The document provides information about the phylum Mollusca. It discusses that Mollusca is the second largest phylum and includes animals such as snails, squids, clams, and octopuses. It then summarizes the key characteristics of several mollusc classes, including Bivalvia (clams and mussels), Gastropoda (snails), and Cephalopoda (octopuses and squids). The document also describes some unique adaptations of molluscs and their life cycles.

2. Introduction
•Mollusca is the Latin
for “soft”
•Includes snails, squids,
nautilus, octopus,
clams, etc.
•2nd largest phylum
after Phylum
Arthropods (Animal
Kingdom)

3. Phylum Mollusca; the most malleable body plan in the animal kingdom

6. •Have adapted to a wide variety of habitats
•Terrestrial, marine, benthic, and
accomplished swimmers.
•Molluscs evolved in the sea and most molluscs
are still marine.
•Some gastropods and bivalves inhabit
freshwater.
•A few gastropods (slugs & snails) are
terrestrial

7. Edible mussels
(Mytilus edulis)
•Humans use molluscs in a variety
of ways:
•As food –
mussels, clams, oysters, abalon
e, calamari
(squid), octopus, escargot
(snails), etc.
•Pearls – formed in oysters and
clams.
•Shiny inner layer of some shells
used to make buttons.

8. “If a grain of sand, parasite or other foreign particle
becomes trapped between the mantle and the shells’ inner
surface, a pearl may form over a period of years. Natural pearl
formation is a fairly rare event; perhaps only one oyster in 1,000 is
likely to harbor a valuable pearl naturally”

11. Mollusc Phylogeny
• ~50-80K extant
species
• ~40K extinct
species
• Fossil records
from precambrian
period of
proterozoic eon
(>570my BP)

12. The first molluscs probably arose
during Precambrian times.
Diverse molluscs found in the early
Cambrian.
It is likely that molluscs split off from
the line that led to annelids after coelom
formation, but before segmentation
appeared.

13. o“Hypothetical Ancestral Mollusc”
oProbably lacked a shell or crawling
foot.
oProbably small (about 1 mm).
oLikely was a worm-like organism with a
ventral gliding surface.
oProbably possessed a dorsal mantle, a
chitinous cuticle and calcareous scales.

15. 1. Dorsal Epithelium forming a mantle which
secretes calcareous spicules or one or more
shells
2. Cuticular band of teeth(radula) in the
esophagus used for feeding (not present-
lost? in bivalves)
3. Ventral body wall muscles develop into a
locomotory or clinging foot

16. • Most but not all mollusc have shells consisting
primarily of calcium carbonate set in a protein
matrix.
• The Molluscan body plan includes
– Head-foot
– Visceral Mass -containing digestive, circulatory,
respiratory and reproductive organs.

17. An example of mollusc illustrating their common body plan

18. The Head-Foot Region
• Most molluscs have well developed head ends
with sensory structures including
photosensory receptors that may be simple
light detectors or complex eyes (cephalopods)

19. The Head-Foot Region
• The radula is a
rasping, protrusible
feeding structure
found in most
molluscs (not
bivalves).
– Ribbon-like
membrane with
rows of tiny teeth.

20. The Head-Foot Region
• The foot of a mollusc
may be adapted for
locomotion, attachme
nt, or both.
• Pelagic forms may
have a foot modified
into wing-like
parapodia.

21. Muscular foot

22. Mantle and Mantle Cavity

23. When present, the calcareous shell is secreted by the mantle
and is lined by it. It has 3 layers:
•Periostracum – outer organic layer helps to protect inner
layers from boring organisms.
•Prismatic layer – densely packed prisms of calcium
carbonate.
•Nacreous layer – iridescent lining secreted continuously
by the mantle – surrounds foreign objects to form pearls
in some.

24. Pearl is formed between the mantle and the shell.

25. • Many molluscs have an open circulatory
system with a pumping heart, blood vessels
and blood sinuses.
• Most cephalopods have a closed circulatory
system with a heart, blood vessels and
capillaries.

26. • Most molluscs are
dioecious, some are
hermaphroditic.
• The life cycle of many
molluscs includes a free
swimming, ciliated
larval stage called a
trochophore.
– Similar to annelid
larvae.

