5-7: Phylum Mollusca

Habitat (Gastropoda)

Gastropods live on land and in both salt and fresh water. Those that live on land are considered terrestrial, and those that live in salt water are considered marine.

Feeding (Cephalopoda)

In cephalopods, the foot is modified to help in the digestive process. For example, the octopus's foot is divided into eight parts or tentacles. The nautilus has 80 to 90 tentacles, while the squid has 10 tentacles. Attached to the tentacles are suction cups or multiple harpoon-like structures that both help in capturing their prey--all cephalopods are ravenous predators. They bring their food to their mouth using their tentacles. Cephalopods have beak-like jaws that help tear their food to pieces. Once the food is torn to shreds, those shreds are pushed into their mouth with the radula. Cephalopods eat fish, worms, other mollusks, and crustaceans. How are gastropod tentacles different from cephalopod tentacles? Gastropods have a pair of tentacles on top their head region with eyes on the tips that are sensitive to light and touch. The cephalopod tentacles are actually a modified mollusk foot that contains suction cups or harpoon-like structures.

Which best describes the siphon in mollusks?

It is a structure that water shoots out of to propel the mollusk, and it releases dark liquid in some squids and octopuses.

Which of the following describes the radula of mollusks?

It is a tongue-like structure with backward curving teeth located in the mouth.

Which of the following best describes the job of the nephridia in mollusks?

Nephridia release fluid waste from the coelom.

Shells (Gastropoda)

Snails and slugs belong to class Gastropoda. Snails have a single shell, and slugs have evolved into having no shell.

Reproductive

Some mollusks are hermaphrodites--individuals that produce both eggs and sperm. Other mollusks are individual males and females. Some oysters and slugs don't stick to one particular gender but rather switch back and forth between being male and female. All mollusks engage in sexual reproduction. Recall that in sexual reproduction, male sperm fertilize female eggs. Some mollusks release their gametes (eggs and sperm) into the water for fertilization to occur. Some terrestrial mollusks and cephalopods release sperm into the female internally for internal fertilization. All mollusk fertilized eggs develop into a trochophore, as pictured. The trochophore is a larval stage in mollusk development. Recall that the larval stage is a developmental stage that occurs after fertilization. The trochophore is covered in cilia so that it can swim freely. What larval stage do all mollusks have in common? All mollusks have a trochophore larval stage.

Feeding (Bivalvia)

All bivalves are filter feeders. They lack the radula that all other mollusks have. They have cilia that bring water over their gills, which are coated in mucus, as seen in the photo. The mucus traps small organisms and food particles. The food then flows to the mouth where digestion begins. Besides the digestive system, what other system do mollusk gills belong to? Gills are part of the respiratory system.

Habitat (Bivalvia)

All bivalves live in water, either fresh or salt, although most live in salt water. What is another name to describe salt water habitats? Marine is another name for salt water habitat. Pictured is a giant clam, which lives in the Red Sea.

Habitat (Cephalopoda)

All cephalopods are marine animals, that is, they live only in salt water. Pictured is a squid in its habitat, the ocean.

Locomotion (Cephalopoda)

All cephalopods swim by means of jet propulsion. Cephalopods let in water through their mantle and then expel it quickly through a tube structure called a siphon. It is this drawing in and pushing out of water that allows cephalopods to move quickly. Some cephalopods also release a dark fluid or ink from their siphons when they are escaping predators or when they feel endangered. This release of ink clouds their trail of escape, which is exactly what the octopus is doing in the image. The cuttlefish releases a special ink called sepia that has been used for centuries by artists.

Senses (Gastropoda)

All gastropods have a pair of tentacles on their head region, as pictured. At the end of each tentacle is an eye, which is sensitive light and touch.

Feeding (Gastropoda)

All mollusks (except for bivalves) have a radula. What is a radula? A radula is a tongue-like structure, located in the mouths of mollusks. On its surface are backward curved teeth. The image on the left displays how a radula works. The gastropod sticks out its radula and scrapes the food source (pictured in blue), then returns the radula to its mouth to swallow the food. This mechanism is a problem for humans when it comes to their crops and gardens. The radula of many snails and slugs shred the leaves of important plants. To the right is a close up image of the teeth, which are very sharp. Some gastropods are strict herbivores that scrape algae off rocks. Others are ferocious predators, such as the oyster drill, which uses its radula to bore a hole in the shell of other mollusks--bivalves. It then sucks out the soft body of the bivalve.

