I. Evolutionary Perspective Chapter 12 Molluscan Success Some of the world s best predators Large brains Complex sensory structures Rapid locomotion Grasping tentacles Tearing mouthparts Have been around for about 500 million years II. Molluscan Characteristics Have a coelom Provide room for organ development Surfaces for diffusion of gases, nutrients, and wastes Storage Elimination of reproductive products Hydrostatic support Body of two parts: Head-foot and visceral mass Mantle that covers visceral mass and secretes a shell made of calcium carbonate Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, & releases reproductive products Bilateral symmetry Protostome characteristics including trochophore larvae, spiral cleavage, and schizocoelous coelom formation A. Regions of Molluscan Body Coelom reduced to cavities surrounding the heart, nephridia, & gonads Open circulatory system (except in Cephalopods) Most have radula used for scraping food Head-foot Contains mouth, nervous and sensory structures, elongate foot used for attachment and location Visceral mass Contains the organs for digestion, circulation, reproduction, excretion Positioned dorsally to head-foot 1
Mantle Attached to visceral mass and secretes the shell (if one is present) Mantle Cavity Space between mantle and the foot Opens to the outside of the body Functions in gas exchange, excretion, elimination of digestive wastes, & release of reproductive products Radula Tonguelike structure covered with teeth which is used to scrape food into the mouth III. Class Gastropoda Slugs & snails Torsion - ability to turn the body 180 degrees. Provides advantages: Allows the head to enter the shell first Allows clean water to enter the mantle cavity Allows the snail to sense changes coming from the direction is moving 2
Feeding and Digestion Scrape algae & other small organisms into mouth using radula Food is trapped in mucous strings called a protostyle Digested in the stomach and passed out in the form of fecal pellets Gas Exchange Aquatic snails have gills Land snails have extremely reduced or no gills Some species have a siphon that is in an inhalent tube Open Circulatory System Not all fluid is enclosed in vessels Collects in spaces called sinuses and bathes cells Hydraulic skeleton Fluid in tissue spaces extends into body structures for support Nervous System Six ganglia located in head-foot Eyes at base of tentacles Statocysts in the foot Osphradia (chemoreceptors) detect sediments, chemicals and prey Excretory Paired nephridia Reproduction Marine snails are dioecious Monoecious species have internal fertilization and exchange sperm Eggs develop into trocophore larva that are free-swimming Then develop into veliger larva that have a foot, eyes, tentacles, and shell Then metamorphosis into the adult form IV. Class Bivalvia clams, oysters, muscles, and scallops Shell & Structures Valves - convex halves of shells Umbo - rounded hump at the hinge of the shell. It is the oldest part of the shell Adductor muscles hold the shell closed Main defense against predators Mantle secretes the shell If a sand grain or parasite becomes lodged between the shell and the mantle, nacre is secreted and a pearl will form 3
Gas Exchange Gills covered with cilia trap water into tubules Those that burrow have a siphon for breathing Filter feeding Gills trap food in cilia and move it to food grooves which carry it to stomach Open circulatory system Blood flows from heart to tissue sinuses, nephridia, gills, and back to the heart Nervous System 3 pairs of interconnected ganglia associated with esophagus, the foot and posterior adductor muscle Scallops have photoreceptors with eyes with a lens and a cornea Statocysts for balance Reproduction & Development Most are dioecious External fertilization with eggs developing into veliger larva called a glochidium Attaches itself to a fish and finishes developing into an adult clam V. Class Cephalopoda Octopus, squid, cuttlefish, and nautiluses Anterior portion of foot modified into tentacles Foot modified for jet-propulsion Shells are only present in the nautilus 4
Locomotion Squid and cuttlefish use jet-propulsion for all movements Octopi only use jet-propulsion for a means of escape Feeding & Digestion Locate prey by sight and capture is using the adhesive glands on tentacles Have jaws and a radula that are beak shaped Octopi have salivary glands to inject venom Closed Circulatory System All fluid is contained in vessels 3 chambered heart Nervous System They have a well-developed brain capable of memorization and logic The eye is similar to the vertebrate eye Can form images, distinguish shapes, and discriminate some colors Have chromatophores Pigment cells that change color to allow the cephalopod to blend in with its surroundings Ink glands allow the cephalopod to escape predators Reproductive & Development Dioecious Male have sperm contained in packets called spermatophores One tentacle is modified to place the sperm packet into the oviduct Eggs are fertilized and attach to the substrate Vampire Squid http:// footage.shutterstock.com/video.html? id=9115 http:// www.teachertube.co m/viewvideo.php? video_id=96736 5
VI. Class Polyplacophora Also known as the chitins Have a reduced head, flattened foot, and a shell divided into 8 dorsal valves Separate sexes and external fertilization VII. Class Scaphopoda Tooth shells or tusk shells Conical shell with openings at both ends Spends most of its time buried in the marine substrate Have a radula and tentacles for feeding VIII. Class Monoplacophora Limpets Undivided, arched shell and flat foot Little is know about their embryology IX. Class Aplacophora Known as the solengasters and Caudofoveata Cylindrical molluscs that lack a shell and crawl on their ventral surface Some species live burrowed in the sand 6