27. The trochophore larval stage is followed by a free-swimming veliger larva in most species

29. Mollusc Taxonomy
Major classes
Polyplacophora – the chitons
Aplacophora-
Monoplacophora
Gastropoda- snails and slugs
Bivalvia- clams, mussel and oysters
Scaphopoda- tusk shells
Cephalopoda- octopus and squids

30. Class Polyplacophora
Greek: many plate bearing
Defining characteristics: Shell forms as a series of 7
to 8 separate plates
•Use radula to scrape algae off
rocks
•Live on rocky intertidal zones
•Gills are suspended from roof of
mantle cavity.
•Water flows from anterior to
posterior ; linear digestive tract
Tonicella lineata

31. • Pair of osphradia serves as sense organ.
• Light sensitive esthetes form eyes in some species – pierce plates.
• Blood pumped by a three-chambered heart.
– Travels through aorta and sinuses to gills.
• Pair of metanephridia carries wastes from pericardial cavity to exterior.
• Sexes are separate.
• Trochophore larvae metamorphose into juveniles without a veliger stage.

33. Class Aplacophora
•Quite small– usually only a few millimeters
•Entirely marine
•No true shell
•The body is unsegmented and bears
numerous calcareous spines or scales
embedded in an outer cuticle
Greek: no shell bearing
Defining Characteristic:
Cylindrical, vermiform body
with the foot forming a
narrow keel

34. Two subclasses:
Neomeniomorpha or Solenogastres
Chaetodermomorpha or Caudofoveata
Solenogastres Caudofoveata

35. •Caudofoveata are small (1-30
mm), mainly deep sea molluscs.
They are worm-like, lacking
shells or distinct muscular feet;
they instead have scales and
calcareous spines
called sclerites, for movement.
•live by burrowing through soft
sediment and feed by lying
vertically in the sediment with
just the mouthparts exposed
and taking in passing organic
detritus.

36. Subclass Solenogaster
•do not have true ctenidia, although their gill-like structures resemble them
•during development many solenogastres are covered by a spiny scleritome
comprising spines or scale-like plates; this has been likened to the halwaxiid
scleritome

37. Solenogasters feed on
cnidaria and
ctenophores, either sucking
their bodily fluids or eating
their tissue.

38. Class Monoplacophora
Greek : one shell bearing
Defining characteristics:
1. 3 to 6 pairs of ctenidia, 6 to 7 pairs of nephridia
2. Multiple (usually 8) pairs of foot (pedal) retractor
muscles
-Was known only from fossils until 1952
-Entirely marine and all collected from depth at least
2000 m
-A single unhinged cap shaped shell is present
-The largest species is about 37 mm

39. Class Gastropoda
Greek: stomach foot
Defining Characteristics:
1. Visceral mass and nervous system become
twisted 90-180 ⁰ (exhibiting torsion) during
embryonic development
2. Proteinaceous shield on the foot
(operculum) to which columellar muscles
attaches
______________________________________
Gastropoda is the largest of the molluscan
classes.
– 40,000 to 75,000 living species.
– Include snails, slugs, sea hares, sea
slugs, sea butterflies.
– Marine, freshwater, terrestrial.
• Benthic or pelagic

40. • Gastropods show bilateral symmetry, but due to a twisting
process called torsion that occurs during the veliger larval
stage, the visceral mass is asymmetrical.

41. Torsion is an anticlockwise twisting of most of the body (the visceral mass) through 180⁰ during
early development

42. • The shell of a gastropod is
always one piece –
univalve – and may be
coiled or uncoiled.
– The apex contains the
oldest and smallest
whorl.
– Shells may coil to the
right or left – this is
genetically controlled.

43. • Coiling is not the same as
torsion.
• Early gastropods had a
planospiral shell where
each whorl lies outside the
others.
– Bulky
• Conispiral shells have each
whorl to the side of the
preceding one.
– Unbalanced
• Shell shifts over for better
weight distribution.

44. • herbivores and feed by
scraping algae off hard
surfaces using the radula.
• scavengers of dead
organisms, again tearing off
pieces with radular teeth.
• carnivores and have a
radula modified into a drill
to bore through the shells of
other molluscs. They use
chemicals to soften the
shell.

45. • Snails in the genus Conus feed on fish, worms, and
molluscs.
– Highly modified radula used for prey capture.
– They secrete a toxin that paralyzes their prey.
• Some are painful, even lethal, to humans.

46. • Flamingo tongue snails
feed on gorgonians.
• Mantle is brightly
colored and envelops
the shell.

