Phylum Mollusca

Characteristics

Unsegmented, soft body

Bilaterally symmetrical

Gastropods, Bivalves, Cephalopods (and others)

Body is covered by a mantle which can be thin (bivalve) or thick and muscular (octopus)

Locomotion is by ventral “foot” or arms/tentacles

Classes of Molluscs

Class Gastropoda - snails, sea slugs

Class Bivalvia - clams, mussels, oysters

Class Cephalopoda - nautilus, squid, octopus

Class Gastropoda

Gastropod - “stomach footed”

Univalve (snail) or no shell (nudibranch - sea slug)

Shell usually coiled

Mantle - thin layer of tissue that secretes the shell

Radula (ribbon of teeth) to scrape algae, or eat prey

Chitin - tough material the radula is made from

Class Bivalvia

Clams, mussels, oysters, scallops

Two valves (shells), attached by a hinge

Umbo - the bump at the dorsal end of the valve, formed first

Growth rings - rings on shell that determine age

Two adductor muscles keep valves closed

Ctenidia - gills to absorb oxygen and to filter food from the water (filter feeders)

Inhalant/exhalant siphons - where water enters and leaves mantle cavity

“Foot” - used to dig in order to burrow

Clams

Use their shovel shaped foot to burrow in sand. When buried, water is drawn in and out of the incurrent and excurrent siphons

Scallops

They swim by repeatedly opening and closing their shells and rapidly ejecting water from their siphon

Small blue eyes to detect motion and light

Mussels

Secrete strong sticky fibers called byssal threads for attachment to hard surfaces (not all)

Oyster Reefs

Oysters form reefs by cementing their shells to a hard surface or other oysters

Oyster reefs can serve as barriers to storms and tides, preventing erosion

Oysters remove pollutants through filtration and provide habitat for many species

Pearls

Many bivalve species make pearls around foreign objects (like a parasite or sand) that get caught between the mantle and the shell.

Bivalves secrete layers of iridescent NACRE (a pearly protective coating) over foreign object

Reproduction in Gastropods

hermaphrodites

cross fertilization

lay eggs

Reproduction in Bivalves

spawning

external fertilization

lay eggs

Cephalopods

Nautilus - external shell

Squid - internal shell

Cuttlefish - internal shell

Octopus - no shell at all

“Head-footed” foot is modified into arms/tentacles attached to head

Large eyes/advanced vision

Thick mantle (outer layer of skin) protects the organs

Strong capacity for learning, color/texture changing ability

Gills in mantle cavity

Jet Propulsion

Siphon is fleible and can point toward arms or tail

Cephalopods fill the mantle cavity with water and force it out of the siphon

Squid are propelled in OPPOSITE direction of the flow of water out of their siphon

Nautilus

“Living Fossil” - same as it was 500 million years ago

Shell is chambered to control its buoyancy

90 short, suckerless tentacles

Capture crabs and fish, live in deep water, has siphon

Shell may be up to 10 inches in diameter

Squid

Body elongated, covered by mantle, two triangular fins

8 arms and 2 tentacles

sharp beak

tentacles shoot out

Pen - reduced internal shell

Chromatophores - color changing cells on skin

Octopus

Bottom dwellers

Crawl and swim (siphon)

Chromatophores

Eight arms with suction cups

NO shell

Beak-like jaw, venomous bite

Ink sac - dark ink and mucus

Cuttlefish

Flattened body, paired fins span length of mantle (skirt)

Chromatophores

8 arms/2 tentacles/siphon

Beak, ink sac

W-shaped pupils

Cuttlebone - gas filled internal shell, buoyancy

Circulatory System in Bivalves and Gastropods

Open circulatory system (blood bathes tissues directly)

A single, two chambered heart

Circulatory System of Cephalopods

Closed circulatory system (blood enclosed in vessels)

3 hearts - 2 gill hearts and 1 for the rest of the body

Cephalopod Reproduction

Males use a modified arm to transfer a SPERMATOPHORE (a packet of sperm) to a female

Female octopuses protect their eggs until birth and usually die afterward from lack of nutrition

Other Molluscs (not in Gastropoda, Bivalvia, or Cephalopoda)

Tusk Shell

Shell tapered like an elephant tusk.

Thin tentacles w/ adhesive tips

Chiton

Eight overlapping plates that cover a slightly arched dorsal surface.