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Phylum Mollusca Fact Sheet Phylum Mollusca constitutes as one of the largest phyla of animals: - # of living species # of individuals Significant characteristic is the possession of a coelom (a fluid-filled cavity that develops within the mesoderm): - Functions as hydrostatic skeleton Provides space within which internal organs can be suspended All mollusks have a soft body: - Usually protected by a hard, calcium-containing shell In some cases shell has been lost over the course of evolution (slugs, octopuses) or greatly reduced in size and internalized (squid) All mollusks have the same fundamental body plan: - Head-foot: contains both sensory and motor organs Visceral mass: contains well developed organs of digestion, excretion, and reproduction Mantle: specialized tissue that enfolds the visceral mass and secretes the shell Excluding cephalopods, mollusks have an open circulatory system. Cephalopods have a closed circulatory system because they complete vigorous activities that require the cells to be supplied with large quantities of oxygen and food particles. All members except for bivalves have a radula for feeding. A radula is a ribbon-like set of hooked teeth. Class Bivalvia - 7,500 living species Derive name from the two parts the shell is divided into Adductor muscles are used close the shell swiftly and tightly in times of danger Lack defined heads, no radula, foot is laterally compressed, large mantle cavity with gills, eyes may be present elsewhere on body Have open circulatory system, poor sensory organs, simple nervous system Separate sexes, fertilization can be internal or external depending on the water Most adults are sedentary, herbivorous filter feeders, cilia on gills bring in food particles, mostly microscopic algae Class Gastropoda - Largest group of mollusks Have either a single shell or no shell Common in both freshwater and saltwater Land-dwelling snails do not have gills, but the area in their mantle cavities once occupied by gills is rich in blood vessels, and the snail's blood is oxygenated there More mobile life than bivalves Have a ganglionated nervous system with as many as six pairs of ganglia connected by nerve cords, concentration of nerve cells in the tentacles, at the anterior end In some, eyes are highly developed Some have separate sexes but most are hermaphrodites Have an operculum to protect their soft bodies from environmental hazards or predators Class Cepholapoda - Most evolutionarily advanced animals to be found among the invertebrates 600 living species, bilaterally symmetrical Rival the vertebrates in complexity and, in some cases, in intelligence Only modern shelled cephalopod, Nautilus Mantle has more flexibility, due to freedom from shell Effect of this is the jet propulsion by which cephalopods dart through the water Have sacs from which they can release a dark fluid that forms a cloud, concealing their retreat and confusing their enemies Have well-developed brains, composed of many groups of ganglia, in keeping with their highly developed sensory systems and their lively, predatory behavior Separate sexes, internal fertilization, courtship behaviors are complex, males fight for access to females Class Polyplacophora - 900 living species Aka chitons Shell consisting of eight overlapping plates Bilaterally symmetrical Well-developed foot surrounded by a groove for gills Head lacks eyes and tentacles Has light sensitive areas and chemical receptors for finding food and heading in the right direction Make a living by grazing algae from rocks Many live in shallow waters but few occur in depths greater than 5,000 meters