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Phylum Mollusca Introduction Mollusk – slugs, snails, octopus, squid, clam, oyster 50,000 living mollusk species and about 35,000 fossil species. The number of known species is second only to the Phylum Arthropoda. Introduction The majority of mollusks are marine, but large numbers of species occupy freshwater and terrestrial habitats. Some mollusk swim while others are sedentary. Introduction Oysters produce pearls. Some mollusk shells are highly valued by collectors. In times past these shells were used as money and today are used ornamentally for such items as buttons and jewelry. Common characteristics shared by mollusk Soft, flexible body Bilateral symmetry Eucoelomates (body cavity containing internal organs) -Digestive system with two body openings – a mouth in the head and the anus emptying into the mantle cavity. Most have a radula (except bivalves). The radula is usually toothed (may be a single or could be thousands) and is adapted for scraping, tearing, stabbing, and cutting. In some the teeth are hollow and poisoncontaining and are used as weapons. Thick muscular foot, primarily used for locomotion Mantle (thin membrane surrounding coelom); also secretes the shell in those forms that possess one. A well-developed head, which may bear sensory tentacles (except bivalves) An open circulatory system (except cephalopods, which is closed) with a heart, blood vessels, and hemocyanin. When oxygenated such blood is bluish in color; when deoxygenated the blood is colorless. Respiration - Gas exchange is via gills, lungs, or the body surface. Secrete mucus – uses include locomotion, food entrapment, and prevention of water loss. Classes of Mollusks (3 of 7) 1.Gastropoda –includes snails, limpets, slugs, conchs 2.Bivalvia – oysters, clams, scallops; made of two hinged shells, 3.Cephalopoda – squid, octopus, nautilus GASTROPODS - largest and most diverse class (about 40 000 or 80%); Most have a single coiled shell (made of calcium carbonate) into which the body can be withdrawn. Shells vary in color, shape, ornamentation, and size according to the species. Slugs are shell-less. Shells range in size form microscopic to a 24 inch marine snail ( Pleuroploca gigantean). A sea hare, Aplysia, can reach 3 ft. Feeding Habits – Many are herbivores (snails), with multi-toothed radulas for scraping algae from various substrates. Some are carnivores – feeding on small fish, worms, plankton and on smaller mollusks. The conch feeds on fish and annelid worms that they first paralyze with poison contained in their hollow radula teeth. The poison is also toxic to humans, causing paralysis and sometimes death. Muscular foot is used for movement in most species. Reproduction terrestrial – hermaphrodites Aquatic – external fertilization Nervous system – simple; small brain; nerves coordinate movement and behavior. BIVALVES Two shells, of similar size, held together by a hinge ligament coming together at the umbo; shell covers soft body; muscles open and close (adductor muscle) Shells can range in size – the freshwater seed shells are among the smallest know, being less than 0.1 inch in length, while the shell of the giant clam may exceed 4 ft in length (500 lbs). No distinctive head or radula Feeding habits – most are sedentary filter feeders. Draw water in, water moves over the gills, then exits through; as waste moves over gills, food and sediment trapped in the mucus is sorted and then eliminated Most have a single pair of gills for respiration and for trapping minute food particles. Separate sexes with external fertilization. Most are marine, but many live in brackish water in streams, ponds, and lakes. CEPHALOPODS – “headfooted” Most complex of the mollusks. All are marine. Have tentacles w/suckers instead of a single foot. They are used for movement and capturing food. Have radula and sharp, beak-like jaws to tear apart food. They are carnivorous. Eating fish, bivalves and crabs. Have a closed circulatory system. Only the nautilus has a shell. Move by jet-propelled swimming (66 ft/second) Reproduction –Internal fertilization, then eggs are laid. Tentacles transfer sperm into female’s body. Nervous system – Complex brain w/ ability to learn (capturing prey or avoiding harmful situations). Highly developed sense of vision Respiration – gills Most have chromatophores, special pigment cells, in their skins, which allow them to change color rapidly. May have a sac that secretes an ink-like substance called sepia; this is expelled when the animal is alarmed, hiding the animal as it flees. Cephalopods are worldwide in distribution and are found in all depths of the ocean .