PHYLUM MOLLUSCA

snail giant clamNautilus

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)

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

CharacteristicsBody Plan

• Foot, mantle, visceral mass

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

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

Characteristics

• Radula (tongue-like rasping organ) usually present

Circulatory System

• Open circulatory system (closed in cephalopods)

• Heart, blood vessels and sinuses

• Respiratory pigments in blood

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

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

Generalised Mollusc

Reproduction

• Mostly dioecious; some monoecious

• Spiral cleavage of egg to produce free-swimming larva– Trochophore or veliger larva (aquatic

molluscs)

• Some with direct development

Trochophore• Free-swimming larva that emerges from egg in primitive

molluscs

prototroch

mouth

anus

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

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

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

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

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)

http://academics.hamilton.edu/biology/preynold/Scaphopoda/photo.html

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

Class: Gastropoda

• Both trochophore and veliger larval stage

• Bilaterally symmetrical, but because of torsion, visceral mass has become asymmetrical

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

Practical

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

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

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)

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

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

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

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

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

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

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

Class: Cephalopoda

• Foot modified into funnel for expelling water from mantle cavity (jet propulsion)

• Shell often reduced or absent

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

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

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

Class: Cephalopoda

• Subclass: Ammonoidea– Totally extinct – End Cretaceous (65

MYA)

Class: Cephalopoda

• Subclass: Coleoidea (most have 8 arms and 2 tentacles)

• Cuttlefish, squid and octopus

cuttlefish

squidoctopus

cuttlefish

Class: Cephalopoda

Order: Sepioidea• Cuttlefish• Rounded or compressed

bulky body bearing fins• Arms and tentacles bear

suckers but tentacles only at their ends

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

Class: Cephalopoda

Order: Octopoda• No shell; 8 Arms; no tentacles; short compact

body; suckers not stalked and do not bear horny rims

Evolutionary Relationships

• Molluscs are allied with both annelids and arthropods

• Molecular evidence suggests they are more closely related to the annelids