Mollusks and Annelids

1. Mollusks and Annelids Bio II Rupp

2. Phylum Mollusca Characteristics • Mollusk—from the Latin meaning “soft” • Many have hard shells for protection • Approx. 112,000 species • Range from sedentary feeders to fast moving predators • Coelomates • Muscles can move without affecting gut • Circulatory system is not interfered with • Trochophore—larval stage • Free-swimming • Mouth and anus • Cilia for movement and food gathering • Annelids and mollusks both have, which shows common ancestry

3. Trochophore Mollusk and annelid larval stage

4. Mollusk Body Plan • Two main regions • Head-foot • Contains sensory organs in head • Foot contains muscle for locomotion • Visceral mass • Contains heart, digestive organs, excretion organs, and reproduction organs

6. Mollusk Body Plan con’t. • Coelom around heart • Mantle covering the visceral mass—aka—epidermal layer • Mantle secretes CaCO3 • Shells protect, but decrease surface area • Gills evolved • Gills are protected by mantle cavity • Bilateral symmetry—apparent in nervous system • Ganglia are connected by nerve cords • Light • Touch • Chemicals

7. Mollusk Body Plan con’t. • Radula • Main feeding adaptation • Tongue-like strip covered in backwards facing teeth • Adapted to mollusk lifestyle • Terrestrial snails eat grass • Aquatic snails eat algae or drill shells • Cone shell has a harpoon-like radula

8. Mollusk Body Plan con’t. • There are 8 classes, we will focus on 3 • Gastropoda • Bivalvia • Cephalopoda

9. Table 37-1

10. Class Gastropoda • Gastro means “stomach” and poda means “foot” • 90,000 species • Snails, abalones, conchs—one shell • Slugs and nudibranchs—no shell

11. Class Gastropoda con’t. • Torsion—the twisting process that relocates the mantle cavity • Occurs in larval development • Places mantle cavity near the head • Allows head to be retracted • Coiling of the shell is unrelated

12. Class Gastropoda con’t. • Movement occurs by wave-like contractions of the foot • Open circulatory system • Hemolymph • Hemocoel—blood cavity

13. Snails • Terrestrial, marine, and freshwater • Eyes on the end of tentacles—they can retreat into head • Gill-breathers • Land snails have modified gills for gas exchange • Need to stay moist • Dormancy—mucous plug—prevents desiccation

14. Slugs • Respire through modified mantle cavity • Live in moist shady places to prevent desiccation

15. Nudibranchs • Marine • No shell • Name means “naked gill” • Respiration occurs across the entire body • Numerous extensions create greater surface area

16. Pteropods • Name means “wing foot” • Use the foot to swim instead of crawl

17. Class Bivalvia • Name means “two shells” • Clams, oysters, scallops • Adductor muscles for closing their shells • Contraction closes • Relaxation opens

18. Class Bivalvia con’t. • Each valve or shell has three layers • Thin outer layer to protect against acidity of sea water • Thick middle layer of CaCO3 • Inner layer has “mother of pearl” coating to create a smooth surface for the soft body

19. Class Bivalvia con’t. • Typically sessile • Use their foot as an anchor • Fill it with hemolymph to create a hatchet shape • Contract foot and pull themselves into the sand

20. Class Bivalvia con’t. • Filter feeders • No radula • No distinct head • Nervous system has three ganglia pairs • Mouth • Digestive system • Foot • All are connected • Respond to chemicals and touch • Some have eyes along mantle—calico scallop

21. Clams • Live buried in mud or sand • Mantle cavity is sealed except for the siphons • Cilia generate current to pull water in • Water exits through excurrent siphon • Gills capture organic matter in mucous • Gills are also used for gas exchange with hemolymph • Typically separate sexes • Marine clams use external fertilization • Freshwater clams use internal fertilization • Sperm in incurrent siphon • Larvae exit the excurrent siphon • Eventually settle on bottom and mature

24. Oysters • Make permanent attachment to hard surface early in life • Commercially grown for food and pearls

25. Scallops • Motile • Use shells in a clapping motion to swim

26. Teredo or Shipworm • Not a filter feeder • Bores into driftwood or boat timbers and ingests the particles • Wood is broken down by symbiotic bacteria

