Mollusks are invertebrates with soft, unsegmented bodies that are often protected by a hard outer shell. In addition to a soft body often covered by a shell, a mollusk has a thin layer of tissue called a mantle that covers its internal organs, and an organ called a foot. Mollusks have bilateral symmetry and a digestive system with two openings.
Most species of mollusks have an open circulatory system, in which the blood is not always inside blood vessels. Most mollusks that live in water have gills, organs that allow oxygen from the water to enter the body.
Biologists classify mollusks into groups based on physical characteristics such as the presence of a shell, the type of shell, the type of foot, and the type of nervous system.
Gastropods are the largest group of mollusks, and include snails and slugs. Gastropods have a single external shell or no shell at all. Gastropods can live nearly everywhere on Earth. Some gastropods are herbivores. Still others are carnivores. All have an organ called a radula (plural radulae), a ﬂexible ribbon of tiny teeth. Gastropods usually creep along on a broad foot.
Clams, oysters, scallops, and mussels are bivalves. Bivalves are mollusks that have two shells held together by hinges and strong muscles. Bivalves live in all kinds of watery environments. Bivalves do not have radulas. Most bivalves are ﬁlter feeders; they strain their food from the water. Most bivalves are omnivores, animals that eat both plants and animals. As adults, most bivalves stay in one place or move slowly. Bivalves cover sand or grit lodged in their shells with a protective coat, which eventually forms a pearl.
Octopuses, cuttleﬁsh, nautiluses, and squids are cephalopods. A cephalopod is an ocean-dwelling mollusk whose foot is adapted to form tentacles around its mouth. Cephalopods are carnivores and use a radula to scrape the ﬂesh of their prey. Cephalopods have large eyes and excellent vision. They have the most complex nervous system of any invertebrate. Cephalopods swim by jet propulsion.
The major groups of arthropods are crustaceans, arachnids, centipedes and millipedes, and insects. Arthropods are invertebrates that have an external skeleton, a segmented body, and jointed attachments called appendages.Arthropods have bilateral symmetry, an open circulatory system, a digestive system with two openings, and sexual reproduction.
All arthropods have a waxy exoskeleton, or outer skeleton, that protects the animal and helps prevent evaporation of water. An exoskeleton cannot grow with the animal. Arthropods shed their outgrown exoskeletons in a process called molting.
Arthropods have segmented bodies and jointed appendages that give them ﬂexibility and enable them to move. Sometimes the segments are joined into distinct sections, such as a head, midsection, or hind section. They have specialized appendages that perform certain functions. For example, an antenna (plural antennae) is an appendage on the head that contains sense organs.
A crustacean is an arthropod that has two or three body sections, ﬁve or more pairs of legs, and two pairs of antennae.
Crustaceans live in watery environments and must have gills. They obtain food in many ways. Some are scavengers that eat dead plants and animals. Some are predators, eating animals they have killed.
Most crustaceans begin their lives as microscopic, swimming larvae. Crustacean larvae develop into adults by metamorphosis, a process in which an animal's body undergoes dramatic changes in form.
Arachnids are arthropods with two body sections, four pairs of legs, and no antennae. The ﬁrst body section is a combined head and midsection. The hind section, called the abdomen, contains the arachnid's reproductive organs and part of its digestive system. Spiders, mites, scorpions, and ticks are all arachnids.
Centipedes and millipedes are arthropods with two body sections and numerous pairs of legs. Centipedes have one pair of legs attached to each segment, and some centipedes have over 100 segments. Millipedes, which may have more than 80 segments, have two pairs of legs on each segment.
Insects are arthropods with three body sections, six legs, one pair of antennae, and usually one or two pairs of wings. The three body sections are the head, thorax, and abdomen. Sense organs, such as the eyes and antennae, are located on an insect's head. Most insects have two large compound eyes that are especially keen at seeing movement. Most also have simple eyes, which can distinguish between light and darkness. An insect's thorax, or midsection, is the section to which wings and legs are attached. The abdomen contains many of the insect's internal organs.
