Phylum Mollusca

General Characteristics Soft-bodied invertebrates Internal or external shell Second largest animal phylum Include snails, slugs, clams, squids, and octopi Fully-lined coelom Show cephalization

General Characteristics Free-swimming, larval stage called a trochophore Development very similar to annelids which shows common ancestry Bilaterally symmetrical Most have separate sexes that cross-fertilize eggs Gills between mantle and visceral mass used for gas exchange

General Characteristics Includes four classes: Polyplacophora (chitons) Gastropoda (snails, slugs, nudibranchs, and conchs) Bilvalvia or Pelecypoda (clams, oysters, and mussels) Cephalopoda (squids, octopus, and nautilus)

Body Plan of Mollusks Four main parts: foot, mantle, shell, and visceral mass Foot: muscular; used for crawling, burrowing, or capturing prey Can be flat (snails), spade-shaped (clams), or modified into tentacles (squid and octopus)

Body Plan of Mollusks Body covered with protective mantle: thin layer of tissue that covers most of the mollusk’s body; may or may not produce a shell Visceral mass: body organs lying below and protected by the mantle (all internal organs) Shell: made by glands in mantle that secrete calcium carbonate

Feeding Can be herbivores, carnivores, filter feeders, detritivores, or parasites Snails and slugs use radula for feeding Radula: flexible, tongue-shaped structure covered with tiny teeth Herbivores use radula to scrape; carnivores use radula to drill through shells or flesh

Feeding Clams, oysters, and scallops filter feed using feathery gills Mucus produced by gills traps plankton as water comes in through siphons Siphon: tubelike structure through which water enters and leaves Octopi and squid use tentacles and sharp bird like beak to capture and eat prey

Respiration Aquatic species use gills inside mantle to exchange gases Terrestrial species use mantle cavity lined with blood vessels to capture air and exchange gases Lining must be kept moist to allow for diffusion of oxygen and CO2 exchange

Circulation Can be open or closed circulatory system Open system: blood is pumped by simple heart into sinuses (large saclike space) then to gills, then back to heart; slower system; found in snails and clams Closed system: blood is pumped by heart through blood vessels; faster system; found in faster-moving mollusks

Excretion Nitrogen-containing wastes (ammonia) is released from blood through nephridia

Response/Nervous System Can be simple or very complex Bivalves have small ganglia, few nerve cords, and simple sense organs Cephalopods have well-developed brains, good memory, and complex eyes; Can be taught to do many tasks: open jar, perform for reward

Movement All use foot for movement Snails/slugs use mucus and rippling motion of foot Aquatic species use siphons to suck in water and shoot it out “jet propulsion”

Reproduction Sexual reproduction External fertilization Release enormous amounts of eggs and sperm into water Free swimming larvae In tentacled species, fertilization is internal and female lays fertilized eggs

Class Polyplacophora Common name is chiton All marine Shell divided into 8 over-lapping plates Live on rocks along seashore Feed on algae

Class Gastropoda Head has a pair of retractable tentacles with eyes located at the ends Have a single shell or valve (snails) or none (slugs) Known as univalves

Class Gastropoda PTEROPODS Called "sea butterflies“ Marine Have a wing-like flap for swimming

Class Gastropoda NUDIBRANCHS Marine slug Lacks shell Can be poisonous

Class Bivalvia Sessile or sedentary Includes marine clams, oysters, & scallops and freshwater mussels Filter feeders Have two-part, hinged shell (2 valves) Have muscular foot that extends from shell for movement Scallops clap valves together to move

Class Bivalvia Shell secreted by mantle & made of 3 layers --- outer horny layer protects against acids, middle prismatic layer made of calcium carbonate for strength, & inner pearly layer next to soft body Mantle secretes substance called "mother of pearl" to surround irritants like grains of sand Oldest, raised part of shell called umbo

Class Bivalvia Powerful anterior & posterior adductor muscles open & close shell No distinct head Have an incurrent & excurrent siphon that circulate water over the gills to remove food & oxygen Have heart & open circulatory system

Class Bivalvia Nervous system made of 3 pairs of ganglia, nerve cords, & sensory cells that detect light, chemicals, & touch Separate sexes with external fertilization of eggs

Class Cephalopoda Includes octopus, squid, cuttlefish, & chambered nautilus   All marine Most intelligent mollusk Well developed head Active, free swimming predators Foot divided into tentacles with suckers   

Class Cephalopoda Use their radula & beak to feed Closed circulatory system Lack external shell Highly developed nervous system with vertebrate-like eyes Separate sexes with internal fertilization

Class Cephalopoda SQUID Largest invertebrate is the Giant Squid Large, complex brain Ten tentacles with longest pair to catch prey Use jet propulsion to move by forcing water out their excurrent siphon Chromatophores in the skin can help change squid color for camouflage

Class Cephalopoda Can squirt an inky substance into water to temporarily blind predators Have internal shell called pen Female lays eggs in jellylike material & protects them until hatching

Class Cephalopoda OCTOPUS Eight tentacles Similar to squid Crawls along bottom looking for prey

Class Cephalopoda CHAMBERED NAUTILUS Has an exterior shell Lives in the outer chamber of the shell Secretes gas into the other chambers to adjust buoyancy

Economic Importance of Mollusks Used  by humans for food Pearls from oysters Shells used for jewelry Do crop & garden damage Serve as intermediate hosts for some parasites such as flukes