In this topic you will learn about animals that do not have backbones.

Invertebrates come in a variety of shapes and sizes. However, the one thing they have in common is that they lack a backbone. More than 95 out of every 100 animals are invertebrates. Invertebrates can be divided into smaller groups based on characteristics.

Sponges are the simplest invertebrates. A sponge's body is shaped like a sack with an opening at the top. Water flows into the sponge through holes in its body. Water and waste leave through the opening at the top.

Cnidarians, such as jellyfish, are invertebrates that use poisonous stingers on tentacles to capture prey and for protection. Cnidarians have simple tissues and a mouth. They also have radial symmetry.

There are several kinds of worms. Flatworms have flat, ribbonlike bodies with a head and a tail. Roundworms have a slender, rounded body with pointed ends. The hookworm and vinegar eel are roundworms. Earthworms, sandworms, and leeches are segmented worms. They have more complex bodies than other worms. Their blood travels through blood vessels.

Most seashells come from mollusks. Mollusks are soft-bodied invertebrates. Some, like snails and slugs, live on land. Others, like clams and oysters, live in water.

Echinoderms are spiny-skinned animals. Echinoderms include starfish, sand dollars, sea cucumbers, and sea urchins. Echinoderms have an internal supporting skeleton called an endoskeleton.

The largest group of invertebrates is made up of arthropods. An arthropod is an invertebrate with jointed legs and a body that is divided into sections. Arthropods have an exoskeleton. An exoskeleton is a hard covering that protects an invertebrate's body. Included among arthropods are spiders, lobsters, shrimp, centipedes, and insects.

Most of the animals on Earth are invertebrates. They are important because they are a food source for other animals. People also depend on them for many things.