Copyright Pearson Prentice Hall Biology Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 Mollusks Photo Credit: © Jeffrey L. Rotman Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall What Is a Mollusk? What are the defining features of mollusks? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall What Is a Mollusk? What Is a Mollusk? Mollusks are soft-bodied animals that usually have an internal or external shell. Mollusks include snails, slugs, clams, squids, and octopi. Many mollusks share similar developmental stages. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall What Is a Mollusk? Many aquatic mollusks have a free-swimming larval stage called a trochophore. The trochophore larva is also characteristic of annelids, indicating that these two groups may be closely related. Mollusks are an incredibly diverse group, including clams, octopi, and snails, as well as the lesser known cuttlefish, shown here. Many mollusks have a larval stage called a trochophore), shown here, which has at least one band of cilia encircling its body. All mollusks have a soft body, and most mollusks have an internal or external shell. Copyright Pearson Prentice Hall

Form and Function in Mollusks Mollusks have true coeloms surrounded by mesoderm tissue. They have complex, interrelated organ systems that function together to maintain the body as a whole. Copyright Pearson Prentice Hall

Form and Function in Mollusks What is the basic body plan of mollusks? Copyright Pearson Prentice Hall

Form and Function in Mollusks Body Plan The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass. Copyright Pearson Prentice Hall

Form and Function in Mollusks The muscular foot takes many forms flat structures for crawling spade-shaped structures for burrowing tentacles for capturing prey The body plan of most mollusks includes a foot, mantle, shell, and visceral mass. Early mollusks may have looked like the animal shown here. As they evolved, their body parts became adapted for different functions. Squid Snail Clam Copyright Pearson Prentice Hall

Form and Function in Mollusks The mantle is a thin layer of tissue that covers most of the mollusk's body. The shell is made by glands in the mantle that secrete calcium carbonate. Just beneath the mantle is the visceral mass, which contains the internal organs. Copyright Pearson Prentice Hall

Form and Function in Mollusks Feeding  Mollusks can be herbivores, carnivores, filter feeders, detritivores, or parasites. Copyright Pearson Prentice Hall

Form and Function in Mollusks Snails and slugs feed using a flexible, tongue-shaped structure known as a radula. Hundreds of tiny teeth are attached to the radula. The radula is used to scrape algae off rocks or to eat the soft tissues of plants. Snails use a radula for feeding. The teeth of a radula give it the look and feel of sandpaper. Beneath the radula is a stiff supporting rod of cartilage. When the mollusk feeds, it places the tip of the radula on its food and pulls the sandpapery layer back and forth. Teeth Radula Copyright Pearson Prentice Hall

Form and Function in Mollusks Clams, oysters, and scallops use gills. Food is carried by water, which enters the incurrent siphon. A siphon is a tubelike structure through which water enters and leaves the body. Copyright Pearson Prentice Hall

Form and Function in Mollusks The water flows over the gills and leaves by the excurrent siphon. Excurrent siphon The anatomy of a clam is typical of bivalves, or two-shelled mollusks. The mantle and part of the foot have been cut away to show internal organs. The adductor muscles are used to open and shut the two exterior shells. The gills exchange oxygen and carbon dioxide between the body and the surrounding water. The arrows show the path of water over the gills. Incurrent siphon Copyright Pearson Prentice Hall

Form and Function in Mollusks Respiration  Aquatic mollusks breathe using gills inside their mantle cavity. Gills Copyright Pearson Prentice Hall

Form and Function in Mollusks As water passes through the mantle cavity, oxygen in the water moves into blood flowing through the gills. At the same time, carbon dioxide moves in the opposite direction. Copyright Pearson Prentice Hall

Form and Function in Mollusks Land snails and slugs respire using a mantle cavity that has a large surface area lined with blood vessels. Copyright Pearson Prentice Hall

Form and Function in Mollusks Circulation  Some mollusks have open circulatory systems; other mollusks have closed circulatory systems. Copyright Pearson Prentice Hall

