This document provides an overview of the phylum Mollusca. It discusses that Mollusca is divided into 8 classes, with the most diversity found in Gastropoda and Bivalvia. The key characteristics of molluscs are described, including their versatile body plan, mantle, visceral mass, and shell. The general anatomy and functions of molluscan systems like respiration, circulation, feeding, and reproduction are also summarized. Details are then provided on the defining features of major molluscan classes, including Gastropoda, Bivalvia, and Cephalopoda.

1. Mollusca

3. Phylum Mollusca
Diversity - 90,000 species (Hickman et
al. 2017)
- 8 classes: Caudofoveata,
Solenogastres, Polyplacophora,
Monoplacophora, Gastropoda,
Bivalvia, Scaphopoda,
Cephalopoda

5. Phylum Mollusca

6. Phylum Mollusca

7. Phylum Mollusca

8. Phylum Mollusca

9. General Characteristics
Most versatile body plan of all animals
- Divided into 8 classes
- Most diversity is in two (Gastropoda and Bivalvia)
- Microscopic to 20m in length
- Most are less than 10 cm
- Sessil to fast swimmers
- Occupy marine, fresh, and terrestrial biospheres
- Only Gastropoda and Bivalvia in freshwater
- Only gastropoda in terrestrial areas
- Must be moist and with calcium in soil

10. General Characteristics
Form and Function
- Despite wide variety of body plans, some basic features shared
- Mantle - secretes shell (or becomes outer body covering)
- Visceral mass - most internal organs (embedded in tissue)
- Shell - external or internal
- Radula - specialized feeding organ
- Foot - used for locomotion

11. General Characteristics
Body in two parts
- Head-foot
- Visceral mass

12. General Characteristics
Body in two parts
- Head-foot
- More active region of a mollusc
- Contains feeding, sensory, and locomotor organs
- Radula - tongue-like rasping organ
- Present in all except bivalves and solenogastres
- File-like, may have 250,000 teeth
- New teeth continually produced
- Supported on cartilage-like rod - odontophore
- Works like a conveyor belt to deliver food to digestive tract

13. General Characteristics
Radula

14. General Characteristics
Radula

15. General Characteristics
Body in two parts
- Head-foot
- Foot is present in all molluscs (used for
locomotion)
- Creeping movements
- Slime trail
- Digging
- Attachment
- Byssal threads
- For some, modified into proboscis

16. General Characteristics
Body in two parts
- Head-foot
- Foot is present in all molluscs (used for
locomotion)
- Creeping movements
- Slime trail
- Digging
- Attachment
- Byssal threads
- For some, modified into proboscis

17. General Characteristics
Body in two parts
- Head-foot
- Foot is present in all molluscs (used for
locomotion)
- Creeping movements
- Slime trail
- Digging
- Attachment
- Byssal threads
- For some, modified into proboscis

18. General Characteristics
Body in two parts
- Head-foot
- Foot is present in all molluscs (used for
locomotion)
- Creeping movements
- Slime trail
- Digging
- Attachment
- Byssal threads
- For some, modified into proboscis

19. General Characteristics
Body in two parts
- Head-foot
- Foot is present in all molluscs (used for
locomotion)
- Creeping movements
- Slime trail
- Digging
- Attachment
- Byssal threads
- For some, modified into proboscis

20. General Characteristics
Body in two parts
- Visceral mass - contains digestive, respiratory, and reproductive organs
- Depends primarily on ciliary tracts for functioning
- Mantle - body wall of molluscs.
- Made of epidermis
- May have sense organs and glands
- Glands secrete mucus, cement, and shell (some)
- Mantle may create mantel cavity
- Cavity houses respiratory organs
- For some, mantle cavity may offer protection

21. General Characteristics
Mantle and mantle cavity

22. General Characteristics
Mantle and mantle cavity

23. General Characteristics
Mantle and mantle cavity

24. General Characteristics
Body in two parts
- Visceral mass - contains digestive, respiratory, and reproductive organs
- Shell - secreted by mantle
- Much variability in form and structure
- Made of calcium carbonate
- 3 layers
- Periostracum
- Outer, thick in freshwater; thin or absent in marine
- Prismatic
- Middle, very dense
- Nacreous
- Inner, glossy and secreted throughout life

