LECTURE 6:

PHYLUM-Mollusca

PHYLUM: MOLLUSCA (CHARACTERISTIC FEATURES)

CLASS: POLYPLACOPHORA (CHITONS)

CLASS: BIVALVIA

CLASS: GASTROPODA

CLASS: CEPHALOPODA
 PLAN
General characteristics: Archetype features
1. External features (shell, foot mantle cavity)
2. Body wall (mantle); coelom reduced
3. Digestive system & the Radula
4. Respiratory system: Ctenidia
5. Circulatory system
6. Excretory system
7. Nervous system
8. Reproductive system

Specialised features of the four major classes

INTRODUCTION
Mollusca: Latin meaning “soft”- soft-bodied animals
2nd largest invertebrate phylum: 100,000 living
species; 45,000 fossils
Mostly marine.
Includes the “smartest invertebrate” the octopus
Largest invertebrate the giant squid Architeuthis sp.
(18 m long)
Includes: oysters; clams; squids; octopus; snails; slugs
Inhabit a variety of habitats: most live in the sea
INTRODUCTION
Mollusca: Latin meaning “soft”- soft-bodied animals
2nd largest invertebrate phylum: 100,000 living
species; 45,000 fossils
Mostly marine.
Includes the “smartest invertebrate” the octopus
Largest invertebrate the giant squid Architeuthis sp.
(18 m long)
Includes: oysters; clams; squids; octopus; snails; slugs
Inhabit a variety of habitats: most live in the sea
Architheutis
INTRODUCTION TO THE MOLLUSCS.

Evolutionary position unclear.

Protostome, bilateral phylum with similar larval stage to annelids but not
segmented
Before annelids or after arthropods??- Text.

Varied size - giant squid (Architeuthis sp.) 18 m long – small bivalves

<1 cm.

Most (80%) have shell- diameter < 5/10 cm.

Tremendous diversity.
Mollusks

Mollusks
Annelids

Complete
Gut

2 layers,
acoelomate

2 layers

Phylogenetic tree
Bewildering diversity of form-

Plate 1. Molluscan diversity: (from top left to right) chiton- Tonicella lineata;
marine snail- Calliostoma annulata; nudibranch- Chromodoris kuniei;
Pacific giant clam- Panope abrupta; octopus- Octopus briareus.
Though not superficially apparent all molluscs are built on the same
body plan.
Same structural components: Text- LIFE: (pg. 636 or 700)
Archetype mollusc (generalised mollusc)

•Not a fossil or primitive member.

•Means the original model or prototype.

•Generalized plan which allows description of the phylum.

Combined several traits of the different members of the phylum.

The Archetype or hypothetical mollusc.
GENERAL CHARACTERISTICS USING THE ARCHETYPE.
1. EXTERNAL FEATURES
Ovoid shape.
Bilateral symmetry.
Flat ventral surface- foot
Dorsal surface- covered by a shell
Not highly cephalized.

Epidermis (mantle) is folded & underlies shell.

Mantle overhangs foot- posterior mantle cavity
Mantle cavity has 1 or more pairs of gills.
2. BODY WALL & SHELL
Ciliated epidermis- body wall or mantle.

Mantle edge has 3 folds.

Shell secreted by outermost fold of mantle.

Shell has layers:
•Periostracum (the outermost)
- proteinaceous & tanned.
•Inner layers vary from 2 - 4 & are
comprised of CaCO3 crystals
embedded in a protein matrix.
•Prismatic layer
- CaCO3 crystals deposited vertically.
•Nacreous layer
- CaCO3 crystals deposited
horizontally.
•Ratio of organic (protein) material :
inorganic (CaCO3) may be 12: 88 or
72:28.
EUCOELOMATES-
Coelom is reduced surrounds the heart & part of the
intestine.
Large body spaces are actually haemocoel (pseudocoel).
 3. NUTRITION & DIGESTION.
Feed by scraping food particles off a hard substrate using a Radula.
•Rasping tongue-like organ.
•Occurs in all except the bivalves.
•No other phylum has a radula.
•Many re-curved chitinous teeth in rows along a membraneous belt in a
radula sac.
•This belt or ribbon is stretched
along a cartilaginous rod-
(odontophore).

•Up to 250,000 teeth.