48. • Pulmonates lack gills.
– Have a highly vascular area in
mantle that serves as lung.
– Lung opens to outside by
small opening, the
pneumostome.
– Aquatic pulmonates surface
to expel a gas bubble and
inhale by curling, thus
forming a siphon.
In Terrestrial
gastropods: The
mantle cavity serves as
a “lung”

49. • Most have a single nephridium and well-
developed circulatory and nervous systems.
• Sense organs include eyes, statocysts, tactile
organs, and chemoreceptors.
• Eyes vary from simple cups holding photoreceptors
to a complex eye with a lens and cornea.
• Sensory osphradium at base of the incurrent
siphon may be chemosensory or
mechanoreceptive.

50. • Respiration in many
performed by ctenidia in
mantle cavity.
• Derived prosobranchs lost
one gill and half of
remaining gill.
– Resulting attachment to
wall of mantle cavity
provided respiratory
efficiency.

51. • Slugs lack
Pneumostome shells
• Mantle
thickened
• Pneumostome
– Air intake
into mantle
cavity

52. • Monoecious and dioecious species.
• Young may emerge as veliger larvae or pass
this stage inside the egg.
• Some species, including most freshwater
snails, are ovoviviparous.

53. Greek: Anterior Gill
Defining Characteristic:
1. Mantle Cavity generally anterior
due to torsion
•Prosobranchia includes most marine
snails and some freshwater and
terrestrial gastropods.
•Largest of three subclasses; mostly
marine
•Generally free-living and mobile; some
have evolved sessile or even parasitic
lifestyles
•Warm water cone snails (Conus sp.) are
carnivorous produce potent venoms
•Most primitive of gastropods; two other
subclasses evolved from prosobranch
like ancestors

54. Greek: Posterior Gill
Defining Characteristic:
1. Mantle cavity lateral or posterior due to detorsion,
or lost
Opisthobranchia includes
sea slugs, sea hares, sea
butterflies, and canoe shells
Sea hare Aplysia has large anterior
tentacles and a vestigial shell.

55. – Most are marine, shallow-water.
– Partial to complete detorsion -
anus and gill(s) are displaced to
right side.
– Two pairs of tentacles, one pair
modified to increase chemo-
absorption.
– Shell is reduced or absent.
– Monoecious
Compared to Prosobranchia:
1. A trend toward reduction or loss of the
shell
2. Reduction or loss of the operculum
3. Limited torsion during embryogenesis
4. Reduction or loss of the mantle cavity
5. Reduction or loss of the ctenidia

56. Locomotion is generally made by cilia
and pedal waves along the ventral
surface of the foot
Some opisthobranchs, ex. sea hares,
can swim in short spurts by flapping
lateral folds called parapodia

57. Latin : Lung
Defining Characteristic:
1. Mantle cavity highly vascularized and other
modified to form a lung
Compare to other subclasses, only few of
these species are marine and those few
species occur only in intertidal and in
estuaries

59. Class Bivalvia
Latin: Two valved [Greek: hatched foot]
Pelecypoda
Defining Characteristics:
1. Two valved shell
2. Body flattened laterally
-Includes clams, scallops,
mussels , shipworms and oyster
Mostly sessile filter feeders.
No head or radula.
Two major subclasses:
•Protobranchia
•Lamellibranchia
-and one very small subclass;
The Septibranchia

60. • Bivalves are laterally
(right-left) compressed
and their two shells are
held together by a
hinge ligament on the
dorsal surface.
• The Umbo is the oldest
part of the shell,
growth occurs in
concentric rings
around it.

61. • Part of the mantle is
modified to form
incurrent and excurrent
siphons.
– Used to pump water
through the organism
for gas exchange and
filter feeding.
– Sometimes used for jet
propulsion.

63. • Shipworms can be destructive to wharves & ships.
• The valves have tiny teeth that act as wood rasps and
allow these bivalves to burrow through wood.
• They feed on wood particles with the help of symbiotic
bacteria that produce cellulase and fix nitrogen.

64. • Bivalves move around by extending the muscular foot
between the shells.
• Scallops and file shells swim by clapping their shells
together to create jet propulsion.

65. • Like other
molluscs, bivalves
have a coelom
and an open
circulatory
system.
• The mantle cavity
of a bivalve
contains gills that
are used for
feeding as well as
gas exchange.

66. • Scallops have a row of small blue eyes along the mantle
edge. Each eye has a cornea, lens, retina, and pigmented
layer.

