1. Chapter 11
Molluscan Success

2. Evolutionary Perspective
Triploblastic
Coelomate
Very successful
100,000 living species
Relationships to other animals
Lophotrochozoans

4. Molluscan Characteristics
Body of two parts: head-foot and visceral mass
Mantle that secretes a calcareous shell and covers the visceral mass
Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and release of reproductive products.
Bilateral symmetry
Trochophore larvae, spiral cleavage, and schizocoel coelom formation
Coelom reduced
Open circulatory system (except Cephalopoda)
Radula usually present

5. Body Organization Head-foot Elongate Mouth Attachment and locomotion
Visceral mass
Dorsal to head-foot
Organs of digestion, circulation, reproduction
Mantle
Enfolds body
Secretes shell
Mantle cavity
Gas exchange, excretion, elimination of digestive wastes and reproductive products
Radula
Supported by odontophore
Rasping food

8. (a)
(b)

9. Class Gastropoda Snails and slugs 35,000 living species Torsion
180o counterclockwise twisting of visceral mass, mantle, and mantle cavity during development
Possible adaptive significance
Head enters shell first.
Clean water enters anteriorly oriented mantle cavity opening.
Mantle sensory organs move to head region.

11. Class Gastropoda Shell coiling Locomotion Earliest fossils, one plane
Modern, asymmetrical
More compact
Internal organs asymmetrical and sometimes no longer paired
Locomotion
Flattened foot
Cilia propel over mucous trail
Muscular waves

12. Class Gastropoda Feeding and digestion
Most scrape algae and attached organisms
Herbivores, predators, scavengers
Digestive tract
Ciliated
Food incorporated into mucous mass called protostyle.
Gas exchange
One or two gills in mantle cavity
Land snails (pulmonates)
Mantle cavity richly vascular for gas exchange with air

13. (a)

14. Other Maintenance Functions
Open circulatory system
Blood bathes tissues in sinuses.
Heart
Single ventricle and single auricle
Functions
Transports nutrients and gases
Hydraulic skeleton
Nervous system
Six ganglia plus nerve cords
Sensory structures
Eyes at base or end of tentacles
Statocysts in foot
Osphradia in mantle cavity

15. Other Maintenance Functions
Excretion
Single nephridium
Result of shell coiling
Discharges into mantle cavity or adjacent to mantle cavity (pulmonates)
Ammonia (aquatic species)
Uric acid (pulmonates)

16. Reproduction Dioecious or monoecious External fertilization Copulation
Some dioecious marine species
Copulation
Sperm transfer may be mutual or one-way.
Eggs shed singly, in strings, or in masses
Larval stages
Trochophore
Veliger

17. Gastropod Diversity Subclasses Prosobranchia Opisthobranchia Pulmonata
20,000 species
Mostly marine
Opisthobranchia
2,000 species
Sea hares, sea slugs
Shell, mantle cavity, and gills reduced or lost
Pulmonata
17,000 species
Freshwater or terrestrial
Vascular mantle cavity serves as lung

18. (b)
(a)
(c)

19. Class Bivalvia Clams, oysters, mussels, scallops 30,000 species
Shell and associated structures
Single shell consisting of two hinged valves
Mantle sheetlike and covers laterally compressed body.

20. .

21. Gas Exchange, Filter Feeding, and Digestion
Sedentary filter feeders
Loss of head and radula
Expansion of cilia-covered gills into folded sheets (lamellae)
Cilia create water currents into and through mantle cavity.
Gas exchange in water tubes
Food trapped along gill surface and transported to food grooves and labial palps
Digestion
Crystaline style and gastric shield

25. Other Maintenance Functions
Open circulatory system
Mantle and gills oxygenate blood
Nephridia
Below pericardial cavity
Open to suprabranchial chamber
Nervous system
Three pairs of interconnected ganglia
Sensory receptors at mantle margin

26. Reproduction and Development
Most dioecious
Gonads within visceral mass
External fertilization
Trochophore and veliger larval stages
Freshwater in family Unionidae
Parasitic larval stage
Glochidium

28. (c)

30. .

32. Gooduck

33. Class Cephalopoda Squid, octopuses, cuttlefish, and nautiluses
Foot modified into circle of tentacles or arms and incorporated into siphon
Head in line with visceral mass
Muscular mantle

35. Class Cephalopoda Shell Locomotion Feeding and Digestion
Reduced or absent except in nautilus
Locomotion
Jet propulsion using muscles of mantle compressing water within mantle cavity and siphon
Feeding and Digestion
Predators
Tentacles, jaws, radula
Digestive tract muscular with large digestive glands

36. .

37. Other Maintenance Functions
Closed circulatory system
Nervous system
Large brains
Complex sensory structures
Eyes
Statocysts
Chromatophores
Color changes involved with courtship and other displays
Ink glands

38. The cephalopod eye.

39. Learning
Unparalleled in comparison to any other invertebrate and many vertebrates
Evolved in response to predatory lifestyles

40. Reproduction and Development
Dioecious
Male produces spermatophores
Transfers to female’s mantle cavity using modified tentacle (hectocotylus)
Eggs deposited singly or in masses attached to substrate.
Eggs tended by parents.

41. Class Polyplacophora Chitons
Reduced head, flattened foot, shell consisting of eight dorsal valves, muscular mantle extends beyond margin of shell
Feed on attached algae
Ladderlike nervous system
Dioecious with external fertilization

42. (a)

43. Class Scaphopoda Tooth shells or tusk shells Marine, burrowing
Conical shell open at both ends
Dioecious with trochophore and veliger larvae

44. Class Scaphopoda.

45. Class Monoplacophora Marine Undivided arched shell Broad, flat foot
Serially repeated pairs of gills and foot retractor muscles
Dioecious

46. Class Monoplacophora.

47. Class Solenogastres Marine substrates Lack shell Crawl on ventral foot
Figure 11.23
Class Solenogastres
Marine substrates
Lack shell
Crawl on ventral foot
Minute calcareous spicules
Carnivores

48. Class Caudofoveata Deep sea Wormlike Feed on foraminifera
Lack shell, foot, and nephridia
120 species

49. Further Phylogenetic Considerations
More than 500 million years old
Lophotrochozoa
Shell and muscular foot not ancestral
Solenogaster spicules may be similar to ancestral “shell”.
Muscular foot first seen in Polyplacophora.
Quickly diversified into modern classes

50. Cladogram showing possible evolutionary relationships among the molluscs.