Invertebrates. What You Already Know

Transcription

1 Invertebrates What You Already Know Animals can be grouped based on their behaviors or their physical traits. One way scientists classify animals is by whether or not they have backbones. An animal with a backbone is a vertebrate. Vertebrates look different from one another, but they all have a backbone. Fish, birds, and mammals are some groups of vertebrates. Animals without backbones are called invertebrates. They have structures other than bones that give them their shape. Some invertebrates have a soft sac filled with liquid that supports them. Others have a shell or other hard covering on the outside of their bodies. Some invertebrates live in water. Some live on land. Sea stars, sea jellies, and clams are examples of invertebrates that live in water. Earthworms, scorpions, and slugs are land invertebrates. Bees, ants, and beetles are insects. Insects are part of the largest group of invertebrates called arthropods. Spiders, though not insects, are also arthropods. Invertebrates are all around us on land, in water, and even in the air. Keep reading to find out more about invertebrates.

2 Arthropods There are more kinds of arthropods than any other invertebrate. Arthropods have a hard outer body covering called an exoskeleton. The exoskeleton supports and protects the animal s body. An arthropod can bend and move easily because its exoskeleton is jointed. This means that each part of the hard covering is connected in a way that allows movement. The exoskeleton is also waterproof. It keeps the animal s body from drying out. The group of animals called arthropods is made up of smaller groups. Insects, spiders, centipedes, and lobsters are each in different arthropod groups. Arachnids Spiders may look like insects, but they are members of an arthropod group called arachnids. Arachnids have two main body parts and eight legs. When arachnids grow, they shed their exoskeleton. Then they grow a new, bigger exoskeleton. Many spiders make a thin thread called silk. Some spiders spin webs with their silk. Scorpions are arachnids that live in warm, dry places. A scorpion has a tail that curves over its head. It uses its tail to sting. If you see a scorpion, be careful.

3 Insects There are more kinds of insects than any other kind of animal on Earth. Scientists have identified almost a million different kinds of insects. Insects have six legs and a body that has three main sections. An insect s body is made up of a head, thorax, and abdomen. For their small size, insects can be very strong. They have strong muscles attached to their exoskeletons. Most insects have two pairs of wings. The front wings cover and protect the back wings. The back wings are used for flying. One kind of dragonfly is the fastest flying insect. Its speed has been measured at up to 58 km per hour (36 mi per hour). Insects have adaptations that help protect them. An adaptation is a trait that helps a living thing survive in its environment. For example, some moths have markings on their wings. This adaptation makes them look like another kind of animal. Some insects blend in with their surroundings. This helps them to stay safe.

4 Invertebrates in the Ocean Many invertebrates live in water. Some live in freshwaters, such as rivers, lakes, or ponds. The oceans are home to many different kinds of invertebrates. Sea invertebrates include sponges, sea stars, sea jellies, corals, lobsters, and clams. Some of these invertebrates grow to be very large. A giant clam can weigh up to 600 pounds. Other sea invertebrates can be smaller than a fingernail. Crustaceans Most crustaceans live in the ocean. Examples include lobsters, crabs, and crayfish. Hermit crabs, sow bugs, and pill bugs are crustaceans that live on land. Crustaceans have hard outer shells that protect their soft bodies. Like other arthropods, crustaceans have jointed legs. Crabs and lobsters have large front legs that act as pincers. They use the pincers to catch food or to fight predators. Sponges Sponges may look like plants, but they are sea animals. A sponge cannot move about freely. It takes in seawater through tiny holes in its body. Then it traps bits of food that are in the seawater.

5 Echinoderms An echinoderm is an invertebrate with a soft body covered by a spiny exoskeleton. In fact, the term echinoderm means spiny skin. Sea stars, which are often called starfish, and sea urchins are echinoderms. Sea stars have five arms and rows of tube feet that help them move. Sea urchins are protected by the long spines that cover them. Stingers Sea jellies, or jellyfish, are invertebrates with soft bodies. The body of an adult sea jelly is called a medusa. It is shaped like an umbrella. The animal moves by opening and closing its body to squeeze out water. Some kinds of sea jellies use long body parts called tentacles. Tentacles have stinging cells that help to catch prey. Anemones look like flowers. They have tentacles that sting. When an animal is stung by an anemone, it cannot move.

6 Coral Coral is an invertebrate that lives in warm, salty waters. Most corals are made up of many tiny animals called coral polyps. Parts of the polyps form a hard covering made up of calcium carbonate, or limestone. Corals live in colonies. When a coral dies, it leaves behind its hard exoskeleton. Over time, these hard coverings can build up. They can form large structures called reefs. Coral reefs become homes for other sea life. Mollusks A mollusk is an invertebrate with a soft body. Its body is surrounded by a soft layer. This layer often makes a hard shell that protects the body. Some mollusks are bivalves. Bivalves have a shell that has two halves that are hinged. The hinge lets the shells open and close. Clams and mussels are bivalves. Other mollusks are gastropods. They have one foot and crawl slowly along the ocean floor. Snails and slugs are gastropods. Octopuses and squids are mollusks with no shell. They can move through the water very quickly.

7 Worms Worms are invertebrates that have long, soft bodies and no legs. Some worms live in soil. Others live in water. Some worms even live inside other animals. Earthworms Earthworms have a segmented body. That means its body is made up of sections. Earthworms live in soil. They swallow soil along with bits of food in the soil. Their waste makes soil better for plants to grow in. Parasites Some kinds of worms are parasites. A parasite is an organism that lives on or in another living thing, usually causing it harm. Parasites feed on plants and animals. Some parasite worms can live inside humans. Roundworms, flatworms, and tapeworms are types of parasites. A leech is a segmented worm that is a parasite. Marine Worms Some worms that live in the ocean eat plants. Others are hunters. The bristle worm lives in water or in moist soil. Its body is made up of segments. It has bristles that look like tiny hairs on both sides of its body.

8 Glossary arachnid bivalves crustacean echinoderms exoskeleton gastropod mollusk parasite an arthropod that has eight legs a mollusk with a shell that has two hinged halves an arthropod with a soft body covered by a hard shell an invertebrate with a soft body covered by a spiny exoskeleton the hard outer body covering of arthropods and some other invertebrates a mollusk with a single foot an invertebrate with a soft body covered by a soft layer that may produce a hard shell an organism that lives on or in another living thing, usually causing it harm