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 1st –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)Muscular wavesbonus snail Modified foot for clingingfoot Swimming (sea butterfly & sea hare)sea butterfly sea hare

Feeding Scrape algae using radula Herbivores Scavengers Parasites Predators (whelk proboscis)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 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 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