67. Suspended organic matter enters incurrent siphon.
Gland cells on gills and labial palps secrete mucus to entangle particles.
Food in mucous masses slides to food grooves at lower edge of gills.
Cilia and grooves on the labial palps direct the mucous mass into mouth.
Some bivalves feed on deposits in sand.

68. • Bivalves usually
have separate
sexes.
• Zygotes develop
into
trochophore, vel
iger, and spat
(tiny bivalve)
stages.

69. In freshwater clams,
fertilized eggs develop
into glochidium larvae
which is a specialized
veliger.
Glochidia live as
parasites on fish and
then drop off to
complete their
development.
Glochidia – Unique Larval Stage of
Freshwater Bivalves

70. Greek: first gill
Defining Characteristics:
1. Gills small, functioning primarily as gas
exchange surfaces
2. Food collecting by long , thin, muscular
extensions of tissue surrounding the mouth
(palp proboscides)

71. -Entirely marine, and all species live in soft substrate
- feed on sediments taken in and the organic fraction is digested
(deposit feeding)
-Palp boscides, long, thin muscular extensions of the tissue
surrounding the mouth is responsible for food collection , not in
the gills
-Much more common in deep water

72. Greek: plate gill
Defining Characteristics:
1. Gills modified to collect suspended food particles, in
addition to serving as gas exchange surfaces
2. Secretion of proteinaceous attachment material
(usually in the form of threads) by a specialized
gland (the byssus gland) in the foot
Most bivalves are lamellibranchs. Majority are marine, and
some belong to Unionidae, freshwater bivalve species.
Commercially important for many years as food (e.g.,
Oysters and scallops)
Widely used to assess environmental pollution

73. Greek: Fence gill
Defining Characteristic:
Gills highly modified to form a muscular
septum, which pumps water through the
mantle cavity for respiration and feeding
Small groups of carnivorous bivalves that feed on
zooplanktons and on pieces of decomposing animal
tissue
All species are marine and found in very deep water
The septibranch ctenidium is highly modified, lacking
filaments and forming a muscular septum

74. Septibranchs feed as organic vacuum (suck small crustaceans
and annelids).
The stomach is lined with hardened chitin, to grind up
ingested food.

75. Class Scaphopoda
Greek: Spade foot
Defining Characteristics:
1. Tusk-shaped, conical shell, open at both
ends
2. Development of anterior, threadlike,
adhesive feeding tentales
Young group, first appearing in the fossil records in the middle
Ordovician (450 years ago)
300-400 species,
Entirely marine, lives sedentary lives in sand or mud substrate
Possess no ctenidia

76. The scaphopod shell is
never spirally wound, but rather
grows linearly as hollow, curved
table; hence known as “tooth
shell” and “tusk shell”.

77. -capture small food particles, including Foraminiferans ,from the
surrounding sediment and water using specialized, thin tentacles known
as captacula.

78. Rear end, through which water is
driven
into the pallial cavity by ciliary action
Burrowing foot pulling the
animal into the ground

79. Scaphopoda shells (many of them from
Vancouver Is, British Columbia) were the
shells used to make the North American
Indian trade money "Wampum“. Scaphopod
shells have been used as decoration, jewellery
and money by many peoples.

80. Scaphopods have separate sexes, and external
fertilisation. They have a single gonad occupying
much of the posterior part of the body, and shed
their gametes into the water through the
nephridium.
Once fertilised, the eggs hatch into a free-
living trochophore larva, which develops into
a veliger larva that more closely resembles the
adult, but lacks the extreme elongation of the
adult body.

81. Class Cephalopoda
Greek: Head foot
Defining Characteristic:
1. Shell divided by septa, with
chambers connected by the
siphuncle: a vascularized strand of
tissue contained within a tube of
calcium carbonate (shell reduced or
lost in many species)
2. Closed circulatory system
3. Foot modified to form flexible arms
and siphon
4. Ganglia fused to form a large brain
encased in a cartilaginous cranium

82. • Cephalopods include
octopuses, squid, nautiluses
and cuttlefish.
• Marine carnivores with beak-
like jaws surrounded by
tentacles of their modified foot.
– Modified foot is a funnel for
expelling water from the
mantle cavity.

83. Cephalopods are the supreme testament to the
impressive plasticity of the basic molluscan body plan.
Ctenidia and a radula are present in all cephalopod
species
The head and associated sensory organs are extremely
well-developed.
Nautilus locomotes by jet propulsion, expelling water
from the mantle cavity through flexible, hollow tube
called the siphon or funnel
Cephalopods swim by expelling water from the mantle
cavity through a ventral funnel.
They can aim the funnel to control the direction they
are swimming.

