Animals are multicellular, living organisms in the Kingdom Animalia. With few exceptions, all animals breathe oxygen, eat organic matter, can move voluntarily, can reproduce voluntarily, and secrete body secretions. In fact, they have evolved over millions of years from a very early ancestor with a bony labyrinth that served as a nervous system and home to the first vertebrates. During the last million years or so, creatures have become complex enough to have complex brains, nerves, muscles, and even glands.
Although all vertebrates (all animals with backbones) belong to the class of vertebrates, most animals retain their terrestrial (land) habitats and feed on a variety of plants and animal tissues such as fur and blood. The class of mammals includes all the living species of all the classes of animals and reptiles. A few invertebrate animals and some protozoa (males & females of certain mollusks and snails) also belong to this class of animals. In addition, there are fishes, amphibians, protozoa, ungulates (amphibians with legs and tail), and other classifications of animals that have both land and aquatic modes of transportation.
An animal’s reproductive system is complex, including the ovary, uterus, fallopian tubes, cervix, vulva, and several other components. The development of an animal begins in the egg and continues through fertilization, hatching, birth, and death. Throughout an animal’s life, a variety of sex cells (sex organs) exist and reproduce sexually. The distinction between a male and a female animal is based primarily on these sex cells and their respective roles in reproduction.
Most animals have both a male (Y chromosome) and a female (X chromosome) sperm. They sperm has the ability to travel to several locations on the female’s body to fertilize the eggs, while the X sperm has the ability to penetrate the female’s thick mucus layer and reach the egg. Some animals, including humans, have a single X chromosome, while other animals, such as frogs and chickens, have multiple X chromosomes. Sexual differentiation within an animal body is often caused by gene regulation mechanisms. Gene regulation is important in animals because some chromosomes, such as the Y chromosome, are considered harmful to animals, while others, such as the X chromosomes, are beneficial.
An animal’s sensory organs are located in areas of the head and body. Some animals have highly developed sensory organs, whereas other animals possess more developed senses. The former have better vision than other animals, but they do not have highly developed sense organs that can detect high frequency sound waves or the movements of prey. Certain animals have highly developed sense organs such as cats, but cats lack other features such as the tongue or the capability to “feel” temperature.
An animal’s heart is considered to be classified as either a muscle or a gland. A muscle is defined as any tissue present in an animal that produces energy for movement, while a gland is defined as any tissue present in an animal that produces a surplus of hormones for growth, maintenance, and development. The heart is present in all land animals. In land animals, such as horses, cattle, ostriches, and zebras, the heart is present in the chest area. In water animals, such as ducks and some forms of whales, the heart is present in the abdomen.