Coelom

All mollusks have a coelom, which is a true body cavity containing fluid and completely lined with mesodermal tissue. The coelom surrounds all the organs, as pictured in the cross-section of the animal above. This trait is a characteristic of all complex animals. The simplest animals lack a coelom, including placozoans, poriferans, and cnidarians.

Foot

All mollusks have a foot, which is a highly muscular region specialized for locomotion.

Visceral Mass

All mollusks have a visceral mass, which is a centralized region of the body in which all the organs are located, as indicated in the image.

What type of symmetry do all mollusks have?

All mollusks have bilateral symmetry.

Radula

All mollusks, with the exception of bivalves, have a radula, which is a tongue-like structure located inside the mouth. The radula is specialized to scape objects for food--for example the algae off rocks or leaves into shreds. The radula is able to scrape because there are teeth on its surface, curved backwards. The mollusk sticks out its radula to scrape for food then returns with the food to the mouth to be swallowed.

Shell

Almost all mollusks have one or two shells. The shell acts as a strong and hard exoskeleton to support and protect the soft body underneath it. The mantle secretes thin sheets of protein and a hard mineral called calcium carbonate, which are the materials that compose the shells. What other animal contains a structure with calcium carbonate? Calcareous sponges (poriferans) contain calcium carbonate within their skeletons.

Shells (Bivalvia)

Bivalves, as their name implies, have two valves or shells. Notice all the baby clams in the picture have two shells. What part is involved in creating the shells of mollusks? The mantle creates the shells of mollusks. The mantle is also responsible for the pearl formation in bivalves. When an irritant like a grain of sand gets lodged in between the mantle and the shell of a bivalve, the mantle responds by coating the irritant with nacre, the same substance that forms the inner part of the shell. Nacre is also known as "mother of pearl." Eventually a pearl is formed by layers and layers of nacre coating the irritant. The valves in bivalves are held together by a hinge and are connected to the soft body inside by special muscles called adductor muscles. When the adductor muscles contract, the valves close.

Which of the following classes belongs to phylum Mollusca?

Cephalopoda

Senses (Cephalopoda)

Cephalopods have a highly developed head region with a complex nervous system. Cephal is Greek for head. This is why cephalopod is the name for those mollusks that have the most developed brains. They are the most intelligent of all invertebrates and are the only invertebrate that can be trained to distinguish different shapes and patterns. Many aquarium-dwelling octopuses can figure their way out of their tanks while searching for food. Some cephalopods have been seen climbing aboard ships and cracking open and eating the crabs on board. Cephalopods also have highly specialized eyes. Some even have color vision. The world's largest invertebrate, the giant squid, also has the largest eye, bigger than some vertebrate eyes.

There are three major classes of mollusks--Gastropoda, Bivalvia, and Cephalopoda. There are some features common to mollusks, such as having the same five organ systems--respiratory, excretory, digestive, reproductive, and circulatory. There are six body parts that mollusks also share. What are these common body parts?

Mollusks have a visceral body, foot, mantle, coelom, radula, and one or two shells.

Shells (Cephalopoda)

Most cephalopods have evolved to not have shells, although there are a few exceptions. The nautilus is the only cephalopod that still has a single shell exoskeleton, as pictured. The cuttlefish also has a small internal shell that has human uses, such as jewelry making and a calcium dietary supplement for many pets including birds, reptiles, and hermit crabs.

Mollusca

The animal phylum that includes the mollusks, which are the second most abundant invertebrates in the world. Examples of mollusks include snails, slugs, squids, octopuses, oysters, clams, and scallops.

Digestive

The digestive organs are located within the visceral mass of the mollusk body. Mollusks with a radula stick it out to scrape food off different surfaces like leaves and rocks. They then return the radula to their mouth, carrying the food with it. The food is swallowed and goes down their digestive tract (colored in green in the picture). The food is broken down and the nutrients are taken up by the cells lining the digestive tract. These cells then transport the nutrients to the blood. Some mollusks, bivalves, lack a radula. Instead, bivalves have cilia that they use to pull water over their gills. Their gills are coated in a mucus that catches small organisms and food particles. The captured prey then flow into their mouth and through the digestive tract. Digested food then exits the mollusk body through the anus, a pore on the outside of the mollusk's body.

