Invertebrates

Invertebrates are animals without backbones. There is a tremendous diversity of invertebrates, which includes protozoa (single-celled animals), corals, sponges, sea urchins, starfish, sand dollars, worms, snails, clams, spiders, crabs, and insects. Over 98% of the nearly two million currently known species are invertebrates. The diversity of invertebrates also includes their size, which ranges from less than a millimeter to several meters long. Invertebrates also display diverse body forms, means of locomotion, and feeding habits.

Invertebrates are an essential part of every ecosystem on Earth. Indeed, the human race is absolutely dependent on invertebrates; our species would cease to exist without them. They are responsible for the decomposition of organic waste, which allows the recycling of the chemicals. Invertebrates also are involved with the pollination of plants, and are crucial as links in food chains where herbivores convert the energy in plants into energy that is available to animals higher up the food web.

Most invertebrates live in water or have some stage of their life in water. The external layers of aquatic invertebrates are generally thin and are permeable to water, allowing the exchange of gas, although some have specialized respiratory structures on their body surface. Aquatic invertebrates feed by ingesting directly, by filter feeding, or actively capturing prey.

Some groups of invertebrates such as earthworms, insects, and spiders live on land. These invertebrates need to have special structures to deal with life on land. For example, because air is less buoyant than water, earthworms have strong muscles for crawling and burrowing while insects and spiders move by means of several pairs of legs. Drying out on land is a problem so earthworms must secrete mucous to keep their bodies moist, while insects and spiders are waterproof and are physiologically adapted to conserve water.