Mollusks Squishy Squad Categories Gastropoda ex snail Bivalvia

Mollusks Squishy Squad

Categories • Gastropoda (ex: snail) • Bivalvia (ex: clam) • Cephalopoda (ex: squid) • • • Schaphopoda Monoplacophora Polyplacophora Aplacophora Caudofoveta

Evolution • There are fewer cephalopods now. – Only 550 species – Vertebrate competition? • Overall, Mollusks are very successful. – 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 – Mouth – Nervous structures – Sensory structures • Foot – Attachment – Locomotion

Body plan – visceral mass • Contains organs for – 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 – 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 1 st – Some have operculum 2. Clean water enters mantle cavity 3. Sensory organs in front

Waste exits above head? • Evolutionary adaptations – 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 – Less room at tight end – Paired organs single organs

Locomotion • Crawling – 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 – 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 – Move blood to move body – Retract quickly, expand slowly

Nervous System • Eyes – base on top or bottom of tentacle – Simple pit of photoreceptors – Lens and cornea • Statocysts on foot • Ophradia – chemoreceptors – 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) – Oral shield and radula for food intake • One pair of gills

Caudofoveata • Dioecious – Sexual reproduction – Fertilized eggs are brooded – Larvae swim freely

Solenogastres • 250 species of marine animals • Similar to caudofoveata • BUT!!! – 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 – 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 – 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 – Ciliated adhesive knobs on tentacles (captacula) • Extend from head • Food carried to head • From radula to gizzard

Scaphopoda • No tentacles, eyes, or osphradia • Dioecious