PHYLUM MOLLUSCA snail giant clamNautilus. Characteristics Soft body Unsegmented Bilateral symmetry...
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PHYLUM MOLLUSCA
snail giant clamNautilus
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Characteristics
• Soft body• Unsegmented• Bilateral symmetry• Usually definite head• Marine, freshwater and terrestrial habitats• Ventral body wall forms muscular foot
– Locomotion• Dorsal body wall forms mantle, enclosing mantle cavity• Shell (secreted by mantle) • Coelom limited mainly to area around heart (pericardial
cavity)
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Coelom
• Body cavity surrounded by mesoderm
• Major step in evolution of larger, more complex life forms
• Benefits:– More stable arrangement of organs
– Alimentary canal more muscular and highly specialised without interfering with other organs
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CharacteristicsBody Plan
• Foot, mantle, visceral mass
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Head-Foot Region
• Well developed head – Bears mouth and specialised sensory organs– Radula inside mouth
• Muscular foot – Variously modified and adapted for
locomotion, attachment to a substrate or a combination of functions
– Often secretes mucous
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Visceral Mass Region
• Visceral mass contains digestive tract, circulatory organs, paired kidneys, reproductive organs
• Mantle– Protective sheath; outgrowth of dorsal body
wall– Encloses a space (mantle cavity)– Cavity usually houses respiratory organs,
which develop from the mantle– Products from digestive, excretory and
reproductive systems empty into cavity– Mantle secretes the shell– Gaseous exchange
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Characteristics
• Radula (tongue-like rasping organ) usually present
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Circulatory System
• Open circulatory system (closed in cephalopods)
• Heart, blood vessels and sinuses
• Respiratory pigments in blood
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Respiration
• Gaseous exchange by:– Gills (one or two)– Lung– Mantle (exposed surface)– Body surface
Excretory System
• One or two kidneys (metanephridia) • Open into coelom (pericardial cavity)• Usually empty into mantle cavity
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Nervous System
• Several pairs of ganglia with connecting nerve chords
• Sensory organs of touch, smell, taste, balance and vision (in some); eyes highly developed in cephalopods
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Generalised Mollusc
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Reproduction
• Mostly dioecious; some monoecious
• Spiral cleavage of egg to produce free-swimming larva– Trochophore or veliger larva (aquatic
molluscs)
• Some with direct development
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Trochophore• Free-swimming larva that emerges from egg in primitive
molluscs
prototroch
mouth
anus
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Veliger
• In many molluscs the trochophore is passed into the egg, and a veliger hatches to become the only free-swimming stage
• Has the beginnings of foot, shell and mantle
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Class: Caudofoveata Class: SolenogastresWormlike Wormlike
Marine Marine
Reduced head Reduced head
Have no shell but covered with calcareous scales or spicules
Have no shell but covered with calcareous scales or spicules
Mostly burrowers Mostly free-living on bottom sediments
Sexes are separate Hermaphroditic (monoecious)
Radula present but may be reduced
Usually radula is absent
Gills present Gills absent, secondary respiratory structures may be present
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Class: Monoplacophora
• Thought extinct until 1952, when living specimens were dredged up from bottom of ocean
• Small with rounded shell and creeping foot (limpet-shaped)
• Mouth with characteristic radula
foot
mouth
gills
shellmantle
anus
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Class: Polyplacophora
• Chitons• Flattened dorsoventrally • Convex dorsal surface with 8
plates• Head with sensory organs is
reduced• Radula projects from the mouth
and scrapes algae from rock surfaces
• Sexes separate• Trochophore develops into
juvenile
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Class: Scaphopoda
• Tusk shells or tooth shells• Marine; benthic; sedentary• Slender body covered with a mantle and
a tubular shell open at both ends• Foot protrudes through larger end of shell
– used to burrow into mud or sand• Small end of shell exposed• Gills absent, gaseous exchange via
mantel• Long tentacles extend from the head
region– Captacula- feeding
• Dioecious; trochophore larvae (ancestral)
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Class: Gastropoda
• Largest and most diverse class• Marine, freshwater or terrestrial• Includes snails, slugs, limpets,
whelks, nudibranchs and others• Head well developed with one or
two pairs of tentacles, which may bear eyes
• Shell, when present, is always one piece and may be coiled or uncoiled
naudibranchs garden slug
limpet
whelks
snail
abalone
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Class: Gastropoda
• Both trochophore and veliger larval stage
• Bilaterally symmetrical, but because of torsion, visceral mass has become asymmetrical
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Torsion
• Only in gastropods• Twisting process that moves the mantle cavity (originally
