The word “animal” refers to any eukaryotic organism in the Kingdom Animalia. With few exceptions, all animals breathe oxygen, consume organic matter, can move, reproduce sexually, and have a living memory. People are animals too, although to most people they seem much more like bacteria or plants than they do animals. We all know that we are animals, but only certain types of animals are considered “people.”
Plants, fungi, bacteria, protozoa, and viruses are part of the Kingdom Animalia, too. All animals members are alive, but they tend to be very different from eukaryotic animals (organisms that have a nucleus), unicellular organisms (single-celled organisms), and multicellular organisms (including animal cells). A good example of an animal with an exoskeleton is a worm. A few vertebrates, birds, reptiles, and even some amphibians are classic examples of animals with a skeletal system. All animals have bodies, although not all of them are alike.
All animals have nerve and sensory organs, but they vary greatly in what kinds of those organs they have. Nervous and reproductive organs are present in all animals, but in different animals depending on the type of animal. Most amphibians and some insects have both sexes, whereas reptiles and amphibians have only one sex. Most mammals have a nervous system and a centralized circulatory system, while most eukaryotes and plants have a centralized digestive system. A few animal species are only capable of using one sense organ; these are the cetaceans (whales, dolphins, and seals), birds (including humans), and most fishes.
All animals have a limb or joint system, but this varies greatly among animal groups. limbs are present in all animals, but in different degrees and types. The limbs of animals with closely-related bodies (such as fish) tend to be very similar in structure, but those of independent creatures such as fishes are more variable and species. Only the extremities of multicellular organisms are completely devoid of limbs, which are present in all animals.
All animals have mouths to feed on, but not all animals have chewing teeth. Most animals have teeth for simple scraping, but some forms of the trichophytic tree also have additional teeth for piercing and chewing into hard objects. Herbivores and parasites have both mouth and teeth and belong to different categories of animal science. Primates belong to the family of animals with a single jaw and jaws that grow multiple offspring by means of sexual reproduction. Multituberculates are marsupials and belong to an entirely different group of animals with two jaws, each with a tooth for a distinct purpose.
Every form of organism that ever lived has gone extinct, because it could no longer survive. Ecosystems may be preserved in a single complex system of interactions among living organisms. This is not the case, however, with the extinction of species. It is estimated that only less than five percent of all genera are still alive today. Most of these extinct organisms had important roles in the Earth’s ecosystem, and were involved in one way or another in the evolutionary development of plants and animals.