25. General Characteristics

26. Homeostatic Structures and Functions
Respiration
- Most molluscs have folded, ciliated gills (ctenidia)
- Used for feeding in bivalves
- Bivalves may have extended siphon
- Some molluscs respire through skin
- Many terrestrial snail lack gills (pulmonates)
- Mantle is modified into saclike lung for breathing air
- Pneumostome present

27. Homeostatic Structures and Functions
Respiration

28. Homeostatic Structures and Functions
Respiration

29. Homeostatic Structures and Functions
Respiration

30. Homeostatic Structures and Functions
Circulation
- Most have an open circulatory system
- Blood is not entirely contained within vessels
- Adequate for slow moving animals
- Cephalopods have closed circulation
- Heart is simple and has few vessels
- 2 auricles and 1 ventricle
- Coelom is reduced and becomes haemocoel surrounding heart
- Blood has both oxygen carrying cells and white blood cells
- No thrombocytes
- Haemocyanin (Cu) - most, blue in color
- Hemoglobin (Fe) - fewer, red in color

31. Homeostatic Structures and Functions
Circulation

32. Homeostatic Structures and Functions
Circulation

33. Homeostatic Structures and Functions
Feeding and Digestion
- Detritus feeders, burrowers, borers, grazers, carnivores, filter feeders, etc.
- Radula aids in feeding
- Most digestive organs are embedded in visceral mass
- Complete digestive system
- Foregut
- Midgut
- Hindgut

34. Homeostatic Structures and Functions
Feeding and Digestion
- Foregut - receives and prepares food
- Buccal cavity
- Mouth
- Radula
- Salivary glands
- Esophagus

35. Homeostatic Structures and Functions
Feeding and Digestion
- Midgut - storing region, crushing region
- Stomach
- Digestive glands
- Crystalline style
- Grinds food in some

36. Homeostatic Structures and Functions
Feeding and Digestion
- Crystalline style

37. Homeostatic Structures and Functions
Feeding and Digestion
- Hindgut - absorption of nutrients, formation of feces
- Intestine

38. Homeostatic Structures and Functions
Excretion
- 1 pair of metanephridia
- Term kidney is used, but different from vertebrate kidney)
- Some excretion through body walls and gills

39. Nervous System
Central nervous system - ring of ganglia
in head area
- Paired ganglia extend to other
parts of the body
- Nervous system is simple
compared to vertebrates
- Cephalopods rival some
mammals in reasoning and
learning

40. Reproduction and development
Most are dioecious
- Many gastropods (snails and slugs) are monoecious
Marine formes produce larval stage known as trochophore
- For some, a secondary larval form (veliger) will form
Cephalopods have direct development

41. Class Polyplacophora
Where found
- Generally on rocky surfaces in
intertidal regions
- Move very little over lifetime
- Clings to rocks with broad foot
- Capable of rolling up like armadillo
or pangolin for protection
- At low tide, can press margin of
mantle against substrate
- Lessens water loss

42. Class Polyplacophora
General morphology
- Small, 2-5 cm
- Dorsoventrally flattened
- Convex dorsal surface
- 7 to 8 plates on dorsal side (mantle
may cover)
- Generally dull in color
- Head reduced
- Mantle cavity extended along sides
of foot

43. Class Polyplacophora
Feeding
- Complete digestive tract
- Use radula to scrape algae
- Magnetite in radula makes stronger
- One predaceous species

44. Class Polyplacophora
Reproduction
- Sexes are separate in most
- Larva produced as trochophore
- No veliger stage

45. Class Gastropoda
General characters
- Largest and most diverse class of
molluscs
- Variety of body forms and names
- Snails, limpets, slugs, whelks, conchs,
periwinkles, sea slugs, sea hares, sea
butterflies.
- Generally slow moving or
sedentary with heavy shells

