Phylum Mollusca (Chapter 27.4)Please set up your notebook for Cornell Notes

MollusksCharacteristics

Soft-bodied w/ external or internal shellHave a free swimming larval stage called trochophore

Also appear in annelids

Form and function

Body plan

Bilaterally symmetrical

Four parts

Foot flat structure for crawling, shovel-shaped for burrowing or tentacles for capturing prey

Mantle thin layer of tissue that covers the body

Shell made by glands in the mantle that secrete calcium carbonate

Reduced or lost in some groups

Visceral mass internal organs

FeedingHerbivores, carnivores, filter feeders, detritivores or parasitesSnails and slugs have a raspy, tongue shaped structure known as a radula

Can scrape algae off rocks, drill thru shells and tear tissues

Octopi and some sea slugs use sharp jaws to eatSome octopi use poison to subdue prey

Clams, oysters and scallops are filter feedersSiphon a tube-like structure that brings water in and out of the body

Respiration

Aquatic mollusks use gills inside the mantle cavity

Terrestrial mollusks use a large mantle cavity lined with blood vessels

Lining must be moist

Circulation

Some mollusks have an open circulatory system (ie. Snails and clams)

Other mollusks have a closed circulatory system (ie. Octopi and squid)

Excretion

Nephridia remove ammonia from the blood and release it outside of the body

Response

Simple nervous system small ganglia, few nerve cords and eyespots

Clams, oysters

Complex nervous system well developed brains and can remember things for long periods

Octopi, squid, cuttlefish

MovementSnails secrete mucus along base of foot then use rippling motionOctopi use jet propulsion by drawing water into mantle and expelling from siphon

ReproductionSexual

External fertilization release large number of eggs and sperm into water

Snails and two shelled mollusksInternal fertilization

Tentacled mollusks and some snailsSome mollusks are hermaphrodites but fertilize other organisms eggs

Classes of mollusks

Class Gastropoda shell-less or single shelled, move by using foot on ventral surface

Snails, slugs, nudibranches (sea slugs)

Nudibranches recycle nematocysts from cnidarians to use for protection

Snails withdraw into shell for protection

Class Bivalvia two shelled mollusksClams, oysters, mussels, scallopsTend to stay in one placeFilter feeders

Class Cephalopoda soft-bodied, head attached to a foot that is divided into tentacles

Octopi, cuttlefish, squid, nautilus

Small internal shell or no shell

The only cephalopod with shell is a nautilus

Large eyes