Among animals are those with backbones, or vertebrates, and those without backbones,, or invertebrates. Invertebrates share common traits, such as being multicellular, or made of more than one cell. And unlike plant cells, animal cells have no cell walls. Other than sponges, cells within invertebrates are organized into tissues, or cells that share a common function.

Most invertebrates are mobile, meaning they are able to move freely. But some, such as clams and oysters, spend their adult lives anchored, or attached to something solid, like rocks or even the bodies of other animals. An invertebrate’s body shows symmetry. Bilateral symmetry, or two-sided symmetry, means that if you were to draw a line down the middle of the animal’s body, the two sides, or halves, would look identical.

Most invertebrates reproduce sexually, meaning male and female invertebrates produce sex cells. The cells join to form a new organism.

Invertebrates depend on other living things for food. Some animals filter food from water. Others hunt for and capture food.

There are different kinds of invertebrates, including sponges, cnidarians, mollusks, arthropods, insects, worms, and echinoderms. The simplest of all of these animals are the sponges.

Sponges and Cnidarians


Sponges look like a bag full of holes. They lack a head, arms, and legs, and once a young sponge has settled in one spot, it never moves again. Sponges live on the floor of the ocean. They take in tiny bits of food that they filter from passing water. Cnidarians include corals, hydras, and jellyfish. These animals have tentacles that they use to sting or trap prey. The tentacles push food into a body cavity where it is digested.

Corals often form large colonies. The colonies secrete a stony skeleton, called a reef. Coral reefs provide homes for a wide variety of animals in warm, ocean waters.



Worms is a term that describes a huge variety of animals. Different kinds of worms live in the soil, in the ocean, and as parasites.

Just one kind of worm, the roundworm, may have as many as half a million species. Roundworms are small and can be found everywhere. Scientists counted 90,000 tiny roundworms ,in just one rotting apple. An acre of ground may contain up to 10 million.

Earthworms are among the most advanced worms. They have a mouth at the head end, an anus for waste removal at the tail end, and a digestive tract in between. Short, stubby bristles help them move. Although earthworms have no eyes or ears, they have a nervous system and respond to both light and sound.

Every earthworm has both male and female reproductive organs. They mate in pairs, exchanging sperm so that each individual fertilizes the other’s eggs. Sometimes an earthworm can fertilize its own eggs.

Role of Worms in the Environment

Many roundworms are parasites, meaning they live inside the body of a larger animal and feed off it. Examples include hookworms and eye-worms. They cause diseases in livestock and in humans.

In contrast, earthworms are beneficial. By burrowing through the soil, they help break up soil particles and mix oxygen into the soil. They also return nutrients to the soil through their wastes.



Mollusks are generally made up of a soft body and shell. Slugs, squids, and octopuses also fit into this group, although they don’t have shells like the others. Mollusks have no skeletal system, but they have all the other basic organ systems (muscular, digestive, circulatory, nervous, excretory, respiratory, and reproductive). Their bodies include a foot, a mantle, a shell, and a mass of body organs.

Role of Mollusks in the Environment

Many mollusks feed on bits of decaying plants and animals, which helps clean up their environment. Scientists sometimes use certain mollusks to measure the level of pollutants in their sea environments.

Some mollusks, such as snails and slugs, eat plants, including many farm crops. But mollusks help humans, too. Squid, clams, and oysters are mollusks that are important food sources for humans and other animals. The shells and pearls produced by some mollusks are used for jewelry.


Arthropods are animals with legs that bend. Spiders, ticks, lobsters, and insects are common arthropods. They also have eyes and antennae for sensing the world around them. They have a hard outer covering known as an exoskeleton.

The material that forms the exoskeleton is not living tissue and cannot grow with the rest of the animal. As a result, arthropods must shed their exoskeletons to grow. The process is called molting. Before molting, the animal produces a soft, new exoskeleton under the old one. During molting, the old exoskeleton splits open and the animal crawls out. After a short wait, the new exoskeleton swells and hardens into the animal’s new size. Although exoskeletons work well for small animals, they would be too heavy for large animals.

Insects, the largest group of arthropods, not only respond to the world around them, they adapt, or change, and so quickly that they have become among the most successful animals on Earth. This success is related to their ability to live on land, in air, and in water. Scientists have identified more than 700,000 species of insects, and no one thinks that the list is complete. Insects also reproduce at an amazing rate. A house fly may lay one hundred eggs at a time. The young will hatch, develop, and lay their own eggs in as few as ten days.

Insect Body Structure


All insects have six legs and three body regions: a head, a thorax, and an abdomen. A mouth and one pair of antennae are located on the head. They may or may not have wings.

Insect mouths are specialized for eating. Some, such as the grasshopper, have mouth structures designed for chewing. Others, such as the mosquito, have structures designed for piercing and sucking. The butterfly has a long tube for sipping nectar from flowers. Insects’ legs may also be specialized. They may be suited for walking, jumping, swimming, or clinging to other animals.

Some insects have tiny, simple eyes. They see poorly and make up for it by their sense of touch. Other insects see more clearly with compound eyes, which are eyes made up of many small parts. Insects have no noses, and instead smell with their antennae, or feelers.

Insect Life Stages


Like jellyfish and corals, insects live their lives in stages. They go through either three or four distinct stages as they develop. The process is called metamorphosis. All insects begin as eggs. Insects that live in four stages hatch into worm like creatures called larvae. In this stage, the larvae eat constantly and grow quickly. They then go into a resting stage (the pupa), in which they wrap themselves in a cocoon. Inside the cocoon, the tissues of the larva change into the tissues of an adult insect. When the insect comes out of the cocoon, it is an adult.

Insects that go through only three stages of metamorphosis skip both the larva and pupa stages. Instead they hatch into nymphs. The nymphs are much smaller than the adults. They have no wings, and their reproductive structures are incomplete. With each molting, however, a nymph is more and more like an adult.

Role of Insects in the Environment

Many farm crops depend on insects to pollinate them. Insects are also an important part of the food chain, providing food for many animals, and even humans in some cultures. Yet insects are also pests, and much worse. Locusts, beetles, and termites destroy crops and lumber. The bites of mosquitoes, ticks, and lice spread many serious diseases.