•Function: scrape substrate &

convey food to mouth.
•Stomach

•Cone-shaped.
•Food particles bound onto a mucous
string.
•Beating of style sac cilia move string
into stomach.
•Crystalline style rich in the enzyme
amylase (only in some molluscs).
Archetype Gut
Crystalline
style
Digestion continued:
•Food sorted & smaller particles pass to 2 digestive organs
(diverticula).
•Hindgut
- important for faeces formation
- need to excrete compact faecal pellets.
 4. RESPIRATION- Bipectinate gills
Ctenidia(um)

Beating of lateral cilia draws

respiratory current through ctenidium.

 Frontal & abfrontal cilia remove

excess particulate material from gills
i.e. prevent fouling.

Diagram of a molluscan ctenidium.

Large black arrows indicate
direction of ventilating current
produced by lateral gill cilia. Small
red & blue arrows show blood flow
within gill.
 Respiration- Counter current system

Blood enters via the afferent vessel &

slowly percolates across gill filament.

Oxygenated blood taken up by an

efferent vessel.

Counter- current system.

5. CIRCULATION

Open blood vascular system.

1 pair of auricles & a ventricle (muscularised chambers).
Respiratory pigments present:
•Myoglobin
•Haemoglobin
•Haemocyanin.
Heart- 3 chambered
 6. EXCRETION.
 One pair of metanephridia.
 Nephridiopore located in mantle cavity, where
gills are.
7. NERVOUS SYSTEM.
 Simple: nerve ring, pedal cords & visceral cords.
 Sense organs- Archetypal

1) Tentacles - tactile
2) Statocysts - orientation.
3) Osphradia- New.
- areas of sensory epithelium found on mesentery supporting
each gill.
- detect amount of sediment in the inhalant current.
Pecten sp. (Bivalve- scallop) – blue eyes
8. REPRODUCTION
Dioecious.

Metanephridia transfer sperm & eggs to the outside.

Some with gonoduct or gonopore.

Fertilization is external (in the sea).

Two distinctive larval stages.

 Reproduction Contd.
Trochophore

•1st larval stage

Identical to annelid trocophore.

2nd stage- Veliger- with paired velum-

wings.

Diagram of a generalized
trocophore larva.
2nd larval stage-
Veliger larva
 Molluscan classes
 8 classes (only 4 in detail):
 CLASS:
1. Polyplacophora - most primitive.

2. Bivalvia – 2nd largest class.

3. Gastropoda – largest class.

4. Cephalopoda - most advanced.

CLASS POLYPLACOPHORA (Chitons)
Similar to Archetype
Exclusively marine- found on rocky
shores around Jamaica.
Dorsoventrally flattened & survive stuck
onto the rock surface.
Bilaterally symmetrical.
Oval shape.

The fuzzy chiton.

Shell consists of 8 articulating plates-
overlap posteriorly.

Thick mantle (girdle) extends over the

dorsal surface.

Mantle cavity (pallial groove)

surrounds the foot

Numerous gills in the mantle cavity.

Sense Organs

•Subradula organ- Chemosensory; protruded to “taste” substrate.

•Esthetes
- sensory cell clusters occupying pits in the shell plates.
- concentrated on the anterior shell plates.
CLASS: BIVALVIA
Clams, oysters- 35,000 spp.

Seems very different from archetype

structurally.

Laterally compressed bivalve shells.

Radula lost.

Plate 1.The flat tree oyster

Isognomon alatus.
BIVALVIA
2 shells halves (called valves) hinged dorsally & gape
anteriorly.

Body completely enclosed in shell & mantle.

Hinge made up of tanned protein.

Adductor muscle (catch)

•Keeps valves closed.
•Smooth muscles constantly under tension.
Striated muscles (for quick closure).
 BIVALVIA

Pearl production

•Foreign object (sand particle or

parasite) gets in space between shell
and mantle.
•coated with nacre (iridescent inner
layer of shell) becomes a pearl.
NUTRITION – No radula, filter feeders
1 pair of highly folded gills.
Enlarged for filer feeding- Hypertrophy
Cilia collect particles.
Bound in mucous and channeled to the mouth.
Palps or paired lips flanking the mouth.
Internal systems: respiration, circulation,
excretion, reproduction
Tutorial Question
Q # 6. Use the archetype plan to describe the general
features of the phylum Mollusca.
Class that shows the greatest deviation
from the Archetype
Gastropoda Cephalopoda Bivalvia
- Cephalizaton - External
- External/Shell features
- Dorsal elongation & - Movement - Sessile
- Shell coiling - Feeding - Feeding
- Torsion - Respiration/
- Asymmetry Circulation
- N. system
- Reproduction
CLASS: GASTROPODA
Largest class: > 55 000 extant species (yr- 2000)
15 000 fossil species (extinct species)

Conch; snails; slugs: wide geographical distribution- sea; freshwater; land

2nd largest class in the animal kingdom: surpassed only by the insects -
 CLASS- GASTROPODA

TREMENDOUS VARIETY OF
GASTROPODS
 Gastropoda- Why so successful?

