There are millions of different kinds of animals in the world, yet they all have two basic characteristics in common. The first is that animals are multicellular organisms whose cells are organized according to their functions. In nearly all animals, groups of similar cells form tissues such as blood, muscle, and skin. Tissues are arranged into organs such as the heart, brain, and stomach. The second common characteristic is that all animals are heterotrophs. Animal tissues lack chlorophyll, so they are unable to use the Sun’s energy to manufacture food. For this reason, animals must find and ingest food.
Of the millions of species of animals in the world, 95 percent are classified as invertebrates. As the numbers suggest, invertebrates form a highly successful group that has adapted to every niche of the environment. The principal characteristic of animals in this group is the absence of a backbone, a column of vertebrae around a central nerve chord. To support and protect their bodies, many invertebrates are equipped with hard, external skeletons.
On the reef, the statistics of invertebrate success hold true, and the greater part of reef animals are invertebrates. They include very simple creatures, such as sponges, corals, anemones, jellyfish, and worms, as well as more complex groups, such as clams, snails, octopuses, lobsters, and starfish. While the most primitive reef invertebrates are barely more than colonies of cells, the advanced ones possess organs as sophisticated as those of humans.