Biology 1 of 43
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What Is a Mollusk? What are the defining features of mollusks? 3 of 43
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. 4 of 43
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. 5 of 43
Mollusks have true coeloms surrounded by mesoderm tissue. They have complex, interrelated organ systems that function together to maintain the body as a whole. 6 of 43
What is the basic body plan of mollusks? 7 of 43
Body Plan The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass. 8 of 43
The muscular foot takes many forms flat structures for crawling spade-shaped structures for burrowing tentacles for capturing prey Snail Clam Squid 9 of 43
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. 10 of 43
Feeding Mollusks can be herbivores, carnivores, filter feeders, detritivores, or parasites. 11 of 43
Snails and slugs feed using a flexible, tongueshaped 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. Teeth Radula 12 of 43
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. 13 of 43
The water flows over the gills and leaves by the excurrent siphon. Excurrent siphon Incurrent siphon 14 of 43
Respiration Aquatic mollusks breathe using gills inside their mantle cavity. Gills 15 of 43
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. 16 of 43
Land snails and slugs respire using a mantle cavity that has a large surface area lined with blood vessels. 17 of 43
Circulation Some mollusks have open circulatory systems; other mollusks have closed circulatory systems. 18 of 43
In an open circulatory system, blood is pumped through vessels by a simple heart. Heart 19 of 43
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. 20 of 43
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. 21 of 43
Excretion Cells of the body release nitrogen-containing waste into the blood in the form of ammonia. 22 of 43
Nephridia remove ammonia from the blood and release it outside the body. Nephridium 23 of 43
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. 24 of 43
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. 25 of 43
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. 26 of 43
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. 27 of 43
Groups of Mollusks What are the characteristics of the three main classes of mollusks? 28 of 43
Groups of Mollusks Groups of Mollusks The three major classes of mollusks are gastropods bivalves cephalopods 29 of 43
Groups of Mollusks Gastropods Gastropods are shell-less or singleshelled 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. 30 of 43
Groups of Mollusks Gastropods include pond snails land slugs sea butterflies sea hares limpets nudibranchs 31 of 43
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 32 of 43
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. 33 of 43
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. 34 of 43
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. 35 of 43
Ecology of Mollusks Ecology of Mollusks Mollusks feed on plants prey on animals filter algae out of the water eat detritus 36 of 43
Ecology of Mollusks Some mollusks are hosts to symbiotic algae or to parasites; others are themselves parasites. Mollusks are food for many organisms. 37 of 43
27-4 Continue to: - or - Click to Launch: 38 of 43
27-4 The trochophore larva is a characteristic that mollusks share with a. flatworms. b. roundworms. c. annelids. d. flukes. 39 of 43
27-4 Water enters and leaves the body of a bivalve through a. a siphon. b. cilia. c. a coelom. d. a nephridium. 40 of 43
27-4 The most active group of mollusks is the a. gastropods. b. bivalves. c. cephalopods. d. shell-less gastropods. 41 of 43
27-4 Unlike the other mollusks, cephalopods have a(an) a. closed circulatory system. b. ventral blood vessel. c. open circulatory system. d. dorsal blood vessel. 42 of 43
27-4 The body plan of most mollusks includes all of the following EXCEPT a a. foot. b. mantle. c. shell. d. radula. 43 of 43
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