posterior) to the front of the body, thus twisting the visceral organs through a 90° to 180° rotation
• Occurs in veliger stage
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Practical
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Torsion
• Before torsion, embryo’s mouth is anterior and anus and mantle cavity are posterior
• Change brought about by uneven growth of right and left muscles that attach the shell to the head-foot
• After torsion anus and mantle cavity are anterior and open above the mouth and head
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Coiling
• Coiling of shell and visceral mass is not the same as torsion
• May occur in the larval stage at the same time as torsion
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Coiling
• Apex contains the oldest and smallest whorl
• Whorls become successively larger and spiral about the central axis (columella)
• Largest whorl presses on right side of mantle cavity, leading to a loss of organs on RHS (gill, auricle, kidney)
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Shell
• Shell may be right handed (dextral) or left handed (sinistral)
• Operculum – horny plate that covers shell aperture when body is withdrawn into the shell
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Class: Gastropoda
• Subclass: Prosobranchia– E.g. periwinkles, abalone,
limpets, whelks– Almost all marine – One pair of tentacles– Sexes usually separate– Shell present– Operculum often present– Tored Haliotus midae
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Class: Gastropoda
• Subclass: Opisthobranchia• Sea slugs, sea butterflies and others
– Nearly all marine– two classical groups:
• Tectibranchs - with gill and shell• Nudibranchs - no shell or true gill,
but have secondary gills along the sides between the mantle and foot
– Usually two pairs of tentacles– All monoecious– Some detorsion
Sea butterfly or pteropod
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Class: Gastropoda
• Subclass: Pulmonata– Terrestrial and freshwater snails and slugs– Usually lack gills, but mantle wall has developed a
lung– Monoecious– Aquatic species have one pair of non-retractable
tentacles, at the base of which are eyes– Terrestrial species have two pairs of tentacles with
the posterior pair bearing eyes– Some detorsion
snail
slug
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Class: Bivalvia
• Mussels, clams, oysters, scallops
• Mostly marine; some freshwater or brackish water
• Mostly sedentary filter feeders
• Laterally compressed• 2 shells (valves) held
together dorsally by a hinge ligament
Giant clam
Black mussels; Mytilus edulus
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Class: Bivalvia
• Head greatly reduced• No radula• Wedge-shaped foot• Mantle may be modified into ventral incurrent
and dorsal excurrent siphons in marine forms• Dioecious• Fertilization usually external • Trochophore and veliger larvae• Open circulatory system
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Class: Cephalopoda
• Most complex of the molluscs• Squid, octopuses, nautiluses,
cuttlefish• All marine• All active predators• Anterior margin of head
drawn out into a circle of arms or tentacles used to capture prey (adhesive secretions or suckers)
• Powerful parrot-like beak is used to tear prey apart
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Class: Cephalopoda
• Foot modified into funnel for expelling water from mantle cavity (jet propulsion)
• Shell often reduced or absent
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Class: Cephalopoda
• Chromatophores – Pigment cells in skin that produce colour changes
• Bioluminescence in deep sea forms
• Ink production – Ink sac with fluid containing melanin empties into rectum
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Class: Cephalopoda
• Brain• Complex nervous system• Increased sensory perception
and behavioural complexity• Some species capable of
learning (intelligence?)• Some with closed circulatory
system• Separate sexes• Direct development; no larval
stages
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Class: Cephalopoda
• Subclass: Nautiloidea– Nautilus (chambered Nautilus) only
remaining members– Coiled external shell divided into
gas filled chambers– Relatively simple eyes– 2 pairs of gills– Tentacles contain no suckers and
extend through the shell opening
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Class: Cephalopoda
• Subclass: Ammonoidea– Totally extinct – End Cretaceous (65
MYA)
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Class: Cephalopoda
• Subclass: Coleoidea (most have 8 arms and 2 tentacles)
• Cuttlefish, squid and octopus
cuttlefish
squidoctopus
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cuttlefish
Class: Cephalopoda
Order: Sepioidea• Cuttlefish• Rounded or compressed
bulky body bearing fins• Arms and tentacles bear
suckers but tentacles only at their ends
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Class: Cephalopoda
Order: Teuthiodea• Squid• Body more cylindrical• 8 arms, 2 tentacles• Arms with stalked suckers
with horny rims bearing teeth• E.g. Colossal squid
(Mesonychoteuthis hamiltoni)- largest known invertebrate in terms of mass
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Class: Cephalopoda
Order: Octopoda• No shell; 8 Arms; no tentacles; short compact
body; suckers not stalked and do not bear horny rims
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Evolutionary Relationships
• Molluscs are allied with both annelids and arthropods
• Molecular evidence suggests they are more closely related to the annelids
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