Animals are multicellular, living organisms in the Kingdom Animalia. Although they might not seem so to the unaided eye, all animals are made up of cells which are living organisms and have their own set of cellular processes in place. With a few exceptions, all animals breathe oxygen, consume organic matter, can reproduce, move, and eat food. They also possess feelings, memories, thoughts, feelings, and emotions.
Animals are classified into four kingdoms: Prototheria, Bacteria, Fauna, and Metatheria. Within each of these kingdoms are further divisions: invertebrates, which are classified as fishes and amphibians; metathesis, which are land-dwelling animals such as lizards, salamanders, certain forms of whales and dolphins, birds, and certain forms of insects; protocerids, which are amphibious unicellular organisms; metathesis, which are land-dwelling vertebrates; and social, which are communal animals such as rats and mice. Within the animal kingdom are classifications that are based on similarities and differences among different species. Classifications include carnivores, omnivores, protozoa, and plants. Most animals belong to at least one of the classes discussed.
The major categories of animals are divided into two broad subcategories: animalia without a prokaryotic nucleus. Animals with a prokaryotic nucleus are eukaryotes, which include all bacteria and protozoa and some viruses. Eukaryota have a nucleus and therefore are considered a type of animal, even though the nucleus is not present. Animals without a prokaryotic nucleus are animals without a respiratory system, which includes all vertebrates and amphibians; animals with an exoskeleton, which include all reptiles and mollusks; and animals with muscles, which include all mammals and birds.
Insects are perhaps the most diverse group of the animal kingdom, and are perhaps the best studied of all animals. Some insects are very small, such as aphids, which are also known as red ants; and many insects are large, with a variety of wing types and body sizes. One branch of insect science is entomology, which studies the relationships between animals and their respective environments. Many insects are important to human agricultural production, because they eat pests such as aphids and tobacco. Other insects have less impact on crops, for example dragonflies and ladybugs.
One branch of the animal kingdom that is often overlooked is invertebrates, which include mollusks and crabs, crustaceans, and snails. Unlike vertebrates, invertebrates are usually not visible to the human eye but are vital to the health of other creatures. Examples of invertebrates are arachnids, which are arachnids with a series of hard eyes on their undersides; brachiopods, which are tube-like creatures with hard abdomens and soft fleshy outer layers; and mites, which are small creatures that attach to animal skin.
The third branch of the animal kingdom is the fungi. Fungi belong to the kingdom Animalia, along with all animals and all invertebrates, while they are classed separately on the basis of how they reproduce. They are classed as true fungi if they grow in an external environment and reproduce by means of spores or through a process called mycosis. Some fungi are only able to grow in a specific range of temperature and can therefore be important factors in animal and plant life alike.