46. Class Gastropoda
General characters
- Shells
- Always univalve (single piece)
- May be coiled or uncoiled
- Top is apex and contains the oldest and
smallest whorl
- Whorls become larger as they spiral
down the columella (central axis)
- Open is aperture and may be covered
by an operculum

47. Class Gastropoda
General characters
- Shells
- Always univalve (single piece)
- May be coiled or uncoiled
- Top is apex and contains the oldest and
smallest whorl
- Whorls become larger as they spiral
down the columella (central axis)
- Open is aperture and may be covered
by an operculum

48. Class Gastropoda
General characters
- Shells
- Dextral or sinistral
- Right or left handed coiling
- Genetically controlled and right
predominates

49. Class Gastropoda
Torsion
- Process whereby the anus, shell, and mantle cavity containing the gills moves
from the posterior to anterior
- Occurs during development, but may not remain into adulthood (detorsion)

50. Class Gastropoda
Torsion
- Consequences
- Torsion prevents growth in length; gastropod must grow upward into shell
- Anus now over mouth
- Fouling
- What favored torsion?
- Most likely the advantage of withdrawing the head into the mantle cavity
- Provides protection

51. Class Gastropoda
Coiling
- Ancestral gastropods has a
planispiral (planospiral) shell
- All whorles stacked on top of one
another.
- Bulky arrangement; snail became much
taller as it grew
- Solution was coiling

52. Class Gastropoda
Feeding habits
- Varied, but all utilize the radula in
some fashion
- Herbivores - algae scrapers, grazers,
browsers

53. Class Gastropoda
Feeding habits
- Varied, but all utilize the radula in
some fashion
- Herbivores - algae scrapers, grazers,
browsers
- Borers - bore into other mollusc shells

54. Class Gastropoda
Feeding habits
- Varied, but all utilize the radula in
some fashion
- Herbivores - algae scrapers, grazers,
browsers
- Borers - bore into other mollusc shells
- Predators - use radula to help capture
food

55. Class Gastropoda
Feeding habits
- Varied, but all utilize the radula in
some fashion
- Herbivores - algae scrapers, grazers,
browsers
- Borers - bore into other mollusc shells
- Predators - use radula to help capture
food
- Conus - conotoxin

56. Class Gastropoda
Internal form and function
- Respiration
- Mostly by ctenidium (two gills) located
inside mantle cavity
- Pulmonates lack gills, but have highly
vascularized region
- Pneumostome

57. Class Gastropoda
Internal form and function
- Excretion - single nephridium (kidney)
- Circulatory - open
- Nervous - well developed
- Three pairs of ganglia
- Photoreceptors, statocysts, tactile organs, chemoreceptors

58. Class Gastropoda
Major groups of gastropods
- Prosobranchs - marine snails and some freshwater and terrestrial groups
- Opisthobranchs - sea slugs, sea hares, sea butterflies, bubble shells
- Nearly all marine
- Pulmonates - land snails and freshwater snails
- Ctenidia lost

59. Class Bivalvia
General characters
- Most will feed by creating ciliary
currents that draw water and
suspended food particles over gills
(filter feeding)
- Lack head, radula, and
cephalization (concentration of
sensory structures)

60. Class Bivalvia
General characters
- Shells always in two parts
- Hinge ligament at dorsal side
- Drawn together by adductor muscles
- Umbo is oldest part of shell

61. Class Bivalvia
Body and Mantle
- Visceral mass suspended from dorsal midline
- Muscular foot attached to visceral mass
- Ctenidia (gills) hang on either side of visceral mass and foot
- Mantle covers gills
- Gills lie in mantle cavity
- Shell to mantle to gill to visceral mass to gill to mantle to shell
- Mantle may be extended into long siphons
- Always forms some siphon (excurrent and incurrent)

62. Class Bivalvia
Locomotion
- Most initiate movement by
extending the foot through the gap
of the valves
- Blood is then pumped into the foot
causing it to swell
- Acts as anchor
- Longitudinal muscles shorten the foot
and pull the animal forward.