Excretory

The excretory system is involved in releasing liquid waste from the body. The fluid waste is collected within the coelom and then pulled into a tube called a nephridia, where nutrients are absorbed back into the mollusk tissue. Nephridia comes from the Greek word nephros for kidney. The remaining fluid waste exits the nephridia through a pore and is then dumped into the mantle cavity where it is released from the body. What is the purpose of the excretory system? The excretory system is in charge of getting rid of liquid waste from the body.

Locomotion (Gastropoda)

The foot in snails, as in all mollusks, is used for locomotion. The foot in terrestrial gastropods is highly specialized to get around on land. It secretes a mucus that creates a slippery path, allowing the gastropod to glide along with ease from place to place. A small trail of mucus is visible in the image. What do terrestrial gastropods secrete to allow for easy locomotion on land? Terrestrial gastropods secrete a mucus from their foot for easy movement.

The third major class of mollusks is Cephalopoda, which includes octopuses, squids, nautiluses, and cuttlefish. What are the other two major classes of mollusks, and what animals are included in these classes?

The other two major classes of mollusks are Gastropoda (snails and slugs) and Bivalvia (clams, oysters, mussels, and scallops).

Locomotion (Bivalvia)

The majority of bivalves are sessile. What does sessile mean? Sessile is a characteristic of some animals that remain stationary and planted to one spot for the entire span of their lives. Sessile bivalves remain in place by inserting their muscular foot into the ground. Although most bivalves live a stationary life, there are a few that can move. For example, the deep sea scallop is able to open and close its valves rapidly. When it closes its valves, jets of water push the animal through the water.

Mantle

The mantle is a heavy fold of tissue that encloses the mollusk's body and is what creates the shells of mollusks. The mantle also creates a small empty space called the mantle cavity.

Circulatory

The purpose of the circulatory system is to carry nutrients and oxygen to all the cells within the body. Most mollusks have an open circulatory system, which means there is blood in the body cavity (coelom) that bathes the organs with the necessary nutrients and oxygen. This is different from more complex animals that have closed circulatory systems where all the blood is contained within vessels such as veins, arteries, and capillaries. The only mollusks with closed circulatory systems are squid and octopus. What is the difference between open and closed circulatory systems? Open circulatory systems have blood within the coelom, and the organs are bathed in nutrients and oxygen. Closed circulatory systems contain blood within blood vessels such as arteries, veins, and capillaries. All mollusks have a three-chambered heart, which means their heart has three distinct regions, as opposed to humans, which have a four-chambered heart. The heart pumps the blood to the body cavity when it is oxygenated, and when the blood no longer has oxygen, it is pumped to the lung to get more oxygen.

Respiratory

The respiratory system specializes in taking in oxygen from the environment. It works closely with the circulatory system to transport oxygen to all the tissues in the body. All the mollusks that live in water have gills, which are located within the mantle cavity. Water passes over the gills, bringing with it oxygen. The oxygen is then taken up by the gills and transported to the blood to be carried to all the cells in the mollusk's body. In mollusks that are terrestrial, that is, live on land, there is a simple lung rather than gills. The lung is also located within the mantle cavity. The lung is very delicate and must be kept moist in order to take in oxygen properly, so terrestrial mollusks tend to be more active at night or after a rain storm when the air is moist. During particularly dry times, terrestrial mollusks secrete a thick mucus that creates a plug to block any openings in their body to conserve moisture. How are mollusks that live in water different from mollusks that live on land when it comes to their respiratory organs? Mollusks that live in saltwater and freshwater environments have gills that specialize in taking in oxygen, whereas terrestrial mollusks have a simple lung that takes in oxygen.

Why are some garden gastropods considered pests?

The teeth on their radula tear up the leaves of plants.

Mollusks include snails, slugs, oysters, clams, octopuses, and squids. What are some things all these animals have in common?

They all live in water or moist places. They all have soft bodies. Some have shells (snails, clams, oysters). They are symmetrical and have tissue.

Senses (Bivalvia)

Unlike other mollusks, bivalves have no distinct head region, but they are still able to sense their environment. They have a bundles of nerve cells above their foot that serve as a simple brain and sense organs that are sensitive to light and touch. Pictured is a coral scallop with numerous bundles of nerve cells, which are red colored circles with blue centers. What part of gastropods is sensitive both light and touch? Gastropods have tentacles with eyes attached to the ends that are sensitive to light and touch.

Which of the following mollusks are classified as Gastropods?

snails

Which of the following is a mollusk larvae?

trochophore