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3_14_05
IB 202
5. Phylum Mollusca:
Mollusks have a muscular foot, a visceral
mass, and a mantle
• ~ 150,000 known species of diverse forms,
including snails and slugs, oysters and clams, and
octopi and squids.
• Mostly marine, some fresh water, and some
terrestrial snails and slug.
• Mollusks are soft-bodied animals; most protected
by a hard shell of calcium carbonate.
– Exceptions: slugs, squids, and octopi have reduced or
lost their shells completely during their evolution.
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• Similar body plan- muscular foot (typically for
locomotion), a visceral mass with most of the
internal organs, and a mantle.
– The mantle, which secretes the shell, drapes over the
visceral mass and creates a water-filled chamber, the
mantle cavity, with the gills, anus, and excretory
pores.
– Many mollusks feed by using a straplike rasping
organ, a radula, to scrape up food.
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Teeth of radula
-the hardest material
(magnetite) of
biological origin
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• Most mollusks have separate sexes
– However, many snails are outcrossing
hermaphrodites.
• The life cycle of many marine mollusks
includes a ciliated larvae, the trophophore.
– This larva is also found in marine annelids
(segmented worms) and
some other lophotrochozoans.
Fig. 32.9
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A trochophore larva.
• The basic molluscan body plan has evolved in
various ways in the eight classes of the phylum.
– The four most
prominent are the
Polyplacophora
(chitons),
Gastropoda
(snails and slugs),
Bivalvia (clams,
oysters, and other
bivalves), and
Cephalopoda
(squids, octopuses,
and nautiluses).
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Polyplacophora - chitons
• Marine; oval shapes and shells divided into
eight dorsal plates.
• Muscular foot grips the rocky substrate tightly
and creep.
• Grazers;
use radulas
to scrape and ingest
algae.
Fig. 33.17
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• Gastropoda - > 40,000 species; mostly marine,
but also many freshwater species.
– Garden snails and slugs have adapted to land.
• Distinctive characteristic - During embryonic
development, gastropods undergo torsion in
which the visceral mass is rotated up to 180
degrees, such that
the anus and mantle
cavity are above
the head in adults.
Fig. 33.18
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Diagrammatic Illustration of Torsion in Gastropod Molluscs
• Most gastropods are protected by single,
spiraled shells into which the animals can
retreat if threatened.
– Other species have lost their shells entirely and may
have chemical defenses against predators.
Shell-less nudibranchs
(sea slugs)
Fig. 33.19
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• Many gastropods have distinct heads with eyes
at the tips of tentacles.
Conch
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• Gastropods are among the few invertebrate
groups to have successfully populated the land.
• In place of the gills found in most aquatic
gastropods, the lining of the mantle cavity of
terrestrial snails functions as a lung –
pulmonate snails
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• Class Bivalvia -- clams, oysters, mussels, and
scallops.
• Bivalves have shells divided into two halves.
– The two parts are hinged at the mid-dorsal line, and
powerful adductor muscles close the shell tightly to
protect the animal.
– When the shell is open,
the bivalve may extend
its hatchet-shaped foot
for digging or anchoring.
Fig. 33.20
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• The mantle cavity of a bivalve contains gills
that are used for feeding and gas exchange.
• Most bivalves are suspension feeders, trapping
fine particles in mucus that coats the gills.
– Cilia convey the
particles to the mouth.
– Water flows into mantle
cavity via the incurrent
siphon, passes over the
gills, and exits via the
excurrent siphon.
Fig. 33.21
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• Cephalopods - rapid movements to dart toward
their prey which they capture with several long
tentacles.
– Squids and octopuses use beaklike jaws to bite their
prey and then inject poison to immobilize the
victim.
• A mantle covers the
visceral mass; shell is
reduced and internal
in squids, missing in
many octopi. Only
nautilus has external
shell.
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Fig. 33.22
• Fast movements by a squid occur when it
contracts its mantle cavity and fires a stream of
water through the excurrent siphon.
– By pointing the siphon in different directions, the
squid can rapidly move in different directions.
• The foot of a cephalopod (“head foot”) has
been modified into the muscular siphon and
parts of the tentacles and head.
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• Unique among mollusks, cephalopods have a
closed circulatory system to facilitate the
movements of gases, fuels, and wastes through
the body.
• They have a well-developed nervous system
with a complex brain and well-developed sense
organs.
– This supports learning and complex behavior.
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• Highly developed eye in cephalopods
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6. Phylum Annelida: Annelids are
segmented worms
• All annelids (“little rings”) have segmented
bodies.
• ~ 15,000 species ranging in length from less than
1 mm to 3 m for the giant Australian earthworm.
• Annelids live in the sea, most freshwater habitats,
and damp soil.
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The phylum Annelida - three classes:
Oligochaeta, Polychaeta, and Hirudinea.
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• OligochaeteE.g. earthworm
Coelom is
partitioned by
septa, but the
digestive tract,
longitudinal
blood vessels,
and nerve cords
penetrate the
septa and run the
animal’s length.
Fig. 33.23
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• Digestive system - a pharynx, an esophagus,
crop, gizzard, and intestine.
• The closed circulatory system carries blood
with oxygen-carrying hemoglobin through
dorsal and ventral vessels connected by
segmental vessels.
• In each segment is a pair of excretory tubes,
metanephridia, that remove wastes from the
blood and coelomic fluid.
– Wastes are discharged through exterior pores.
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• A brainlike pair of cerebral ganglia lie above and
in front of the pharynx.
• Earthworms are cross-fertilizing hermaphrodites.
– Two earthworms exchange sperm and then separate.
• Some earthworms can also reproduce asexually
by fragmentation followed by regeneration.
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• Polychaete - Each segment of a polychaete
(“many setae”) has a pair of paddlelike or
ridgelike parapodia (“almost feet”) that function
in locomotion.
– Each parapodium has several chitinous setae.
– In many polychaetes, the
rich blood vessels in the
parapodia function as gills.
Fig. 33.24b
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• Most polychaetes are marine.
• Polychaetes include carnivores, scavengers, and
planktivores.
– The brightly colored
fanworms trap plankton
on feathery tentacles.
Fig. 33.24c
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• Hirudinea - leeches.
• Many leeches feed on other
invertebrates, but some bloodsucking parasites feed by attaching
temporarily to other animals,
including humans.
Fig. 33.24d
– Some parasitic species use bladelike
jaws to slit the host’s skin, while others
secrete enzymes that digest a hole
through the skin.
– The leech secretes hirudin, an
anticoagulant, into the wound, allowing
the leech to suck as much blood as it
can hold.
– Medicinal use of leech.
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