1. Molluscs Bonneville High School Vertebrate Zoology
CHAPTER 16

2. Unit Objectives
Objective 1: Describe the form and function of the Molluscs
Objective 2: Differentiate between the classes of Molluscs (Caudofoveata, Solenogastres, Monplacophora, Polyplacophora)
Objective 3: Differentiate between the classes of Molluscs: (Scaphopoda, Gastropoda, Bivalvia, Cephalopoda)
Objective 4: Describe the Internal form and function of gastropods, bivalves, and cephalopods.
Objective 5: Describe the form and function of the Annelida
Objective 6: Differentiate between the classes of Annelida (Polychaeta, Oligochaeta, Hirudinea, )
Objective 7: Describe the Internal form and function of the following classes: Polychaeta, Oligochaeta, Hirudinea.
Objective 8: Describe the life cycle and reproductive strategies for Molluscs and Annelids

3. Fluted Giant Clam

4. Characteristics Phylum Mollusca
Over 90,000 living species and 70,000 fossil species
Soft body and protostomes
Include chitons, tusk shells, snails, slugs, nudibranchs, clams, mussels, oysters, squids, octopuses, and nautiluses

5. Marine Snail
Nudibranch
Chiton
Pacific Giant Clam
Octopus

6. Characteristics Characteristics:
Herbivorous grazers, predaceous carnivores, filter feeders, and parasites
Most are marine, but some are terrestrial or freshwater aquatic

8. Characteristics Evolution Fossil evidence Some bivalves and gastropods
Indicates molluscs evolved in the sea
Most have remained marine
Some bivalves and gastropods
Moved to brackish and freshwater
Snails (gastropods) successfully invaded land
Limited to moist, sheltered habitats with calcium in the soil
Cephalopods
Evolved to become relatively intelligent

9. Economics Characteristics Many are used as food
Culturing of pearls is an important industry
Snails and slugs are garden pests

10. Mollusc Body Plan: Head-Foot and Visceral Mass Portions
Form and Function
Mollusc Body Plan: Head-Foot and
Visceral Mass Portions
Head-foot region contains feeding, sensory, and locomotor organs (foot)
Visceral mass contains digestive, circulatory, and reproductive organs
Mantle Cavity
Space between mantle (sheath of skin) and body wall is the mantle cavity
Mantle cavity houses the gills or a lung
In most molluscs
Mantle secretes a shell over the visceral mass

11. Generalized Mollusk Anatomy

12. Form and Function Radula Unique to molluscs
Found in all except bivalves
Protruding, rasping, tongue-like organ
Ribbon-like membrane has rows of tiny teeth (up to 250,000) pointed backward
Radula rasps off particles of food from surfaces
Serves as a conveyor belt to move particles to digestive tract
New rows of teeth replace those that wear away
Pattern and number of teeth are used in classification of molluscs
Some specialized to bore through hard material or harpoon prey

13. Radula

14. Form and Function Foot Functions in attachment or locomotion
Modifications include
Hatchet foot of clams
Siphon jet of squids
Secreted mucus aids in adhesion or helps molluscs glide
Snails and bivalves extend the foot by engorgement with blood

15. Form and Function Shell If present, secreted by the mantle
Periostracum
Outer layer - wears away
Composed of hardened protein
Middle prismatic layer
Closely packed prisms of calcium carbonate
Increases with animal growth
Inner nacreous layer
Next to the mantle; the nacre is laid down in thin layers
Aids in Pearl formation
Shiny layer in abalone, nautilus, and bivalve

16. becomes covered with nacre
Bivalve Shell
B. Pearl Formation from a parasite or sand that enters shell into mantle,
becomes covered with nacre

17. Reproduction and Life History
Form and Function
Reproduction and Life History
Most dioecious, some hermaphroditic (monoecious)
Egg hatches and produces a free-swimming trochophore larva
In many gastropods and bivalves
Trochophore is followed by intermediate larval stage, the veliger.

18. Internal Form and Function (Obj 4)
-Torsion: A twisting process that makes the Mollusc visceral mass asymmetrical.
-Coiling: The spiral winding of the shell and visceral mass. It is NOT the same as torsion.
-Ctenidium: Located in the mantle cavity, this acts as gills for some Molluscs
-Lung: A highly vascular area in some Molluscs which aids in respiration it has an opening called a pneumostome.
-Dioecious: Having male and female gonads in separate individuals.
-Monoecious: Having both male and female gonads in the same organism: Hermaphrodite
-Pulmonates: Molluscs which show detorsion and include land and most fresheater snails and slugs. They lost their ctenidia and their vascularized mantle wall has become a lung. They are monoecious.
-Chromatophores: Cells in the skin that contain pigment granules. This can change the color of the skin of some mollusks. They are used for camouflage and communication.
-Ink gland: A gland which most cephalopods and nautiloids have that secretes sepia, a dark fluid containing the pigment melanin. The pigment is deposited into an ink sac that empties into the rectum. The Mollusc can then release the dark cloud to confuse or distract a predator.
Open circulatory system: a pumping heart, blood vessels and blood sinuses. Blood is pumped from the heart into open spaces.
Closed circulatory system: Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. 

