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Chapter

BIO2231: Phylum Mollusca characteristics

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Department
Biology
Course Code
BIO2231
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Various

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PHYLUM MOLLUSCA Characteristics of Phylum Mollusca: o Dorsal body wall forms pair of folds called the mantle, which encloses the mantle cavity, is modified into gills or lungs, and secretes the shell (shell is absent in some); ventral body wall specialized as a muscular foot, variously modified but used chiefly for locomotion; radula in mouth o Live in marine, freshwater, and terrestrial habitats o Free-living or occasionally parasitic o Body bilaterally symmetrical (bilaterial asymmetry in some); unsegmented; often with definite head o Tripoblastic body o Coelom limited mainly to area around heart, and perhaps lumen of gonads, part of kidneys, and occasionally part of the intestine o Surface epithelium usually ciliated and bearing mucous glands and sensory nerve endings o Complex digestive system; rasping organ (radula) usually present; anus usually emptying into mantle cavity; internal and external ciliary tracts often of great functional importance. o Circular, diagonal, and longitudinal muscles in the body wall; mantle and foot highly muscular in some classes (cephalopods and gastropods) o Nervous system of paired cerebral, pleural, pedal, and visceral ganglia, with nerve cords and subepidermal plexus; ganglia centralized in nerve ring in gastropods and cephalopods o Sensory organs of touch, smell, taste, equilibrium, and vision (in some); the highly developed direct eye (photosensitive cells in retina face light source) of cephalopods is similar to the indirect eye (photosensitive cells face away from light source) of vertebrates but arises as a skin derivative in contrast to the brain eye of veterbrates o No asexual reproduction o Both monoecious and dioecious forms; spiral cleavage; ancestral larva a trochopore, many with a veliger larva, some with direct development o One or two kidneys (metanephridia) opening into the pericardinal cavity and usually emptying into the mantle cavity o Gaseous exchange by gills, lungs, mantle, or body surface o Open circulatory system (secondarily closed in cephalopods) of hearts (usually three chambered), blood vessels, and sinuses; respiratory pigments in blood Molluscs: Molluscs are coelomate lophotrochozoan protostomes, and as such they develop via spiral mosaic cleavage and make a coelom by schizocoely. The ancestral larval stage is a trochophore, but development is variously modified within the classes. The coelom in molluscs is limited to a space around the heart, and perhaps around the gonads and part of the kidneys. Although it develops embryonically in a manner similar to the coelom of annelids, the functional consequences of this space are quite different because it is not used in locomotion. Form and Function: Reduced to its simplest dimensions, the mollusk body plan may be said to consist of a head-foot portion and a visceral mass portion. The head-foot is the more active area, containing 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. Modifications of the structures that make up the head-foot and the visceral mass produce the great diversity of patterns observed in Mollusca. Head-Foot: Most molluscs have well-developed heads, which bear their mouth and some specialized sensory organs. Photosensory receptors range from fairly simple ones to the complex eyes of cephalopods. Tentacles are often present. Within the mouth is a structure unique to molluscs, the radula, and usually posterior to the mouth is chief locomotor organ, or foot. Radula: The radula is a rasping, protrusible, tonguelike organ found in all molluscs except bivalves and most solenogasters. It is used for feeding and consists of a ribbonlike membrane on which are mounted rows of tiny teeth that point backward. Complex membranes move the radula and its supporting cartilages (odontophore) in and out of the mouth while the membrane is partly rotated over the tips of the cartilages. 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. As the radula wears away anteriorly, new rows of teeth are continuously replaced by secretion at its posterior end. Foot: The molluscan foot may be variously adapted for locomotion, for attachment to a substratum, or for a combination of functions. It is usually a ventral, sole-like structure in which waves of muscular contraction effect a creeping locomotion. However, there are many modifications, such as the attachment disc of limpets, the laterally compressed ‘hatchet foot’ of bivalves, or the siphlon 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 slide on cilia. In snails and bivalves the foot is extended from the body hydraulically, by engorgement with blood. Burrowing forms can extend the foot into the mud or sand, enlarge it with blood pressure, then use the engorged foot as anchor to draw the body forward. Mantle and Mantle Cavity: The mantle is a sheath of skin, extending from the visceral mass, that hangs down on each side of the body, protecting the soft parts and creating between itself and the visceral mass a space called the mantle cavity. The outer surface of the mantle secreted the shell. The mantle cavity usually houses respiratory organs, which develop from the mantle
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