Phylum Mollusca (Chapter 27.4) Please set up your notebook for Cornell Notes Mollusks Characteristics Soft-bodied w/ external or internal shell Have 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 Feeding Herbivores, carnivores, filter feeders, detritivores or parasites Snails 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 eat Some octopi use poison to subdue prey Clams, oysters and scallops are filter feeders Siphon 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 Movement Snails secrete mucus along base of foot then use rippling motion Octopi use jet propulsion by drawing water into mantle and expelling from siphon Reproduction Sexual External fertilization release large number of eggs and sperm into water Snails and two shelled mollusks Internal fertilization Tentacled mollusks and some snails Some 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 mollusks Clams, oysters, mussels, scallops Tend to stay in one place Filter 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