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Mollusks Squishy Squad Categories • Gastropoda (ex: snail) • Bivalvia (ex: clam) • Cephalopoda (ex: squid) • • • • • Schaphopoda Monoplacophora Polyplacophora Aplacophora Caudofoveta Evolution • There are fewer cephalopods now. o o Only 550 species Vertebrate competition? • Overall, Mollusks are very successful. o o o Over 100,000 species Gastropoda Bivalvia • Triploblastic, coelomic Characteristics 1 • 2 part body: head-foot, visceral mass • Mantle secretes shell, covers visceral mass • Mantle cavity – excretion, gas exchange, elimination, release of gametes • Bilateral symmetry Characteristics 2 • Trochophore larvae, spiral cleavage, schizocoelous coelom formation • Coelom surrounds heart, nephridia, and gonads • Mostly open circulatory system (not in cephalopoda) • Radula used in scraping food Body plan – head-foot • Anterior head o o o Mouth Nervous structures Sensory structures • Foot o o Attachment Locomotion Body plan – visceral mass • Contains organs for o o o o Digestion Circulation Reproduction Excretion • Dorsal to head-foot Body plan – mantle • • • • Outer layer Attaches to visceral mass May secrete a shell (3 layers) Mantle cavity (excretion, gas exchange, elimination, release of gametes) Body plan - radula • Row of teeth on a chitinous belt o o Sits on odontophore (tongue) Moves back and forth to bring food in Gastropods Limpets and Snails and Slugs Oh my! A large class • 35,000 living species • Marine, freshwater, and terrestrial • Helix pomatia • Garden snails • Int. host of human parasites Torsion • Occurs in early development • 180° twisting of VM, mantle, and mantle cavity • Moves gills, anus, & openings from excretory and rep. systems just behind the head Torsion • Plausible advantages 1.Head enters shell 1st Some have operculum 2.Clean water enters mantle cavity 3.Sensory organs in front Waste exits above head? • Evolutionary adaptations o o Waste exits through notches behind head Detorsion Twist 180° Untwist 90° Waste exits to side, behind head Shell coiling • Earliest fossils coiled in one plane • Most modern snails like the one at the bottom right o o Less room at tight end Paired organs single organs Locomotion • Crawling o o Cilia Muscular waves (bonus snail) • Modified foot for clinging • Swimming (sea butterfly & sea hare) Feeding • • • • • Scrape algae using radula Herbivores Scavengers Parasites Predators (whelk proboscis) Digestion • • • • • Tract is ciliated Food trapped in mucus strings Protostyle - mucus mass in stomach Digestive enzymes Wastes as fecal pellets Respiration • Always involves mantle cavity • Modern orgs have one gill • Some have a siphon o o Inhalant tube Burrowers extend it up • Gills lost or reduced in land orgs Open Circulatory System • • • • Leaves the vessels Enters sinuses to bathe cells Heart w/single auricle and ventricle Blood acts as hydraulic skeleton o o Move blood to move body Retract quickly, expand slowly Nervous System • Eyes – base on top or bottom of tentacle o o Simple pit of photoreceptors Lens and cornea • Statocysts on foot • Ophradia – chemoreceptors o o Detect sediment Detect prey The other classes • • • • • Caudofoveta Solenogastres Monoplacophora Polyplacophora Schaphopoda Caudofoveata • • • • • 120 worm-like species Marine 2 - 140 mm in length Orient vertically in sand Gills pointed up Caudofoveata • Feeding Feed on microorganisms and detritus (dead stuff) o Oral shield and radula for food intake o • One pair of gills Caudofoveata • Dioecious o o o Sexual reproduction Fertilized eggs are brooded Larvae swim freely Solenogastres • 250 species of marine animals • Similar to caudofoveata • BUT!!! o o No radula No gills • Bottom dwellers • Feed on cnidarians Monoplacophora • Once thought extinct • 1952 – Neopilina found • 25 species now known • Small w/ low, rounded shell • Mouth has radula • Look like limpets Monoplacophora • Unlike other mollusks – repeating organs o o o o o 3 to 6 pairs of gills 2 pairs of auricles 3 to 7 pairs of metanephridia 1 to 2 pairs of gonads 10 pairs of pedal nerves Polyplacophora • Chitons • (many plate bearers, Coat of mail) • 1000 species • Most 2-5 cm, largest 30 cm Polyplacophora • Dorsoventrally flattened • Convex dorsal surface w/7-8 limy plates or valves • Esthetes – photosensitive Polyplacophora • Blend in w/rocks • Homebodies • Can roll up like an armadillo Polyplacophora • • • • • • Mantle cavity is tube-like Closes at low tide Some can breathe air 3 chambered heart Pair of metanephridia 2 pairs of longitudinal nerve cords • Dioecious Scaphopoda • Tusk shells or tooth shells • From subtidal zone to 6000 m deep • 900 species • Most are 2.5-5 cm • range from 4-25 mm Scaphopoda • Mantle wrapped around VM • Shell open at both ends o o o Foot at wider end Burrows into sand or mud Leaves small end exposed Scaphopoda • Water circulates via foot and cilia movements • No gills (gas exchange occurs in mantle) Scaphopoda • Food Detritus & protozoa from substratum Caught on cilia on foot OR o Ciliated adhesive knobs on tentacles (captacula) o o Extend from head Food carried to head From radula to gizzard Scaphopoda • No tentacles, eyes, or osphradia • Dioecious