Form and Function in Mollusks In an open circulatory system, blood is pumped through vessels by a simple heart. Heart The anatomy of a clam is typical of bivalves, or two-shelled mollusks. Copyright Pearson Prentice Hall

Form and Function in Mollusks Blood leaves the vessels and works its way through different sinuses. Blood passes from the sinuses to the gills, where oxygen and carbon dioxide are exchanged. Blood is then pumped back to the heart. Slow-moving mollusks often have open circulatory systems. Copyright Pearson Prentice Hall

Form and Function in Mollusks Faster-moving mollusks have a closed circulatory system. A closed circulatory system can transport blood through an animal’s body much more quickly than an open circulatory system. Copyright Pearson Prentice Hall

Form and Function in Mollusks Excretion  Cells of the body release nitrogen-containing waste into the blood in the form of ammonia. Copyright Pearson Prentice Hall

Form and Function in Mollusks Nephridia remove ammonia from the blood and release it outside the body. Nephridium The anatomy of a clam is typical of bivalves, or two-shelled mollusks. Copyright Pearson Prentice Hall

Form and Function in Mollusks Response  The complexity of the nervous system and the ability to respond to environmental conditions varies among mollusks. Two-shelled mollusks have a simple nervous system. Copyright Pearson Prentice Hall

Form and Function in Mollusks Octopi and their relatives have the most highly developed nervous system of all invertebrates. Well-developed brains in these animals allows them to remember things for long periods. Copyright Pearson Prentice Hall

Form and Function in Mollusks Movement  Snails secrete mucus along the base of the foot, and then move over surfaces using a rippling motion of the foot. The octopus draws water into the mantle cavity and then forces the water out through a siphon. Water leaving the body propels the octopus in the opposite direction. Copyright Pearson Prentice Hall

Form and Function in Mollusks Reproduction  Some mollusks reproduce sexually by external fertilization. In other mollusks, fertilization takes place inside the body of the female. Some mollusks are hermaphrodites and usually fertilize eggs from another individual. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks What are the characteristics of the three main classes of mollusks? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Groups of Mollusks The three major classes of mollusks are gastropods bivalves cephalopods Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Gastropods Gastropods are shell-less or single- shelled mollusks that move by using a muscular foot located on the ventral side.  Many gastropods have a single shell that protects their bodies. When threatened, they can pull completely into their shells. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Gastropods include pond snails land slugs sea butterflies sea hares limpets nudibranchs Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Bivalves Bivalves have two shells that are held together by one or two powerful muscles.  Common bivalves include: clams oysters mussels scallops Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Cephalopods Cephalopods are typically soft-bodied mollusks in which the head is attached to a single foot. The foot is divided into tentacles or arms. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Cephalopods have eight or more tentacles equipped with sucking disks that grab and hold prey. Most modern cephalopods have only small internal shells or no shells at all. The only present-day cephalopods with external shells are nautiluses. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Cephalopods have complex sense organs that help them detect and respond to external stimuli. Cephalopods distinguish shapes by sight and texture by touch. The eyes of many cephalopods are complex. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Ecology of Mollusks Ecology of Mollusks Mollusks feed on plants prey on animals filter algae out of the water eat detritus Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Ecology of Mollusks Some mollusks are hosts to symbiotic algae or to parasites; others are themselves parasites. Mollusks are food for many organisms. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 The trochophore larva is a characteristic that mollusks share with flatworms. roundworms. annelids. flukes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 Water enters and leaves the body of a bivalve through a siphon. cilia. a coelom. a nephridium. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 The most active group of mollusks is the gastropods. bivalves. cephalopods. shell-less gastropods. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 Unlike the other mollusks, cephalopods have a(an) closed circulatory system. ventral blood vessel. open circulatory system. dorsal blood vessel. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 27-4 The body plan of most mollusks includes all of the following EXCEPT a foot. mantle. shell. radula. Copyright Pearson Prentice Hall

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