84. • Cephalopod fossils go back to Cambrian (570 mya) times.
• The earliest had straight cone-shaped shells.
• Later examples had coiled shells similar to Nautilus.
• Ammonoids were a very successful group, some had quite
elaborate shells.

85. Anatomy of Nautilus

86. • Shells of Nautilus and early
nautiloid and ammonoid
cephalopods were made
buoyant by a series of gas • Nautilus shells differ
chambers. from gastropod
because they are
divided into
chambers. The
animal lives in the last
chamber. A cord of
living tissue extends
through each
chamber.

87. Cuttlefishes have a small curved shell, completely enclosed by the mantle.

88. The squid shell is also internal, but it is little more than a thin, stiff, proteinaceous
sheet, called the pen

89. FUN FACT: The largest
cephalopod Mesonychoteuthis
hamiltoni, (Fig. 17) called the
colossal squid, is longer than a city
bus, while the smallest
cephalopod, Idiosepius notoides, the
pygmy squid, could fit on your
fingernail.

90. General Anatomy Cephalopod
Anatomy

91. All cephalopods have arms, but not all
cephalopods have tentacles .
Octopuses, cuttlefish, and squid have eight
non-retractable arms, but only cuttlefish and
squid (Sepioidea and Teuthoidea) have
tentacles (two each).
Arms usually have cirri (fleshy
papillae/palps), often suckers, and sometimes
hooks (modified suckers) along their
undersides.
 Tentacles are longer than arms, are
retractable, and usually have a blade-shaped
or flattened tip, called a club, which is covered
in suckers

92. Anatomy of an Octopus

93. •Cephalopods have a closed
circulatory system.
•Nervous and sensory systems are
more elaborate in cephalopods
than in other molluscs.
•The brain is the largest of any
invertebrate.
•Most cephalopods’ skin contains
several layers of tiny colored cells
called chromatophores, which
overlay reflective cells called
iridocytes- responsible for
coloration of skin

94. Communication
Visual signals allow
cephalopods to communicate.
Movement of body and
arms
Color changes effected by
chromatophores
Most cephalopods have an ink sac
that secretes sepia, a dark fluid
containing the pigment melanin.
When a predator tries to
attack, the animal ejects the ink
into the water where it hangs
between the animal and the
predator screening a quick
escape

95. Cephalopods have an amazing ability to change
color very rapidly. They accomplish this feat
using numerous pigment-filled bags, called
chromatophores.
Chromatophores
are found in the skin, and
expand and contract to
reveal or conceal small dots
of color (left). Additionally,
an iridescent dermal tissue
can also be manipulated by
some cephalopods to aid in
camouflage, courtship
rituals, or accompany color
changes.

96. Cephalopods
have two eyes
Nautilus spp. Eyes
are simple and
function as on the
pinhole camera
principle; no lens
While other
cephalopods eyes are
incredibly similar to
mammals

97. Octopuses are the only cephalopods with a completely protected
"closed" cornea. This structure is probably the most sophisticated eye of all
invertebrates and is as complex as the vertebrate eye.
Differing eye morphologies in cephalopods. From left, a squid
(Loligo), octopus, cuttlefish, and Nautilus. Note the hyponome below the
octopus eye — this is a muscular tube, that when contracted, expels water in a
jet, propelling the octopus backwards. The hyponome can be aimed in various
directions, giving the octopus finer control over its escape route.

98. Reproduction
• Sexes are separate in cephalopods.
• Juveniles hatch directly from eggs – no free-
swimming larvae.
• One arm of male is modified as an intromittent
organ, the hectocotylus.
– Removes a spermatophore from mantle cavity
and inserts it into female.

99. Reproduction

100. •Most females then lay large yolky eggs in clusters on the
ocean floor or on any other hard substrate.
•Eggs develop by dividing unequally instead of in the spiral
pattern of other molluscs. It is thought this is a derived mode
of development.
•After a period of development within the egg, juveniles
hatch out directly without the swimming larval stage common
to many other molluscs. Most males and females die shortly
after spawning.

102. The brain. Finally, one of the most intriguing aspects
of cephalopods is their intelligence. With a
centralized brain, the largest of all
invertebrates, and highly developed eyes and other
sense organs, they are able to remember and learn
by example or through trial and error.