• Mantle - a single or paired outgrowth of the body
wall that lines the inner surface of the valves of
the shell in mollusks
• Trochophore - small, free-swimming, ciliated
aquatic larva of mollusks and annelids
• Setae - stiff bristle-like hair used for movement in
earthworms
• radula - a band in the mouth of most mollusks that
is set with numerous, minute teeth and is drawn
backward and forward over the floor of the mouth
to break up food
• Crop - a pouch in the foregut of arthropods and
annelids for holding food
More Useful Terms
• Torsion - the rotation of the visceral mass,
mantle and shell 180˚ with respect to the
head and foot of the gastropod. This brings
the mantle cavity and anus to an anterior
position above the head
• Septa - a division of the coelum (a partition
between segments of it) in annelida
• Visceral mass – area which contains most
internal organs
General Information
• Both Mollusks and Annelids have three germ
layers
• Both are also protostomes
• Mollusks move by using a foot that is extremely
muscular
• Mollusks have one coelum while Annelids have
multiple coelums
• Mollusks reproduce sexually while Annelids
reproduce both sexually and asexually
• Both have Bilateral Symmetry
Classes of Mollusca
• Cephalopoda including squid, octopuses, nautilus,
cuttlefish; all marine
• Gastropoda including sea snails with shells and marine
snails without a shell or with a reduced shell; land snails
and slugs, freshwater snails
• Bivalvia including clams, oysters, scallops, mussels
• Polyplacophora including chitons that live on rocky
marine shorelines
• Scaphopoda including tusk shells; all marine
• Aplacophora including solenogasters, and the subclass
Caudofoveata which are deep-sea worm-like creatures
• Monoplacophora deep-sea limpet-like creatures
Cephalopoda
• Predators that include squid, nautiluses,
cuttlefish, and octopuses
• Have a head surrounded by a foot that is
divided into arms and tentacles
• Nautiluses have strong shells, cuttlefish an
internal shell, squid some vestiges of a shell
while octopuses don’t have one
Octopus v. Squid
• No hard shells
• Short life-span, commit
programmed suicide after
reproduction
• Eggs are watched over by
mothers until hatched
• Feed in plankton cloud until
mature
• Crawl with some swimming
• Eight arms
• Extremely good sight
• Thought to be as intelligent as
a dog
• Hard shells
• Eggs are released into water
in cases by most, some tend
their eggs and then die
• Also feed in plankton cloud
• Mostly swim and use
tentacles for hunting
• Eight arms, two tentacles
• Can be much larger than
octopuses, one has been
measured at 59 feet in
length
Gastropoda
• Mostly bottom feeders, some filter feeders
• Most inhabit relatively large shells that serve
as shelter – sea slugs(gastropods) don’t
• Most marine gastropods wander on rocky
parts of the bottom because their foot doesn’t
attach to sediments like sand and mud
Bivalvia
(bivalves)
• Filter feeders anchored to the bottom
• Can live in sedimentary
• Some bivalves dig with a strong muscular
foot and others use their foot to create
strong threads that attach them to rocks
Foot
Polyplacophora
Chiton
Scaphopoda
Tusk Shell
Aplacophora
The Foot of this animal is contained in the
groove above
Solenogastres
Monoplacophora
Monoplacophora were thought ho be
extinct until someone found one in the
fifties, as a result we don’t have many
pictures of them.
Classes of Annelida
• Oligochaeta aka earthworms
– good at tilling the soil
• Polychaeta- each segment has a parapodium for
locomotion
– Parapodium- a pair of paddlelike/ridgelike structures
similar to feet
– The parapodium contains many setae which are made
up of chitin.
Hirundinea aka leeches (freshwater and land
leeches)
– Parasitic and sucks blood of victim. Can suck up to 10
times its weight.
Reproduction (in depth)
• They are hermaphrodites that can cross- fertilize.
–
–
–
–
–
asexually by fragmentation/regeneration
sexually:
1)
males exchange sperm and store it.
2) They make a “mucous cocoon”
3) This cocoon attaches to the side of the worm and
picks up eggs
– 4)
Once egg and sperm meet the cocoon slides off
into the soil to grow
Movement
• Parapodia (made up of setae) that function
as feet to move the annelid along
• Land: crawl/ burrow
• Aquatic: drift or swim
Body Development
•
•
•
•
•
Segmented!!!
1mm to 3m length
Coelom separated by septa
Complete digestive track
Longitudinal blood vessels and nerve cords
that penetrate septa
Specialized Tissue
• Metanephridia- excretory tubes that remove waste
from blood and fluid in coelom
• Digestive system is complex:
–
–
–
–
Crop
Gizzard
Intestine
Pharynx
• Ganglia- similar to brain. Has nerve cells
connected to pharynx ( where food comes in)
Polychaeta
• Have two bristles(parapodia) on each
segment of their bodies, they are like setae
but for aquatic usage instead of terrestrial
• Sometimes called bristle worms
Segments
Chaetae
Oligochaeta
Earthworms
Hirundinea
Leech