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University of Ottawa

MolluscaLophotrochozoaThe phylum includes a diverse assemblage of organisms that includes jetpropelled predatory squids Cephalopoda with visual and intellectual capacity that may rival ours sedentary burrowing clams Bivalvia that filter particulate food from the surrounding water and slowcreeping snails and slugs GastropodaTwo of these characteristics are the mollusc shell and the mantle that secretes the shell The shell usually consists of three layers outer periostracum middle prismatic and inner nacreous layer in the cephalopods it has been reduced to what is called the pen or lost to make these predatory animals lighter and more agileMolluscs were the first animals to scrape organic matter from the substrate using their unique radula essentially a biological file or scraper The scrapings are trapped in mucus and pass through the mouth to a stomach where digestion starts In many molluscs a crystalline style A rodlike structure in some mollusc stomachs made of enzymatic proteins required for digestion Cilia lining the stomach rotate the crystalline style which grinds against the gastric shield to release the digestive enzymes helps to wind the mucus string into the stomach where ciliary sorting fields in the stomach send only appropriately sized food particles into the digestive gland for final intracellular digestion Larger particles pass to an intestine and an anus which opens into the excurrent flow of the mantle cavityThe nervous system varies in different groups but consists primarily of a nerve ring that surrounds the esophagus andpairs of nerve cords one that twoextends into the visceral mass and the second into the foot with both connecting to various ganglia located in the two parts of the body Sensory structures found in the phylum include osphradia to detect water quality statocysts eyespots and imageforming eyes Molluscs may be monoecious or dioecious and lay eggs that undergo spiral cleave and either direct development or hatch as larvae that often develop into a trochophore larva and veliger larva common in many molluscsRadulaThe radula gave molluscs a way of feeding completely different from how any other animal fedWhile all this was going on predatory cnidarians were feeding on other animals in the zooplankton The solution was to scrape it off and molluscs used their radula to do that For molluscs the radula combined with the other traits that you will be looking at allowed this phylum to undergo the extensive adaptive radiation that has made molluscs the second largest animal phylum third if there are as many nematodes as biologists thinkThe radula is a ribbon of teeth supported underneath by the tonguelike odontophore As the radula is extended and retracted it scrapes away at the substrate creating particulate food The usual solution for getting particulate food into the digestive tract is to trap it in mucus then use cilia to propel the foodladen string of mucus into the mouthAs the radula grinds away at the substrate the teeth on the end are worn down Theyre replaced by new teeth added at the back of the radula which as it grows moves forward on the surface of the odontophoreShellandmantleThe shell originated as a defensive strategy by early molluscs to protect themselves using calcareous spicules or spines embedded in their outer epidermisOver time the spicules became larger forming plates that later fused with each other to create a solid shell covering the entire dorsal surface of the animal The specialized epidermis and its gland cells that produced the shell are the mantle
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