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Mollusks and Annelids. Bio II Rupp. 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

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Mollusks and Annelids


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    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

    5. 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

    6. 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

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

    8. Table 37-1

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

    10. 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

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

    12. 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

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

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

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

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

    17. 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

    18. 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

    19. 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

    20. 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

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

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

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

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

    25. 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

    26. 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

    27. 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

    28. 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

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

    30. Cuttlefish

    31. 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

    32. 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

    33. 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

    34. 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

    35. 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

    36. 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

    37. 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

    38. 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

    39. 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

    40. 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