Reading Essentials for Biology Glencoe (794133), страница 74
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They can oftenbe found moving over leaves on a forest floor. Some mollusks,including oysters, live most of their lives attached to the oceanfloor. Oysters also may attach themselves to the underwater partsof docks or boats. Other mollusks, including the octopus, swimeasily in the ocean.Some mollusks have a shell. Other mollusks, including slugsand squids, do not have a hard outer covering. All mollusks havebilateral symmetry, a coelom, a digestive tract with two openings,a muscular foot, and a mantle. A coelom is a fluid-filled bodycavity that is completely surrounded by the mesoderm. Themantle (MAN tuhl) is the membrane that surrounds the internalorgans of the mollusk. In mollusks that have shells, the mantlesecretes the shell.
Some mollusks, like snails, have adapted tolife on land.________________________________________________________________________326Chapter 27READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is a Mollusk?NameDateClassSection27.1Mollusks, continuedAlthough molluskslook different from oneanother on the outside,they are similar inside.Refer to the figure at theright.
Compare the similarities and differencesin the structures of asnail and a squid. Noticethat both the snail andthe squid have a mantle,shell, head, and gut. Thefoot area of the squidhas been modified intotentacles and arms.TentacleArmSquidHeadReduced internal shellMantleGutMantleSnailShellGutHeadFootCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How do mollusks obtain food?Snails and many other mollusks use a radula (RA juh luh) toobtain food.
A radula is a structure located in the mouth of mollusks. Similar to a rough file, the radula is a tonguelike organ withrows of teeth. Mollusks use their radulas to drill, scrape, grate,and even to cut food. Octopuses and squids capture food withtheir tentacles. They use their radulas to tear up the food theyhave caught.
Some mollusks are grazers. Others, including bivalves,are filter feeders. They do not have radulas. Instead, they filtertheir food from the water.How do mollusks reproduce?2. Infer Which of the following best explains whybivalves are filter feeders?(Circle your choice.)a. They do not have amantle.b. They do not have aradula.c. They swim freely inthe ocean.Mollusks reproduce sexually and most mollusks have separatesexes. For most mollusks that live in water, eggs and sperm arereleased at the same time.
External fertilization then takes place.Many mollusks that live on land are hermaphrodites. Hermaphrodites are plants or animals that have both female and male reproductive organs. Many gastropods, the largest class of mollusks,produce both eggs and sperm, and fertilization takes place withinthe animal. Some bivalves also are hermaphrodites, producingboth sperm and eggs.Mollusks have different appearances as adults but they developin similar ways.
In one larval stage, most mollusks resemble spinning tops with tufts of cilia. Cilia are tiny hairlike structures thatbeat in order to produce movement. Most of these larvae willswim freely in the water until they settle down on the ocean floor.They will spend their adult lives on the ocean floor.READING ESSENTIALSChapter 27327NameDateClassSection27.1Mollusks, continuedMost sea snails and bivalves have another stage before reachingadulthood called a veliger. In this stage, the beginnings of thefoot, shell, and mantle are visible.Do mollusks have nervous systems?Mollusks have simple nervous systems.
The function of the nervous system is to coordinate movement and behavior. The moreadvanced mollusks have brains. Most mollusks have paired eyes.The eyes can range from simple cups that detect light to complexeyes with irises, pupils, and retinas. Octopuses have complex eyesthat are similar to the eyes of humans.________________________________________________________________________4. Why do mollusks havegills?________________________________________________________________________5. What is the purpose ofthe nephridia?________________________________________________________________________328Chapter 27Mollusks have well-developed circulatory systems that includeeither a two- or three-chambered heart.
In most mollusks, theheart pumps blood through an open circulatory system. In anopen circulatory system, blood moves through vessels and intoopen spaces around the body organs. In an open circulatory system, body organs are directly exposed to blood that containsnutrients and oxygen.
The blood removes metabolic waste fromthe organs. Other mollusks, like octopuses, have closed circulatory systems. In a closed circulatory system, blood movesthrough the body, but the blood is entirely enclosed in the bloodvessels. The blood moves nutrients and oxygen through theclosed blood vessels. A closed system provides an efficient meansfor gas exchanges within the animal.Respiration and excretion in mollusksMost mollusks have respiratory structures called gills.
Gills arespecialized parts of the mantle. They are a system of tiny strandsthat contain a rich supply of blood for transporting gases. Gillsincrease the surface area where carbon dioxide and oxygen areexchanged. In snails and slugs that live on land, the mantle cavityappears to have become a primitive lung.The excretory structures are called nephridia. Nephridia(nih FRIH dee uh) are organs that remove metabolic wastes fromthe animal’s body. Mollusks have one or two nephridia that collectwaste from the coelom. The coelom is located around the heartonly. Wastes pass from the coelom into the mantle cavity and areexpelled from the body by the pumping of the gills. Rememberthat the gills are respiratory structures.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Do mollusks have a circulatory system?3.
What is the differencebetween an open circulatory system and a closedcirculatory system?NameDateClassSection27.1Mollusks, continuedDiversity of MollusksThere are many kinds of mollusks. Three classes, Gastropoda,Bivalvia, and Cephalopoda include the most common and thebest-known species.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Which mollusks belong to the class Gastropoda?The largest class of mollusks is Gastropoda, or gastropods.Gastropods are stomach-footed mollusks.
The name gastropodcomes from the way the animal’s large foot is positioned underits body. Most gastropods, such as snails, abalones, conches,periwinkles, whelks, limpets, cowries, and cones have a shell.They can be found in freshwater, in saltwater, or in moist landhabitats. Gastropods with shells may be plant eaters, predators,or parasites.Other gastropods, like slugs, do not have shells. They have athick layer of mucus that protects their bodies. Nudibranches arecolorful sea slugs. Nudibranches are protected from predators inanother way. When these sea slugs feed on jellyfish, the poisonousnematocysts of the jellyfish are taken into the tissues of the seaslug.
When a fish tries to eat the sea slug, the nematocysts aredischarged into the predator and the predator is repelled. Thebright colors of nudibranches warn predators of the danger beforethey attack.6. What protects the bodiesof gastropods that do nothave shells?________________________________________________________________________Which mollusks belong to the class Bivalvia?Clams, oysters, and scallops belong to class Bivalvia. Bivalveshave two shells. The two shells are connected with a ligament,called a valve. The valve works like a hinge.
Strong muscles allowthe valves to open and close over the soft body of the bivalve.Most bivalve mollusks live in saltwater, but a few species live infreshwater. Some bivalves are tiny, measuring less than 1 mm inlength. Others, such as the tropical giant clam, can be as large as1.5 m long. Bivalves do not have a distinct head. They use theirlarge, muscular foot for digging and hiding in the sand.Bivalves do not have radula, the rough tonguelike organ ofmany mollusks. They are filter feeders. They obtain their food byfiltering small particles of food from the water in which they live.Bivalve mollusks have several features that help them filter feed.They have gill cilia that beat so water can be drawn through theshells. Water and particles in the water move over the gills. Foodis trapped in the bivalve’s mucus.
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