Three major changes from the

Hypothetical/Archetype mollusc
1. Cephalization: development of a prominent head
2. Dorsal elongation of shell and subsequent coiling
3. Torsion
1. Cephalization

Head with sensory

structures:
•Tentacles
•Eyes
•Statocysts
Brain- concentration
of nervous tissue
(cerebral ganglion)
2. Dorsal elongation & Coiling of the gastropod shell
Planospiral

Helicospiral
 COILING OF THE GASTROPOD SHELL
•Planospiral- symmetrical
•Helicospiral- asymmetrical spirals- shell more compact (most modern
gastropods)
- oblique portage of shell caused loss of the right gill, nephridium, auricle
etc.
(Helicospiral or conispiral coiling- see shells in the demonstration)
Oblique portage of the shell
•Constricts the mantle cavity
on the right side
•Associated structures
present on the left side
only
•Right side obliterated by
the weight of the shell.
•1 gill, 1 auricle, 1
metanephridium etc.
 TYPICAL GASTROPOD SHELL

Conical shape – helicospiral coils.

Bilateral phylum- Asymmetrical

(adult)

Operculum- horny disc bourn on the

foot- closes shell- (seen in
demonstration)
Operculum- lid/door
 3. TORSION
Rotation of main body mass
through 1800 anticlockwise
in relation to head & foot.

Occurs in veliger larva

Unequal growth of two
adductor muscles.
Dorsal views of the archetype
M.C. with Gills, anus & mollusc: (a) pretorsion & (c)
nephridiopores now found post-torsion.
just behind the head.
TORSION Contd.
Effects of Torsion on body structures.
•Gut u-shaped.
•Nervous system
twisted- figure of 8
•Sensory structures
associated with the
mantle cavity
associated with the
head.
Osphradium
 Why do gastropods undergo torsion?
How does it contribute to their success?

1. Allows anterior placement of all sensory structures.

2. Protection of important structures.

3. Better ventilation for gills.

4. Adaptive radiation.
Main problem of torsion: anus now anterior

Fouling: of body, gills & cephalic sense organs.

Solution: varied, but related to changes in water currents,

shell structure and position of the anus.

- holes, notch, changes in the pattern of inhalant &

exhalant currents.

This has led to a range of gastropod groups- diversity and

adaptive radiation.
 SOLVING THE PROBLEM OF FOULING

Oblique
water
current
 Terrestrial Gastropod systems

Blood plexus (lung)

Sealed- small hole
(pneumostome)
 Terrestrial gastropod systems contd.

Respiratory system modified- no gills use capillary network (blood plexus / lung).

Reproductive systems modified- hermaphrodites. Mechanism somewhat like the earthworm.

Terrestrial gastropod systems

Spermatheca
CLASS: CEPHALOPODA
Entirely marine and mostly pelagic
Class- Cephalopoda - 750 spp. includes the octopus and squids.
Most advanced mollusc / invertebrate
Pinnacle of invertebrate evolution.
Example of the most intelligent invertebrate.

Range in size – Usually few cm to 30 cm

One giant squid Architeuthis is 18 m long- largest known invertebrate.

“Beast” by Peter Benchley
“It hovered in the ink dark water, waiting.

It was not a fish.

It was not a mammal, did not breathe air, so it felt no impulse to move to
the surface.

It hovered, nourishing itself with oxygen absorbed from the water pumped
through the caverns of its bullet-shaped body.

Its eight sinuous arms floated on the current; its two long tentacles were
coiled tight against its body.
When it was threatened or in the frenzy of a kill, the tentacles would spring
forward, like tooth-studded whips...”
Cephalopoda
Special features
External features
Movement
Feeding
Respiration & Circulation
Nervous system and sense organs
Reproduction
External features:
Body elongated dorsally (functional orientation)
Head and foot are indistinguishable - Cephalopod means "Head-foot".
Edges of the foot are drawn into arms and tentacles that bear suckers
for holding onto prey.
 External features

-Part of the foot

modified as a siphon for
expelling water from the
mantle cavity.