19. Trochophore larva

20. Classes of Molluscs
Class Monoplacophora-Neopilina
Class Polyplacophora – Chitons
Class Caudofoveata-Chaetodermomorpha
Class Solenogastres-Solenogastres
Class Scaphopoda-Tusk or tooth shells
Class Gastropoda - Slugs, Snails, Nudibranch
Class Bivalvia - Clams, Mussels
Class Cephalopoda - Squid, Octopus, Nautilus

21. Class Monoplacophora
Body bilaterally symmetrical with broad, flat foot;
A single dome-shaped shell
Five or six pairs of gills in shallow mantle cavity
Radula present
Separate sexes. Example: Neopilina

22. Class Polyplacophora: Chitons
Classes of Molluscs
Class Polyplacophora: Chitons
Chitons are somewhat flattened with 7or 8 dorsal plates
Most prefer rocky intertidal surfaces
Chiton radula is reinforced with iron mineral
Scrapes algae from the rocks

23. Mossy Chiton - hairs and bristles aid in defense

24. Class Caudofoveata Wormlike Shell, head and excretory organs absent
Radula usually present
Mantle with chitinous cuticle and calcareous scales
Sexes separate
Was united with solenogasters but now classified separately
Examples Chaetoderma, Limifossor

25. Class Solenogastres Solenogasters Wormlike
Shell, head, and excretory organs absent; radula usually absent
Mantle covered with scales or spicules;
Hermaphrodite
Example: Neomenia

26. Class Scaphopoda Tusk Shells
Body enclosed in a one piece, tubular shell open at both ends.
Conical foot;
Mouth with radula and tentacles, head absent
Sexes are separate
Example: Dentalium

27. Class Gastropoda Class Gastropoda Most diverse class
Over 70,000 living
Forms range from marine forms to air-breathing terrestrial snails and slugs
Shells, if present, are chief defense
Some produce distasteful or toxic secretions
Use process of Torsion

28. Classes of Molluscs Gastropod Shells One-piece (univalve)
Apex is smallest and oldest whorl
Whorls become larger and spiral around central axis
Giant marine gastropods have shell up to 60 cm long
Some fossil forms are 2 meters long
Terrestrial gastropods shells are restricted by soil mineral content, temperature, dryness, and acidity

30. Class Gastropoda Cont. Form and Function Torsion
Developmental process that changes the relative position of the shell, digestive tract and anus
Digestive tract moves so that anus lies above head

31. Class Gastropoda Cont. Coiling Coiling or spiral winding
of the shell and visceral
mass not the same as torsion

32. Abalone - Moon Snail - Feed on kelp -herbivore feeds on clams
and mussels
Radula releases chemicals
to soften shell, so they can
get to their prey

33. Conus - Extends proboscis
to capture prey. Then releases Conotoxins to paralyze (lethal to Humans)
Hours later regurgitate
scales and bones

34. Internal Form and Function
Class Gastropoda Cont
Internal Form and Function
Respiration performed by vascular area in mantle cavity that serves as lung
Most have a single nephridium (kidney) and well-developed open-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. (On tentacle of some)

36. Class Gastropoda Cont Reproduction:
Eggs emitted singly or in clusters, and may be transparent or in tough egg capsules
Young may emerge as veliger larvae or pass this stage inside the egg
Some species, including most freshwater snails, are ovoviviparous

37. Eggs of Mollusks Egg ribbon of Nudibranch
Eggs - resemble grains of wheat

38. Major Groups of Gastropods
Classes of Molluscs
Major Groups of Gastropods
Traditional classification has recognized three subclasses of Gastropoda
Prosobranchia,
Opisthobranchia
Pulmonata

39. Includes most marine snails Have one pair of tentacles, separate sexes
Gastropods - Prosobranchia
Includes most marine snails
Have one pair of tentacles, separate sexes
Diodora aspera
Hole in Apex for water to leave
Flamingo Tongue Snails

40. Marine, Shell is reduced or absent 2 tentacles,
Opisthobranches: sea slugs, sea hares, sea butterflies, and nudibranch
Marine, Shell is reduced or absent 2 tentacles,
Sea Hare
Sea Hare’s defense mechanism- a secretion from its purple gland

41. Nudibranch - calcareous spicules for protection

42. Pulmonates - Snail and Banana slug 2 tentacles: posterior have eyes
Opening to Mantle Cavity

43. Classes of Molluscs Class Bivalvia Mussels, clams, scallops, oysters
Range in size from 1–2 mm in length to the giant South Pacific clams (1m)
Most are sedentary filter feeders
Bivalves lack a head, radula, or other aspects of cephalization
Contain Siphons