Insects eat a wide variety of things. If something is alive, or was once living, some kind of insect will probably eat it. An insect's mouthparts are adapted for a highly speciﬁc way of getting food.
Insects begin life as tiny, hard-shelled, fertilized eggs. After they hatch, insects begin a process of metamorphosis that eventually produces an adult insect. Each insect species undergoes either complete metamorphosis or gradual metamorphosis. Complete metamorphosis has four stages: egg, larva, pupa, and adult. Insect larvae usually look something like worms. After a period of eating and growing, the larva goes into the next stage and becomes a pupa (plural pupae). As a pupa, the insect is enclosed in a protective covering and gradually changes into an adult.
In contrast, gradual metamorphosis has no distinct larval stage. An egg hatches into a nymph that often resembles the adult insect. A nymph may molt several times before becoming an adult.
A food chain is a series of events in which one organism eats another and obtains energy. A study of food chains and other ways that organisms interact with their environment is called ecology.
A food chain starts with a producer, or organism that makes its own food. Then the producers are eaten by consumers. Consumers are organisms that obtain energy by eating other organisms. Some consumers eat producers, and some eat other consumers. A decomposer breaks down the wastes and dead bodies of other organisms. In a food chain, some insects are consumers, some are decomposers, and some are prey for other consumers.
Insects play key roles in food chains because of the many different ways that they obtain food and then become food for other animals. For example, many insects eat crops and wild plants. These insects are then eaten by other animals, especially ﬁshes and birds. Various species of insects break down dead wood, leaves, grass, dead animals, and animal droppings.
Insects are also eaten by humans in many parts of the world. Insect products, such as honey, have been eaten or used by humans through history. Insects interact with other living things in their environments in other ways besides eating and being eaten. Two ways insects interact with other living things are by moving pollen among plants and by spreading disease-causing organisms. Insects and other animals that carry pollen among plants are called pollinators. Insect pollinators include many bees, beetles, and ﬂies. Without pollinators, some plants cannot reproduce. Still other insects spread disease to plants and animals, including humans. These insects often have sucking mouthparts.
Harmful insects are called pests. To try to control pests, people use chemicals, traps, and living things, including other insects. Chemicals that kill pests are called pesticides. There are also alternatives to pesticides, including the use of biological controls. A biological control is a natural predator or disease that is released into an area to ﬁght a harmful insect.
Echinoderms are invertebrates with an internal skeleton and a system of water-ﬁlled tubes called a water vascular system. The skin of most echinoderms is stretched over an internal skeleton, called an endoskeleton. The water vascular system consists of ﬂuid-ﬁlled tubes within the echinoderm's body. Portions of the tubes can squeeze together, forcing water into structures called tube feet. Most echinoderms use their tube feet to move and to capture food.
Almost all echinoderms are either male or female. Eggs are usually fertilized in the water. The fertilized eggs develop into larvae. The larvae eventually undergo metamorphosis and become adults.
There are four major groups of echinoderms: sea stars, brittle stars, sea urchins, and sea cucumbers. Sea stars are predators with ﬁve arms. A sea star feeds by forcing its stomach out through its mouth and between the shells of its prey. Chemicals break down the prey. The sea star sucks in the partially-digested prey.
Brittle stars have long and slender arms, with ﬂexible joints. Brittle stars use their arms to slither along the ocean bottom.
Unlike sea stars and brittle stars, sea urchins have no arms. Moveable spines cover and protect their bodies. A sea urchin's spines cover a central shell that is made of plates joined together. Sea urchins move mostly by using bands of tube feet that extend out between the spines.
The bodies of sea cucumbers are soft, ﬂexible, and muscular. They move by using ﬁve rows of tube feet on their underside. Sea cucumbers use their tentacles to sweep food toward their mouths.
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