27. Class Cephalopoda • Name means “head foot” • Octopuses, cuttlefish, squids, chambered nautliluses • Foot is concentrated in the head, hence the name • Free-swimming

28. Class Cephalopoda con’t. • Predatory lifestyle • Circle of tentacles extending from the head • Jaws resembling a parrot’s beak • Advanced nervous system • Largest brain of any invertebrate • Divided into lobes • Millions of cells • Capable of learning and problem solving

29. Class Cephalopoda con’t. • Well developed senses • Eyes can form images • Tentacles have chemical sensing ability • Closed circulatory system • Separate sexes • Males have specialized tentacles for sperm transfer • Females lay and guard eggs • No trochophore stage

30. Squids • Ten tentacles • Two longest tentacles for prey capture, other eight force food into mouth • Pump water through excurrent siphon for propulsion • Most grow to 1 ft long • Giant squid grows to 60 ft

31. Octopuses • Eight tentacles • Don’t chase prey like squid—crawl and grab • Seldom exceed 3 ft in diameter • Pacific octopus can grow to 8 ft in diameter

32. Chambered Nautilus • Shell • Series of gas-filled chambers separated by partitions • Confined to outermost chamber • Gas in the chambers allows buoyancy

33. Cuttlefish

34. Phylum Annelida Characteristics • Name means “little rings” • 15,000 species • Bilaterally symmetrical • Most distinctive feature is the rings • True coelom • Divided into compartments by partitions • Evolutionary advantage—allows different parts to expand and contract independently • Duplication of organ systems in each segment—injury insurance • Setae and parapodia—allow for division into three classes • Oligochaeta • Polychaeta • Hirudinea

35. Class Oligochaeta • Name means “few bristles” • Live in soil or freshwater • No parapodia • Have a few setae on each segment • Most familiar class member is the earthworm

36. Class Polychaeta • 2/3’s of annelids • Name means “many bristles” • Setae are found on parapodia • Antennae and specialized mouth parts • Only annelids with trochophore stage • Most are marine • Some are free-swimming predators and use strong jaws for feeding • Others feed on sediment

38. Class Hirudinea • 300 species • Live in calm bodies of freshwater or moist vegetation on land • No setae or parapodia • Each end has a sucker • Can crawl using suckers • Can swim with undulating motion • Carnivorous • Invertebrate prey • Blood suckers • Secrete anesthetic and anticoagulant • Can ingest 10x their weight

40. Earthworm Structure and Movement • More than 100 segments • Circular and longitudinal muscles • Movement • Anchor posterior setae • Contract circular muscles to extend • Anchor anterior setae • Contract longitudinal muscles to pull forward

41. Earthworm Feeding and Digestion • Ingest soil while burrowing—use muscular pharynx • Soil moves into the crop—temporary storage area • Soil is moved from crop to gizzard—a muscular gut that grinds the soil to release the nutrients • Nutrients are absorbed by the intestine—typhlosole folds • Undigested materials exit through anus • Maintains soils fertility by decomposition and aeration

42. Earthworm Circulation • Closed system • Blood travels to the posterior through a ventral blood vessel • Blood returns to the anterior heart through a dorsal blood vessel • Aortic arches link the vessels—these arches act as hearts to contract and move the blood

43. Earthworm Respiration and Excretion • Gases are exchanged by diffusion through skin and cuticle—no specialized organs • Diffusion only occurs if skin is moist—mucous and cuticle aid in this effort • Cell wastes are eliminated through nephridia • Each segment, except, the first three, and last one, have nephridia • Coelomic fluid passes through, water is reabsorbed, wastes eliminated

44. Earthworm Nervous System • Chain of ganglia connected by a ventral nerve cord • Most segments contain a ganglion • Nerves branch from ganglia • Several anterior ganglia are fused to form a brain • Light • Touch • Chemicals • Moisture • Temperature • Vibration

45. Earthworm Reproduction • Hermaphrodites that do not self-fertilize • Mating • Press ventral surfaces together • Anterior ends are pointed in opposite directions • Held together by mucous and setae at the clitellum • Sperm is injected and moves to seminal receptacle • Each worm then secretes a tube of mucous and chitin where sperm and eggs meet outside the body • 2-3 week development prior to hatching