1. PHYLUM MOLLUSCA snail giant clam Nautilus
Characterised by a soft body. Very large (second largest animal phylum next to the arthropods) and extremely diverse group, including clams, snails and octopuses.

2. As with the Annelids, Molluscs are true coelomates, with a schizocoelomate body plan.

4. 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)

5. 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

6. Characteristics Body Plan
Foot, mantle, visceral mass

7. Head-Foot Region Well developed head Muscular foot
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

8. 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
Mantle surface also used for gaseous exchange
Jet propulsion (cephalopods)

9. Characteristics
Radula (tongue-like rasping organ) usually present

10. Circulatory System Open circulatory system (closed in cephalopods)
Heart, blood vessels and sinuses
Respiratory pigments in blood

11. Respiration Excretory System 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

12. 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

13. Generalised Mollusc
All systems present and well developed.

14. Reproduction Mostly dioecious; some monoecious
Spiral cleavage of egg to produce free-swimming larva
Trochophore or veliger larva (aquatic molluscs)
Some with direct development

15. Trochophore
Free-swimming larva that emerges from egg in primitive molluscs
prototroch
mouth
anus

16. 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

17. Class: Caudofoveata Class: Solenogastres Wormlike Marine Reduced head
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

18. 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
shell
mantle
anus

19. 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

20. 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)

22. Class: Gastropoda Largest and most diverse class
naudibranchs
garden slug
limpet
whelks
snail
abalone
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

23. Class: Gastropoda Both trochophore and veliger larval stage
Bilaterally symmetrical, but because of torsion, visceral mass has become asymmetrical

24. 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

25. Practical

26. 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

28. 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

29. 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)

30. 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

31. 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

32. 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

33. Class: Gastropoda Subclass: Pulmonata
snail
slug
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
ctenidium - comb-like respiratory structure serving as the gill of certain molluscs.
Terrestrial forms thought to be more advanced; evolved from aquatic forms.

34. Class: Bivalvia Mussels, clams, oysters, scallops
Black mussels; Mytilus edulus
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
Second largest class

35. 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
Second largest class

36. 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

37. Class: Cephalopoda
Foot modified into funnel for expelling water from mantle cavity (jet propulsion)
Shell often reduced or absent
Most complex of the molluscs

38. 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

39. 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
As a result of their more active predatory life styles, they have developed a nervous system of far greater complexity than any other mollusc. Associated with this more complex nervous system is an increased sensory perception and behavioural complexity. Of special interest is their ability to effect rapid skin colour changes due to the presence in the skin of several types of chromatophore which are under nervous as well as hormonal control.
Experiments have shown that some species, at least, are capable of learning and may exhibit a reasonable level of 'intelligence'. As a result, many people consider that the cephalopods represent a zenith, not only of the molluscan line, but also of all the invertebrates.

40. 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
Last remaining cephalopods with an external shell. Slower moving than other cephalopods.

41. Class: Cephalopoda Subclass: Ammonoidea Totally extinct
End Cretaceous (65 MYA)
External shell
First appeared about 400 MYA and went extinct about 65 MYA.

42. cuttlefish
squid
octopus
Class: Cephalopoda
Subclass: Coleoidea (most have 8 arms and 2 tentacles)
Cuttlefish, squid and octopus
Sepiidea – Cuttlefish
Teuthida – Squid; Largest of the cephalopod orders

43. Class: Cephalopoda Order: Sepioidea Cuttlefish
Rounded or compressed bulky body bearing fins
Arms and tentacles bear suckers but tentacles only at their ends
cuttlefish

44. 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

45. Class: Cephalopoda Order: Octopoda
No shell; 8 Arms; no tentacles; short compact body; suckers not stalked and do not bear horny rims

46. Evolutionary Relationships
Molluscs are allied with both annelids and arthropods
Molecular evidence suggests they are more closely related to the annelids