Animals are multicellular, multirellular organisms which form the Kingdom Animalia. With few exceptions, all animals breathe air, eat organic matter, can move, reproduce by means of sexual reproduction, and develop from a rudimentary sphere of jelly-like cells, the placenta, during early development. Although animals and plants do not have their own brains, they are capable of storing and retrieving information. All living things are connected by cord-like fibers called nerves, which convey both visual and tactile information to the brain. The body parts of animals are generally separated into three general sections: the skin, muscle, and connective tissue.
Insects are one of the simplest kingdom Animalia, with most members having wings, an exoskeleton, and a nervous system. They are the only multicellular organisms with a central nervous system and no genitalia. Insect limbs can be found together in a bundle called the caudal trunk, while the head and thorax (back and front) are covered with hairs. Most insects have wings, which are covered with feathers.
Cephalopods are the largest single class of animal with over 300 known species. They are arthropods, meaning they have both a hard outer shell and soft inner one. Cephalopods have paired fins, and the anal fin is especially long and bent backwards to assist in swimming. In addition to the typical Cephalopod diet of shrimp, clams, snails, other invertebrates, and insects, some cephalopods are specialists at eating various types of mollusks, including oysters, snails, walnuts, clams, and other mollusks.
Bacteria and other unicellular organisms make up the second major class of Kingdom Animalia. These animals are eukaryotes, which are part of the Kingdom Animalia, (and they are also theacteria and other multicellular organisms mentioned above). The eukaryotes generally make up their entire reproductive organ. These animals have a single nucleus, an immune system, and no genitalia (including the female reproductive organ). The entire reproductive process is mediated by the specialized sensory organs (sense organs).
The third kingdom is made up of Archaea. This kingdom contains a number of specialized eukaryotic animals, most notably Archaea (which contains just about all the enzymes required for protein metabolism) and eukaryotes (which can secrete the proteins that build up the body). These organisms are composed of both symbiotic and un symbiotic cells (e.g., symbiotic algae secreting enzymes that enable photosynthesis of food), and a variety of different types of cellular membranes. Some eukaryotes secrete their own proteins and grow independently by means of the intracellular membranes they produce, while others are dependent on other organisms for growth and other functions.
Finally, we have the fourth kingdom of animals, which are the Protista. Protista are single-celled animals with a single nucleus and organelles (which we might term organs). Most have paired organs such as the eye and the mouth. The other organs in these animals are generally separated, and there is specialization within each organ. It is worth noting that the arrangement of many of these organs in both animals is similar to the arrangement in humans: the liver and kidney are on the left side of the body in humans, while the spleen and the pancreas are on the right side.