Mollusca and Annelida
Tom McTernan
Brian Munger
Mollusca General Info
• 150,000 known species
• Most mollusks are marine, but some live in
fresh water environments
• Mollusks are soft-bodied animals that are
sometimes protected by a hard shell,
depending on the class
Symmetry in Mollusca
• In the beginning of
the life cycle of a
mollusk, bilateral
symmetry is present,
but this may be lost
by adulthood
Body Development in Mollusks
• 3 main parts: muscular
foot, a visceral mass, and a
mantle
• The muscular foot is used
for movement
• The visceral mass contains
most of the internal organs
• The mantle is a fold of
tissue that secretes the
shell if present, covers the
visceral mass
• The shell secreted by the
mantle is made of CaCO3
Movement
• In Polyplacophora,
Gastropoda, and
Bivalvia, a muscular
foot is used for
locomotion
• In Cephalopods,
siphon made from
the mantle allows jet
propulsion
Germ Layers and Coelom
• Mollusks are triploblastic, the condition of
the blastoderm is divided into 3 layers:
ectoderm, ectoderm, and mesoderm
• Open circulatory system, where the body
cavity is a blood-filled hemocoel filled with
hemophyll (blood and lymph between
organs)
Life Cycle/Reproduction
• Starts with a ciliated
larva, otherwise known
as a trochophore
• No true segmentation
unlike Annelids
• Normal sexual
reproduction is present,
although some are
hermaphrodites
• Most have external
fertilization
Specialized Tissue
• The radula is a tissue unique to the phylum
Mollusca, and is a tongue-like structure that
scrapes up food
• The mantle cavity is a water-filled chamber that
houses the gills, anus, and excretory pores
• Torsion is a process in Gastropoda where one set
of muscles grows faster than the other, which
creates the shell to grow in a spiral pattern
Chiton (Polyplacophora)
Octopus
(Cephalopoda)
Snail
(Gastropoda)
Squid (Cephalopoda)
Scallops (Bivalvia)
Annelids General Info
• There are about 15,000 species of Annelids
that range from less than 1mm to 3m
• They live in the sea, many freshwater
habitats and soil
• Annelida means “little rings” for the
segmentations present in all species
Symmetry in Annelids
• Annelids posses
bilateral symmetry
• They never lose this
symmetry, or gain
another type
throughout their
lifetime
Body Development in Annelids
• The Coelom in an
Annelid is sectioned
off by septa, but the
digestive tract, nerve
cords, and blood
vessels run the
length of the whole
organism
Digestive System
• There are a few
specialized regions in
the digestive system
of an Annelid; the
pharynx, the
esophagus, the crop,
the gizzard, and the
intestine. The
metanephridia excrete
waste from the blood
and coelom fluid our
of the worm
Germ Layers/Coelom
• Annelids are triploblastic, the condition of the
blastoderm is divided into 3 layers: ectoderm,
ectoderm, and mesoderm
• They have closed circulatory systems, and
have true segmentation
• Oligochaeta and Polychaeta have more
spacious coeloms while the coeloms in
leeches (Hirudinea) are filled with tissue
Movement
• While there are three
classes of Annelids, only
two have setae, which are
bristle-like structures that
assist in burrowing
• The parapodia are the
actual extensions on either
side of the organism from
which the setae attach to
• Earthworms (Oligochaeta)
eat their way through soil
Circulatory system
• Consists of networks of
vessels that carry
hemoglobin
• Dorsal and ventral vessels
are connected by other
segmental pairs of
vessels
• Five pairs of vessels and
the dorsal vessel pump
blood throughout the
circulatory system
Life Cycle/Reproduction
• Some Annelids use fission, or breaking off of the main
body to create another organism
• Earthworms cannot reproduce like, this but do have
the ability to regenerate if a segment is amputated
• Some Annelids are hermaphroditic while others are
different sexes within a species
• Polychaeta have ciliated larva, just like Mollusks
• Oligochaeta and Hirudinea are hermaphroditc and
mate throughout the year
• The clitella is the thickened band on a part of a
segmented worm that secretes a viscid sac where
eggs are deposited
Bristle worm
(Polychaeta)
Christmas Tree
Worm (Polychaeta)
Leech (Hirudinea)
Earthworm (Oligochaeta)
Marine Leech (Hirudinea)