phylum+mollusca

THE COELOMATE ANIMALS
 * All groups studied after the pseudocoelomates have a true coelom (eucoelomate)
 * The coelom develops as a fluid-filled space within the mesoderm and serves as a hydrostatic skeleton
 * The body organs lie within the coelom and are held in place by mesodermal membranes, or mesenteries
 * The mesenteries contain blood vessels, sending a rich blood supply to all of the internal organs

PHYLUM MOLLUSCA
 * Nearly 50,000 living species and about 35,000 fossil species (extinct)
 * The term 'molllusca' means soft and refers to the body structure

MOLLUSCAN FACTS * Cephalopods (squids, octopuses) are the largest and most intelligent invertebrates
 * There is a wide range of size from almost microscopic to the giant squid // Architeuthis, // which may be 18 meters long. The shells of giant clams may weigh more than 225 kg (almost 500 pounds)
 * Very diverse group--sluggish to very swift; herbivores, carnivores, filter feeders, detritus feeders, parasites
 * Most are freshwater or marine, only a few snails are terrestrial
 * Widely used as food
 * Pearls are produced by clams and oysters--mainly by a marine oyster found in Eastern Asia, // Meleagrina //

SPECIAL CHARACTERISTICS OF MOLLUSCS 1)The head-foot body plan Molluscs have a well-developed nervous system with various sensory organs for touch, smell, taste, vision, and equilibrium 2)The foot The ventral body wall forms a muscular foot used mainly for locomotion, but also used for attachment in limpets and as a siphon for propulsion in cephalopods 3)The radula A scraping, tongue-like organ called the radula is present in all groups but the bivalves Up to 250,000 teeth can be present and are used for classification 4)The mantle These folds also create a mantle cavity which contains the gills or lungs 5)The visceral mass
 * Most molluscs have a well-developed head, along with a muscular foot **
 * Cephalopods have highly developed eyes **
 * Engorgement with blood extends the foot in some molluscs; mucus is sometimes secreted to produce a slime tract **
 * This organ contains rows of tiny teeth that point backwards and help to scrape food from surfaces **
 * The dorsal body wall forms a pair of folds called the mantle, which is responsible for secreting the shell of most molluscs **
 * This internal part of the body cavity contains the digestive, circulatory, respiratory, and reproductive organs

GENERAL CHARACTERISTICS OF MOLLUSCS none // Optional: // comment for page history 
 * Bilateral symmetry
 * The coelom mainly contains the heart, which is usually three-chambered
 * The circulatory system is open (with blood vessels connected by open spaces called sinuses), except in the cephalopods where the system is closed (a complete network of blood vessels)
 * Digestive system is complex
 * Gases are exchanged by gills, lungs, the mantle, or the body surface
 * One or two kidneys are present
 * Most are dioecious (separate sexes), but monecious forms also exist; the larval forms are called trochophores