-Tentacles - usually
longer & usually only 1
pair.

-Tentacles have suckers

at the ends only.
 External features contd.
Arms are shorter and more numerous (usually 8)
suckers or sucking disks along the length of the arms.

No shell or reduced shells- (only most primitive may have a shell).

Movement.
Move by jet propulsion
at speeds of up to 40 -50 km/hr.
Done by rapid expulsion of water from the mantle cavity (circular muscles)
via siphon.
Funnel or siphon is mobile and can change direction.
Squids use fin undulations to assist with slow swimming.
NUTRITION AND DIGESTION
All raptorial carnivores
Suckers on arms and tentacles hold prey
Tentacles are long and can be shot out with great speed.
Jaws cut prey tissue and pieces transferred to buccal cavity via
radula.
- Secretions in buccal cavity include saliva which contains
powerful proteases which begin breakdown of prey tissue
prior to ingestion.
- Saliva can also secrete poisons to subdue struggling prey.
Include fish, shrimp and other cephalopods.
Digestive system (green)
Respiratory system

System - changed in the cephalopod.

No cilia on gill, gill filaments highly folded.
Ventilation by ciliary beating is inadequate.
Mantle- pumping unit, muscular contractions- ventilation + locomotion.
Water for swimming- ventilation
Exhalent is ventral via siphon.
Intake is dorsolateral.
Loss of counter current system.
Cephalopod gill & water currents
CIRCULATION:
Closed Blood Vascular system
- Central heart well developed major and minor blood vessel, capillary
beds and a venous system.
Ancillary hearts - Branchial hearts associated with each gill.
- Help to create and maintain high blood pressure within gills.
Central heart in a percardium (coelom) - cushions and protects the heart
and other internal organs from movements in body wall.
Circulatory system

- Route of blood: Gills - central heart - rest of body

(anterior and posterior) collected by veins - gills via
branchial hearts.
EXCRETION
Nephridia - two paired sacs along venous route.
As veins pass nephridia branches enter called renal appendages.
Nephridiopore near funnel of siphon.
All live in sea- excrete ammonia.
NERVOUS SYSTEM AND SENSE ORGANS
Very highly organised and centralised nervous system.
All ganglia fused - large brain in cartilaginous cranium.
Large number of nerve cells- 150,000,000
( octopus). Crustaceans only have 30,000.
Great deal of coordination.
Squids have giant nerve fibres which make them capable of rapid movement.
Sense organs
Statocysts for orientation are located near the brain.
Tentacles and arms - tactile sites and chemoreceptors.
Cephalic eyes – well developed image forming structures.
-Most sophisticated eyes of any invertebrate.
-Cornea, lens, retina, but controlled by statocysts and so held in constant position in relation to
gravity.
Cephalopod eye.
Eyes
Image forming, able to discern colour, and can judge distances
(accommodate).
Actually communicate by colour changes in the skin
Chromatophores in the skin controlled by muscles so with
muscular contractions chromatophores expand and colour
disperses or intensifies.
Actually tell others when angry or aroused etc.
Octopus can be taught to distinguish colour and shape.
Some spp. possess a large ink gland
When animal is alarmed it releases this ink as a brown or
black cloud which startles predator and shields escaping
cephalopod.
Chromatophores
REPRODUCTION AND DEVELOPMENT
Dioecious, males tend to be smaller.
Single large posterior gonad.
Sperms encased in a chitinous capsule or spermatophore -stored in a sac
called Needham's sac.
Eggs provided by ovary and encased in albumen by oviducal gland.
Animals usually copulate with arms intertwined.
Spernatophore is a complex structure placed on the mantle wall of the
female near the opening of the oviduct.
Spermatophore
(Needham's sac).
Reproduction contd.
One of the arms of the male modified as an intromittent organ.
Yolky egg capsules released in gelatinous masses.
Fertilised as released from the oviduct.
Usually attached within crevices of the rock.
One brood eggs in a specially protected shell.
Trochophore and veliger stages – not recognisable
Development tends to be direct.
Tiny cephalopod emerges from the egg.
Copulating cephalopods
Tutorial Question
Q# 7. Which molluscan class do you think shows the greatest deviation
from the archetype plan? Describe the peculiar (unique) features shown
by your class of choice.