BIOL 1030 – TOPIC 13 LECTURE NOTES

Topic 13: More Lophotrochozoans: Mollusks (Ch. 33)

  1. Phylum Mollusca (clade) – mollusks
  2. very successful phylum: over 93,000 named, living species
  3. largest number of living species for animals after Arthropoda
  4. mostly aquatic, but over 35,000 terrestrial species (more than all terrestrial vertebrates)
  5. includes snails, slugs, clams, oysters, chitons, cuttlefish, octopi, etc.
  6. from microscopic to 21 m (giant squid – largest invertebrate)
  7. economically important (food, ornamentation, currency, pests)
  1. defining characteristics of mollusks
  2. bilateral symmetry with true coelom; usually have a differentiated head
  3. unsegmented (some argument as to whether lost during evolution or never present in phylum)
  4. three main body parts: muscular foot, visceral mass, and mantle
  5. muscular foot – used for locomotion, food capture, attachment
  6. may be solid or have tentacles or may be a “wing” or fin in pelagic forms
  7. covered with soft epithelium
  8. may secrete mucus to glide on
  9. visceral mass – contains most of the internal organs, including these systems:
  10. digestive system – more complicated than in phyla to this point (true coelom)
  11. complete (separate mouth and anus)
  12. mouth, stomach, intestine
  13. radula – rasping “tongue” with rows of microscopic, chitinous “teeth” used to scrape or drill for food
  14. excretory system – simplest animals with efficient excretory system
  15. nephridia – tubular structures that gather wastes from coelom
  16. walls of nephridia reabsorb useful compounds (sugars, salts, water, etc.)
  17. ability to reabsorb is why the system is considered efficient
  18. wastes discharged into mantle cavity; gills pump wastes out of animal
  19. circulatory system
  20. some have closed system, most have open system
  21. if open, coelom is generally confined to a small area around the heart
  22. heart may have three chambers
  23. some fast cephalopods have auxiliary hearts to speed blood movement
  24. reproductive system and life cycle
  25. most have separate sexes; however, many snails are hermaphrodites
  26. trochophore – ciliated free-swimming larva of many marine mollusks
  27. veliger – second stage in development of most marine snails and bivalves; has beginnings of foot and mantle
  28. mantle
  29. folds (often 2) from dorsal body wall
  30. enclose a cavity between mantle and visceral mass (the mantle cavity)
  31. mantle cavity - may act as lung or enclose gills
  32. gills are filamentous projections of mantle
  33. gills are rich in blood vessels
  34. gills have a large surface area for gas exchange
  35. continuous pumping keeps flow of water into and out of mantle cavity
  36. may have siphons to move water in (incurrent) and out (excurrent) of mantle cavity
  37. bring in oxygenated water and send out deoxygenated water
  38. may be used to bring in food (in bivalves)
  39. may be used for jet propulsion (in cephalopods)
  40. shell (if present) – secreted by mantle, usually on outside, but on inside in some snails, squid, and cuttlefish, and lost in slugs, nudibranchs, and octopi
  41. provides protection
  42. one or two valves (shells)
  43. outer layer - horny protein layer; protects inner layers from eroding
  44. middle layer - densely packed crystals of calcium carbonate (calcite or aragonite)
  45. inner layer - pearly - increases in thickness throughout life of mollusk
  46. pearls form between the inner layer and mantle in bivalves
  1. Mollusk taxonomy
  2. 7 or 8 classes; we will cover 4, all of which appear to be clades
  3. Class Polyplacophora (clade) – chitons
  4. ~1000 living species
  5. marine; shell is segmented with 8 overlapping, calcareous plates
  6. body beneath shell is not segmented
  7. head is greatly reduced
  8. foot used for locomotion and for holding onto substrates
  9. Class Gastropods (clade) – snails and slugs
  10. name means “stomach foot”
  11. ~70,000 living species
  12. mostly marine, but abundant in freshwater, and many are terrestrial
  13. most have a shell
  14. single shell, if present
  15. operculum found in most marine gastropods – a horny plate that forms a covering “door” when the snail withdraws into its shell
  16. coiling (spiral twisting) of shell due to one side of larva growing faster than the other side
  17. shell has been lost in the course of evolution for slugs and nudibranchs (sea slugs)
  18. body clearly divisible into head, foot, and visceral mass
  19. head of most has a pair of tentacles with eyes (tentacles lost in more advanced species)
  20. torsion of visceral mass in development leads to asymmetry of visceral mass; anus ends up practically over the head
  21. terrestrial forms may have mantle cavity serve essentially as a lung – area very rich in blood vessels, allowing efficient gas exchange
  22. feeding – diversity in feeding habits; many are herbivorous (some major agricultural pests); some are active predators
  23. Class Bivalvia (clade) – the bivalves
  24. includes clams, oysters, scallops, and mussels
  25. ~20,000 living species
  26. 2 shells (valves) hinged together
  27. strong adductor muscles used to pull shells together
  28. typically with siphons - most are filter feeders (food trapped on mucus on gills)
  29. complex folded, ciliated gills
  30. no distinct head or radula
  31. most have a strong muscular foot (many different adaptations)
  32. some can move by clapping their shells together (scallops); however, most bivalves are sessile for most of their adult lives, tethered to some substrate by strong threads they secrete or using their foot as an anchor
  33. scallops have many eyes on soft body parts next to the edges of their shells
  34. freshwater mussels the most endangered group in North America, and Alabama has the highest diversity of freshwater mussels in the world
  35. Class Cephalopoda (clade) – the cephalopods (“head foot”)
  36. includes octopi, squids, cuttlefish, and nautilus
  37. ~800 living species
  38. 0-1 shells, internal or external
  39. octopi – no shell
  40. squid and cuttlefish – internal shell remnant used for support
  41. nautilus – chambered shell, superficially resembling snails
  42. highly developed nervous system
  43. giant axons –great for research
  44. elaborate eyes (some up to 40 cm across – largest known eyes)
  45. most intelligent invertebrates; complex behavior
  46. many are skillful hunters
  47. foot modified into grasping tentacles with suckers
  48. built for speed – jet propulsion using siphons (they are fast-moving predators)
  49. most have a closed circulatory system (only mollusks with this)
  50. strong beak for biting; radula used to pull prey in
  51. squid and octopi can release a dark “ink” to cloud water for escaping predators or even for trapping prey
  52. cuttlefish are famous for changing color to match background or for messaging, using chromatophore pigment pouches; most octopi and squid can do this as well

Watch the bizzare behaviour of cephalopods :


The figure below labels body parts of the Gastropods and Chitons. Label the same parts on the Cephalopods and Bivalves.

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