Animals are multi-cellular, eukaryotic organism in the bacterial, placental, and animal kingdom Animalia. The word “animal” comes from the Greek word aus (animus) meaning soul or life. With few exceptions, all animals breathe oxygen, eat organic matter, can move, reproduce sexually, and eat other animals. In fact, there are more animals alive than living people, and they number into the billions of bacteria, molds, protozoa, and fungi. Most animals have nervous systems, and the nerves control many functions of the animal body.
Most animals use at least one set of muscles for walking, moving about, chewing, and dealing with prey. All mammals use at least four sets of muscles for walking, including the spinal cord, muscles of the limbs, and muscles used for digestion. All insects have four sets of jaws and they use mouth parts for breaking and sucking food. All animals have eyelids that allow them to see in the dark, and some have ears that are used to sense sound. All animals have hair tufts that cover their skin; these make the animal appear bushy.
Animal Kingdom includes all animals that belong to the Phylum Chordata, the Class Insecta, the Order Orthoptera, the Subkingdom Animalia, the Family Allcidae, the Genus Atheromatidae, the Suborder Ensifera, and the Family Prototheria. Among all known animals, the three largest are the elephant, the fish, and the bird. The five smallest are the bat, mole, chinchilla, bee, mouse, and fruit bats.
amphibians and reptiles are social animals, having a complex life structure involving both a prothallium (an egg sac) and an anus, where the adult (hatched animal) retreats to hide, usually beneath a leaf or other type of covering. Many amphibians and reptiles have gills, which must be kept moist to facilitate breathing. Amphibians and reptiles also have skin, including the smooth skin of the platyorn species, which serves as the basis for certain classes of reptiles. Amphibians, including snakes, salamanders, dragonflies, crayfish, ickets, millipedes, roaches, ants, termites, bees, wasps, and cockroaches have nerve endings in their skin, unlike the cuttlebones that do not. Insects are the only living creatures with nervous systems.
Most animals have both a sexual and an asexual reproduction system, although not every animal is able to reproduce sexually. Eukaryotic (sugary) animals reproduce asexually, by dividing into asexuallysexually reproducing cells. Sexually reproducing animals have a single set of chromosomes, while asexually reproducing animals divide into sex chromosomes, which can be altered by interference from an external factor. Asexual reproduction involves a series of events, such as the division of an animal’s gametes (which can become sperm and have their own set of chromosomes), the fertilization of a females somatic cell by an intracellular sperm, and then the actual sexual division of a sex chromosome into an x and y chromosome.
amphibians and some insects have sponges as their only ovary. Sponges are small in comparison to the size of an average animal. In animals with sponges, the sponges serve as traps to extract food from under the animal’s body. In insects the sponges serve as a storage medium for food and a breeding ground for larvae and eggs. The work of the sponges is usually by chance, as they are rarely used for any function other than this one. Some insects, such as arachnids, have jointed limbs which use the sponges for a place to sit in.