63. Class Bivalvia
Locomotion
- Most initiate movement by
extending the foot through the gap
of the valves
- Scallops move by jet propulsion
- Rapidly open and close valves to create
a jet of water

64. Class Bivalvia
Gills
- Gas exchange occurs over both
gills and mantle
- Gills appear as elongated “Ws”
- Form lamellae
- Water drawn in through incurrent
siphon
- Passes through pores into tubes and
then to suprabranchial chamber and out
excurrent siphon

65. Class Bivalvia
Gills
- Gas exchange occurs over both
gills and mantle
- Gills appear as elongated “Ws”
- Form lamellae
- Water drawn in through incurrent
siphon
- Passes through pores into tubes and
then to suprabranchial chamber and out
excurrent siphon

66. Class Bivalvia
Feeding
- Most are filter feeders
- As water is drawn in, mucus is secreted
- Entangles food particles
- Mucus and food slide down gillsto food groove at base of gills
- Mucus and food travel along the groove towards labial palps
- Palps direct food into mouth

67. Class Bivalvia
Feeding

68. Class Bivalvia
Feeding
- Floor of stomach is folded into ciliary tracts for sorting food particles
- In most, a styl sace secretes the crystalline style
- Constantly rotting
- Dissolves its surface layer and rolls the mucus mass
- Dislodged food particles are sorted by the base of the stomach
- Large particles to intestine
- Small to digestive gland

69. Class Bivalvia
Feeding

70. Class Bivalvia
Circulation
- Three-chambered heart with open circulation
Excretion
- Pair of U-shaped kidneys (nephridial tubes)
- Empty into suprabranchial chamber
Nervous system
- Three pairs of ganglia; poorly developed sense organs
- Scallops have eyes

71. Class Bivalvia
Reproduction
- Sexes usually separate
- For many, fertilization is external and gametes are shed into water
- Freshwater forms have internal fertilizations
- Many produce glochidia larvae
- Live parasitically on gills of fish
- Some discharge glochidia into water
- Others attract fish and cast glochidia directly at fish gills

72. Class Bivalvia
Reproduction

73. Class Cephalopoda
Squids, octopuses, nautiluses, devilfish,
cuttlefish, and extinct ammonites
- All are marine; all are active
predators

74. Class Cephalopoda
General characters
- Foot is merged with head
- Forms funnel for expelling water and a crown of tentacles
- May be small (2-3 cm) or large (19 m)
- Ancestrally, shell was present and straight
- Derived forms have curved, coiled, reduced, or absent shell
- Found at great depths and shallow intertidal zones

75. Class Cephalopoda
Shell
- Coiled in Nautiloids and extinct
ammonoids
- Although heavy, made buoyant by gas-
filled chambers
- Chambers walled-off from one another
and only last is inhabited
- Cord of living tissue, siphuncle,
connects through all chambers

76. Class Cephalopoda
Shell
- Internal and curved in cuttlefish
- Squids reduced to the pen
- Absent in octopuses

77. Class Cephalopoda
Locomotion
- Squids and cuttlefish are excellent, fast swimmers
- Nautiloids are much slower
- Octopuses are capable of rapid, backward movement, but are better built for
crawling among rocks

78. Class Cephalopoda
Respiration
- All cephalopods (except nautiloids) have one set of gills
- Gills lack cilia
- Rather, water is drawn over by changes to the shape of the mantle cavity
Circulation
- Circulatory system is closed
- Systemic heart served body
- Branchial hearts increase blood pressure at gills

79. Class Cephalopoda
Nervous system
- More elaborate than other molluscs
- Largest brain of any invertebrate
- Senses are well developed
- Eyes are single-lens, large, and complex
- No color vision; but superb underwater resolution

80. Class Cephalopoda
Communication
- Through chromatophores
- Cells will pigments
- Cell shape can be changed, thus the
amount of pigment shown can also be
changed
- Allows for rapid change in
appearance

81. Class Cephalopoda
Reproduction
- Sexes are separate
- Spermatozoa encased in
spermatophores
- Males with specializes tentacles to
retrieve spermatophore and pass it to
female
- Development is direct