Lec 13: Mollusca

• Mollusca is one of the largest animal phyla after Arthropoda.
• Molluscs are coelomate lophotrochozoan protostomes, and as such they develop via spiral mosaic cleavage and make a coelom by schizocoely.
• The name Mollusca indicates one of their distinctive characteristics, a soft body.
• LOPHOTROCHOZOAN- comes from the names of the larval type of the two major animal groups included: lophophorata and trochozoa
• Lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth
• According to fossil evidence, molluscs originated in the sea, and most of them have remained there.
• Only bivalves and gastropods moved into brackish and freshwater habitats.
• Only slugs and snails (gastropods) actually invaded the land.
• The mollusc body plan has a head-foot portion and a visceral mass portion
• The head-foot contains the feeding, cephalic sensory, and locomotor organs. It depends primarily on muscular action for its function.
• The visceral mass is the portion containing digestive, circulatory, respiratory, and reproductive organs, and it depends primarily on ciliary tracts for its functioning
• Two folds of skin, outgrowths of the dorsal body wall, form a protective mantle, which encloses a space between the mantle and body wall called the mantle cavity.
• The mantle cavity houses gills (ctenidia) or a lung, and in some molluscs the mantle secretes a protective shell over the visceral mass.
• Most molluscs have well-developed heads, which bear their mouth and some specialized structures such as sensory tentacles and photoreceptors
• The radula is a rasping, protrusible, tonguelike organ found in all molluscs except bivalves and most solenogasters
• The radula is a ribbonlike membrane bearing a surface of tiny, backward-pointing teeth used for feeding
• The usual function of the radula is twofold: to rasp fine particles of food material from hard surfaces and to serve as a conveyor belt for carrying particles in a continuous stream toward the digestive tract.
• The molluscan foot is usually a ventral, sole-like structure in which waves of muscular contraction effect a creeping locomotion
• The laterally compressed “hatchet foot” of bivalves, or the siphon for jet propulsion in squids and octopuses.
• Secreted mucus is often used as an aid to adhesion or as a slime tract by small molluscs that glide on cilia
• In snails and bivalves the foot extends from the body hydraulically, by engorgement with blood.
• The mantle is a tissue that extends outward from the dorsal midline of the visceral mass.
• The outer surface of the mantle secretes the shell, but the mantle also contains sensory receptors and may be used for gas exchange.
• As the mantle grows outward, it takes on the form of a cape: the sides of the cape wrap around the body, enclosing a space next to the visceral mass. This space is called the mantle cavity.
• Mantle cavity usually houses respiratory organs (gills or lung), which develop from the mantle
• Products from the digestive, excretory, and reproductive systems are emptied into the mantle cavity.
• The gills sit within the mantle cavity and extract oxygen from the water moving through it. Blood inside the gills transports oxygen throughout the body.
• Many molluscs can withdraw their head or foot into the mantle cavity, which is surrounded by the shell, for protection.
• A mollusc ctenidium (gill) consists of a long, flattened axis extending from the wall of the mantle cavity
• Water is propelled by cilia between gill filaments, and blood diffuses from an afferent vessel in the central axis through the filament to an efferent vessel.
• Direction of blood movement is opposite to the direction of water movement, thus establishing a counter-current exchange mechanism
• The shell of a mollusc, when present, is secreted by the mantle and is lined by it
• The periostracum is the outer organic layer, composed of an organic substance called conchiolin, which consists of quinone-tanned protein
• The periostracum helps to protect underlying calcareous layers from erosion by boring organisms.
• The middle prismatic layer is composed of densely packed prisms of calcium carbonate (either aragonite or calcite) laid down in a protein matrix
• The periostracum is secreted by a fold of the mantle edge, and growth occurs only at the margin of the shell.
• The prismatic layer is secreted by the glandular margin of the mantle, and increase in shell size occurs at the shell margin as the animal grows.
• The inner nacreous layer of the shell lies next to the mantle and is secreted continuously by the mantle surface, so that it increases in thickness during the life of the animal.
• Freshwater molluscs usually have a thick periostracum that gives some protection against acids produced in the water by decay of leaf litter.
• In many marine molluscs the periostracum is relatively thin, and in some it is absent.