Invertebrates

            Invertebrates are animals without backbones. Most of the animals of the world are invertebrates.
            Protozoa. These are simple animals that are seen under a microscope. Protozoa are one-celled animals. Some protozoa are: amoeba, paramecium, stentor, and vorticella. Scientists think that some kinds of protozoa, long ago, began to cluster together. Gradually they became attached and formed small colonies. You can find colonial protozoa like Volvox in pond water.
            Sponges. Sponges are simple animals consisting of two layers of cells. The cells of the outside layers have tiny rods that stiffen the cells. They give protection and supoort to the sponge. The cells of the inside layer digest food for both layers. They get the food from seawater that flows through. Divers frind sponges attached to things on the ocean bottom. Sponges don't move from place to place as most animals do. If you get a sponge to examine, you will see only the skeleton. The protoplasm has dried up and has been removed. There are many kinds of sponges, but all are alike in these ways: (1) they are invertebrates; (2) they are many-celled; (3) there are two layers of cells; (4) the two layers are different and do different work.
            Jellyfishes and corals. Jellyfishes and corals belong to another group of water animals. Like the sponges, they are simple animals, made of two layers of cells. The "petals" of the sea anemone are really little arms called tentacles. They can wave around. When a small animal swims by, the sea anemone stings it with poisonous threads given off by some cells in the outer layer of the tentacles. Then the tentacles pass the captured animal through the mouth into a hollow sac. There it is digested and becomes protoplasm. Most of the animals in this group stay attached to one place, but they can move around slowly. Jellyfishes and corals are alike in these ways: (1) they are invertebrates; (2) they have tentacles surrounding an opening called a mouth; (3) they digest food in a hollow sac.
            Worms. There are three main groups of worms: flatworms, roundworms, and segmented worms. Worms are quite complicated animals Scientists think they may have developed from simpler two-layered animals like the jellyfish. Instead of just two layers of cells, worms have many body parts, including simple brains and hearts. These body parts are made of many groups of cells working together at special jobs.
            Earthworms. The earthworm is a segmented worm. Segmented means "divided into sections." Earthworms don't have heads; but some other worms do. Earthworms have a mouth at the front and another opening at the back. The front, like a head, always points forward as the earthworm moves. Earthworms swallow soil at the front. The soil passes through a digestive tube and out the back of the body. The soil contains plant and animal material that is food for the earthworm. The food is digested as it passes through the worm. Undigested materials are passed out through the rear opening. Most worms are alike in these ways: (1) they are invertebrates; (2) they have groups of muscle cells; (3) the digestive system has an opening at the front and at the rear; (4) they have heads; (5) they have special body parts.
            Caterpillars were formerly classified as worms. Their classifications has been changed because it was found that caterpillars were really the larval stage of moths and butterflies.
            Mollusks. Mollusk, meaning soft, is the name of a group of soft-bodied animals. The soft bodies of mollusks are usually covered by hard shells. Mollusks are often called shellfish. Some mollusks have a shell in two parts that open and close by muscles. Others live in a one-piece shell. Some mollusks have no covering shells (such as the octopus and squid).
            Starfish. Starfish and their relatives live in the sea. Many of these animals have sharp spines sticking through their skin. They are called echinoderms, which means "spiny-skinned." Starfish are usually divided into five sections. If you look at the underside of a starfish, you will see thousands of tubes. When they are filled with water, the tubes act like little suckers. They cling to the surface on which the starfish moves. They are called tube feet because they help the starfish move. They can also cling to a mollusk shell and force it apart. Then the starfish can eat the soft animal inside.
            Arthropods. The word arthropod means "jointed-leg." The arthropods are a group of invertebrate animals all of which have jointed legs. Arthropods are the only invertebrates which have jointed legs. Lobsters and shrimp are both arthropods. They have a tough, light, flexible covering that serves as an outside skeleton. This flexible outer covering is called an exoskeleton. It follows the outlines of the animal's body, and it has joints that permit the body to bend and move. Many arthropods live on the land. The outside covering prevents the soft bodies from drying out. The bodies of arthropods are divided into sections. Scientists think that the arthropods may have developed from segmented worms. Arthropods are the largest group of animals in the world.
            Centipedes and millipedes have many legs. Centipedes have one pair of legs on most body segments. Millipedes have two pairs of legs on most body segments.
            Lobsters, crayfish, hermit crabs, ghost crabs, and gooseneck barnacles live in the water. They have segmented bodies, jointed legs, and outer coverings.
            The spotted fever tick, scorpion, sea spider, golden garden spider, and red water mite have four pairs of legs. Some have two body parts, or segments. Others have a single body part.
            The long-horned beetle, blue butterfly, luna moth, crane fly, dragon fly, honeybee, periodical cicada, harlequin cabbage bug, and winged black ant is the largest group of arthropods. All adult insects have bodies with three segments – head, thorax, and abdomen. Here are some other ways that insects are alike: (1) insects have two feelers, or antennae, on the front of the head; (2) insects have two rows of breathing holes along the abdomen. Insects are like other arthropods because they have jointed legs, exoskeleton, and segmented bodies.
            The puzzle of the platypus. Classification is a man-made way of grouping animals. The animals in the world remain the same, whether scientists have classified them or not, whether the groupings are right or wrong. Some living things that are hard to classify are: viruses, coelacanth, euglena, and archaeopteryx. Each of these is an "in-between." Each is between two groups. The duck-billed platypus is another puzzler. It is warm-blooded, has a bill and no teeth, and has webbed feet. Its young hatch from eggs. Its body is covered with hair, not feathers. The mother feeds her young with milk from her body. Scientists have studied the structure of the platypus in great detail. They classify it as a mammal because it is covered with hair, secretes milk, and the embryo gets nourishment from uterus as well as from yolk in egg.