44. Mussels
Scallops
Escaping a Sea Star

46. Northwest Ugly Clams
Siphons
Incurrent brings in Food and Oxygen

47. Class Bivalvia Form and Function
2 shells or valves are held together by a hinge ligament
Valves are drawn together by strong adductor muscles
Umbo is the oldest part of the shell with growth occurring outward in rings
Posterior edges of the mantle folds form excurrent and incurrent openings
In burrowing clams, mantle forms long siphons to reach the water above

48. Class Bivalvia Locomotion Foot is extended out from between the valves
Blood is pumped into the foot
Foot swells and anchors the bivalve in the mud
Shortening of the foot pulls the clam forward
Scallops clap valves to create a jet propulsion

49. Scallop - developed sensory organs along
mantle edges (tentacles and blue eyes)

50. Class Bivalvia Gills Circulatory - Open circulatory system
Both mantle and gills perform gaseous exchange
Siphon used in respiratory
Water enters incurrent siphon
Gas diffused out
Exits through the excurrent siphon
Circulatory - Open circulatory system
3 chambered heart has two atria and one ventricle
Blood vessels line gills to receive oxygen

53. Shipworm Bivalves-
Burrows into wood on
docks and piers

54. Symbiotic relationship with Algae to gain most nutrients
Clam -
Symbiotic relationship
with Algae to
gain most nutrients
Siphonal Area

55. Class Bivalvia Reproduction and Development Sexes usually separate
Gametes discharged in excurrent flow
Fertilization usually external
Embryos develop as trochophore, and veliger larval stages
Freshwater clams have internal fertilization
Sperm enter the incurrent siphon to fertilize eggs in water tubes of the gills
Larvae develop into a bivalved glochidia stage
Attaches to gills of passing fish where they live briefly as parasites
“Hitchhiking” having helped distribute the species

56. Life Cycle of an Oyster

57. Glochidium - freshwater clam larva Attach to fish’s gills by clamping
their valve closed.
Stay for several weeks.
Pocketbook Mussel -
mimics a small minnow, when a
Smallmouth Bass comes to dine, it
releases its glochidia

58. Class Cephalopoda Class Cephalopoda
Squids, octopuses, nautiluses, and cuttlefish
All marine predators
Foot is in the head region
Modified for expelling water from mantle cavity
Range from 2 cm to the giant squid (60 ft)
Largest invertebrate
Nautilus - only one with external shell
Series of gas chambers in shell helps maintain neutral buoyancy

59. A. Feed on a Fish B. Showing Gas filled chambers
Nautilus
A. Feed on a Fish B. Showing Gas filled chambers

60. Cuttlefish

61. Class Cephalopoda Locomotion
Cephalopods swim by forcefully expelling water through a ventral funnel or siphon
Control direction and force of the water, thus determining its speed
Lateral fins of squids and cuttlefishes are stabilizers
Nautilus swims mainly at night
Octopuses mainly crawl on the bottom but can swim
Some with webbing between their arms swim with a medusa-like action

62. Squid - Pen is only remains of shell

63. Respiration and Circulation
Class Cephalopoda
Respiration and Circulation
With higher oxygen demands, cephalopods have a muscular pumping system to keep water flowing through the mantle cavity
Circulatory system has a network of vessels conducting blood through gill filaments (Closed Circulatory System!!)

64. Class Cephalopoda Nervous and Sensory Systems
Cephalopod brain is the largest of any invertebrate
Squids have giant nerve fibers
Sense organs are well-developed
Eyes are complex, complete with cornea, lens, and retina
Can learn by reward and punishment, and by observation of others
Cephalopods lack a sense of hearing but have tactile and chemoreceptor cells in their arms

65. Cuttlefish Eye

66. Class Cephalopoda Communication
Use chemical and visual signals to communicate
Chromatophores are cells in the skin that contain pigment granules
Contractions of the muscle fibers attached to the cell causes the cell to expand and change the color pattern
Color patterns can be changed rapidly
Deep-water cephalopods have elaborate luminescent organs
Ink sac empties into rectum; (Not in Nautiloids)
Contains ink gland that secretes sepia (dark fluid) when animal is alarmed

67. Class Cephalopoda Reproduction Sexes are separate
In male seminal vesicle, spermatozoa are packaged in spermatophores and stored
One arm of male is modified as an intromittent organ, the hectocotylus
Removes a spermatophore from mantle cavity and inserts it into female
Fertilized eggs leave oviduct and are attached to stones, etc.
Large, yolky eggs undergo meroblastic cleavage (not full cleavage)
Hatch into juveniles with no free-swimming larval stage

68. Copulation in Cephalopods -
Male Octopus uses modified arm