Animals are multicellular, living organisms in the microbial, biological kingdom Bacteria. With few exceptions, all animals breathe oxygen, eat organic matter, can move, reproduce sexually, and ingest other forms of matter through digestion. Although animals do not have an eye, nose, excretory system, brain, or other form of sensory perception, they do utilize such things as sight and hearing. In fact, animals may sense certain types of changes in their environment through tactile organs such as the sensitive papillae on the tips of their toes.
The term “animal” refers to any living being with a skeleton, including all forms of vertebrates, amphibians, reptiles, mollusks, arachnids, birds, and mammals. Virtually all of the earth’s animal kingdom can be considered animalia, with the exception of insects. Insects belong to the class Arachnida, which includes spiders, insects, aphids, mosquitoes, lacewings, mealy bugs, termites, spiders, and sawflies. Insects have both external and internal parasites, which depend upon the type of animal they parasitize (whether it is an animal or a plant). Parasites do not require a blood supply to survive, since their hosts, animals that feed on them, provide this supply.
A structure called the animal cell is present in all animals, including humans. The animal cell is a sphere about 4 inches in diameter and made up of three layers: an outer layer covered by a thick epidermis; a middle layer covered by a thin membrane that receives food; and an inner layer that store the accumulated feces. The animal cell has the unique position of being both its shell and its housing. The animal cell walls continuously undergo an active process of cell division and the cell wall becomes shorter and thinner as the animal grows. The size of an animal cell depends upon how large the animal is and how well-developed it is, while the function of an animal cell remains constant whether it is an animal or a plant cell.
Each animal has several types of animal cells. These include prophylactic (or skin) cells that cover the animal’s body; pilosebaceous (or hair) cells that line the head and neck; lymphocytes that circulate lymph within the animal; endocrine cells that produce hormones in the animal; and metalloplasties that assemble the animal’s skeleton. Animals also secrete fluids from their exoskeletons. These fluids include bile and feces. Bile secretions originate in the liver, while feces are excreted in the colon.
All animals share a common ancestry that goes back millions of years. Among the most prominent group of mammals are rodents and insects. Rodents have spread widely throughout the world and are found in all habitats; although they generally are less visible than other kinds of mammal species. Among insects, honey bees and certain forms of mosquitoes are the most important of all insect colonies.
Insects and arachnids constitute the bulk of the animal kingdom with over half the entire animal kingdom made up of insects. A few invertebrates make up the remaining portion of the animal kingdom. The majority of insects are aquatic. Most animals with an external skeleton (including humans) are not necessarily amphibians, but there are invertebrates such as crickets, centipedes, certain types of spiders, leeches, fish, lizards, salamanders, and snails that may be considered to be amphibian. The only known scientifically accurate classification of amphibians is based on the extent of the animal’s ability to retain its watery environment. Amnemics include some of the largest known fish, turtles, chompers, and alligators.