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Phylum Mollusca

Phylum Mollusca. Molluscan Characteristics. Large range in size 1 cm long to 18 m All mollusks… 2 Body Parts- head-foot and visceral mass Mantle that secretes a calcareous shell (made of calcium carbonate) and covers the visceral mass. Molluscan Characteristics Continued. All mollusks…

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Phylum Mollusca

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  1. Phylum Mollusca

  2. Molluscan Characteristics • Large range in size • 1 cm long to 18 m • All mollusks… • 2 Body Parts- head-foot and visceral mass • Mantle that secretes a calcareous shell (made of calcium carbonate) and covers the visceral mass

  3. Molluscan Characteristics Continued • All mollusks… • Mantle cavity functions in • Excretion • Gas exchange • Elimination of digestive wastes • Release of reproductive products • Bilateral symmetry • Coelom reduced to cavities surrounding the heart, nephridia, and gonads

  4. Molluscan Characteristics Continued • All mollusks… • Trocophore larvae (aquatic with cilia) • Open circulatory system in all but one class (Cephalopoda) • Radula usually present and used in scraping food

  5. Molluscan Vocabulary • Head-foot: • Anterior head- contains mouth and sensory structures • Elongated foot- used for attachment and locomotion • Visceral Mass: • Contains organs for digestion, circulation, reproduction, and excretion • Dorsal to head-foot

  6. Molluscan Vocabulary • Mantle: • Attaches to visceral mass • May secrete a shell • Mantle Cavity: • Opens to the outside • Functions in gas exchange, excretion, elimination of digestive wastes, and release of reproductive products

  7. Molluscan Vocabulary • Radula: • Functions in scraping food (teeth and tongue in one structure) • Rows of posteriorly curved teeth

  8. Class Gastropoda Snail • Snails, limpets, and slugs • 35,000 living species • Largest and most diverse molluscan class • Used for food (escargot) • Intermediate hosts for some human parasites • Means “stomach foot” Slug Limpet

  9. Class Gastropoda • Torsion: • 180⁰ counterclockwise twisting of the visceral mass, mantle, and mantle cavity • Allows the head to enter the shell first- protects against predators • Operculum (covering on the foot)- prevents the snail from drying out

  10. Class Gastropoda • Coiled shells- assymetrical • Locomotion- ciliated, flattened foot • Feeding: • May scrape organisms (ex. Algae) using radula • May eat plants, dead organisms, parasites, or predators

  11. Class Gastropoda • Gas exchange: • Occurs with the mantle • Modern gastropods have one gill (compared to 2 in ancestors) • Siphon- inhalant tube that is a rolled extension of the mantle • Open circulatory system • Hydraulic skeleton- fluid supports body structure

  12. Class Gastropoda • Reproduction: • Marine snails • Dioecious • External fertilization • Land snails • Monoecious • Internal fertilization • Eggs hatch into free-swimming trochophore larva

  13. Class Bivalvia NB #97 • 30,000 species • 2nd largest molluscan class • Clams, oysters, mussels, and scallops • Sheet-like mantle & shell • 2 valves • Edible • May form pearls • Filter feeders • Remove bacteria from the water • Conchology- study of mollusk shells

  14. Class Bivalvia • Structures: • 2 convex shell halves- called valves • Hinge along shell made of protein • Teeth- • Tongue & groove modifications that prevents the valves from twisting • Umbo: • Oldest part of the shell • Swollen area near the shell’s anterior margin

  15. Class Bivalvia • Structures: • Shell forms as a single structure (although it looks like two) • Hinge- elastic due to more protein and less calcium carbonate (allows the shell to open when certain muscles relax) • Adductor muscles- strong muscles that close the shell (makes it difficult to pry a shell open)

  16. Class Bivalvia • Pearls • Mantle attaches to the shell around the adductor muscles • If one grain of sand lodges between the shell and the mantle, the mantle secretes nacre around the irritant (which may also be a parasite) • This process eventually forms a pearl • Nacre is also referred to as “Mother-of-Pearl” • Process takes approximately two years

  17. Class Bivalvia • Processes • Gas exchange • Gills form folded sheets (lamellae) • One end attached to the foot and the other end attached to the mantle • Cilia move water into the gills • Digestion • The gills also trap food particles • Cilia move food particles from the gills through ciliated food grooves • Considered filter feeders

  18. Class Bivalvia • Filter feeding • Valuable process for the environment • Average oyster filters approximately 6 liters of water an hour • Pollution, harvesting, and environmental changes have reduced bivalve numbers in certain areas

  19. Class Bivalvia • Open circulatory system • Nervous system • 3 pairs of interconnected ganglia • Some species (ex. Scallops) develop photoreceptors as complex eyes with a lens and a cornea Eyes

  20. Class Bivalvia • Reproduction • Most are dioecious • Few are moneocious • Some are protandric (male early and then develop into a female) • Usually reproduce externally

  21. Class Bivalvia • Habitat and Diversity • Aquatic (either marine or freshwater) • May bury themselves, attaches to substrates, or bore in materials (ex. Wood) • The foot secretes strands that cement the animal to it’s location

  22. Class Cephalopoda • Cephalopod- “head-foot” • Octopuses, squid, cuttlefish, and nautiluses • Most complex mollusks (actually most complex invertebrate) • Anterior foot modified into tentacles to capture prey

  23. Class Cephalopoda • Shell • The only living cephalopod with an external shell is the nautilus • As it grows, the nautilus secretes a new shell and leaves the empty shell behind • An internal shell is present in the cuttlefish • Has gas-filled spaces that increase buoyancy • The shell (called a cuttlebone) is used to make powder for polishing or feed pet birds

  24. Class Cephalopoda • Locomotion • Move with a jet-propulsion system • Force water out of the mantle cavity in a funnel • Muscles on the funnel control the direction of movement • Squids have posterior fins for stability • May swim as fast as 30 km/hr • Crawl around using tentacles

  25. Class Cephalopoda • Feeding & Digestion • Locate prey by sight • Capture prey with tentacles • Tentacles are made of protein- occasionally have a hook • Beak- jaws used to tear food • Cuttlefish & nautiluses- eat small invertebrates • Octopuses- nocturnal & eat snails, fishes, & crustaceans (inject venom) • Squid- fishes & shrimp

  26. Class Cephalopoda • Closed circulatory system • Eyes similar to vertebrate eyes • Ink gland located behind the anus • Ink contains melanin • Black or brown in color • Octopus Video

  27. Class Cephalopoda • Nervous system • Complex lobes • 20th Century experiments demonstrated that cephalopods may be trained to attack, kill, and feed when stimulated • May navigate mazes • Can remember information for up to four months

  28. Class Cephalopoda • Reproduction • Dioecious • Males encase sperm in spermatophores • One male tentacle (hectocotylus) is modified for spermatophore transfer • Male and female tentacles intertwine during copulation • Male inserts hectocotylus into female mantle cavity and releases the spermatophore • Females release eggs • Hatchlings resemble the adults • Adults do not care for the young

  29. Other Mollusks Chiton • Class Polyplacophora • Chitons • Shell that divides into eight valves • Class Scaphopoda • Tooth shells or tusk shells • At least 300 species • Shell is open at both ends • Class Monoplacophora • Thought extinct until 1952 • Class Aplacophora • Lack shells Tusk shell Monoplacophora Aplacophora

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