Animals are multicellular, living organisms in the bacterial, prokaryotic, eukaryote, metallic, and protoplanktonic domains of the Kingdom Animalia. With few exceptions, all animals breathe oxygen, consume organic matter, can move, reproduce sexually, and consume other living organisms. Some animals are warm-blooded and have a skeleton and a digestive tract while other animals are cold-blooded and have no skeleton and no digestive system. A few animals are semi-dormant and are diurnal or nocturnal animals. Most animals have five senses, including smell, touch, taste, hearing, and sight.
Virtually all animals have a brain and a nervous system, although some are extremely simple and lack a nervous system. The major parts of an animal’s body are the legs, claws, head, tail, and organs or muscles. The eyes are located in the front of the head and there are about two hundred known animal eyes.
Cows, horses, pigs, dogs, cats, birds, and reptiles (such as snakes) are the presently recognized animal groups. There are about four hundred million animal species worldwide. Among vertebrates, fish have the only true spinal cord while all other animals develop their nervous systems through gill membranes or through the mouth. Insects have both wings and the exoskeleton (the upper portion of the body covering).
Among invertebrates, fishes have the most complex nervous system along with the most elaborate digestive system. Many insects are social, i.e., they live in social groups of a few thousand to several million animals. Multicellular animals such as algae are examples of social animals.
An animal’s digestive tract is generally divided into two compartments: the pyloric or stomach and the duodenum. The animal body can digest food both at the pyloric rate and at the metabolic rate. Digestive apparatus has organs that secrete digestive enzymes. Metabolism is the process by which the body converts the food that has been consumed into substances of energy. All animals use some form of metabolism to convert food into energy.
Most animals have sex cells or reproductive organs. Sex cells are present both in single and in pairs. There is a great deal of variation between the number of sex cells present in an animal and in its corresponding reproductive traits. Some animals reproduce both sexually and asexually. All living things, including humans, belong to a complex network of interacting animal bodies and their respective environments.
Animals are classified into two broad categories: domestic and wild. Domestic animals are those that are raised commercially for the personal use of humans. Examples of domestic animals are dogs, cats, horses, chickens, rats, birds, and fish. Wild animals are those that are kept on farms for the commercial production of food, fuel, or other products. Examples of wild animals include alligators, lions, cheetahs, hyenas, bears, deer, raccoons, venomous snakes, foxes, monkeys, and alligators. A distinction should be made between wild and tame pets.
Virtually all mammals are multicellular. Most animals have several types of reproductive organs: the ovary, uterus, fallopian tubes, vulva, vagina, and cervix. Each of these organs’ functions in the process of reproduction, though some organs function in conjunction with each other. Some animals possess only one reproductive organ, while others may possess more than one.