Definition of animal. According to definition, an animal is any living being classified as a mammal by strict definition. There are currently three major classes of mammals: amphibians, prototheria, and vertebrates. A few types of mammals include all four quadrant breeds of cats, dogs, horses, rats, porpoises, dolphins, and other cetaceans. Some lizards and snakes also classify as animals, however, these classifications depend more on taxonomic analysis than biological reality.
In comparison, there are over four hundred thousand species of invertebrates and more than seven hundred thousand species of mollusks. The most common animals on earth are both worms and insects. Although more commonly associated with the aquatic or semi-aquatic domain, crustaceans, snails, and other soft-bodied scavengers are also animals. All crustaceans, including clams, oysters, crabs, and snails, belong to the class of mollusks. Other invertebrates that fall into the invertebrate or arboreal domain are amphibians, which include such diverse creatures as frogs and salamanders, both in the fresh and salt domains and fishes which include such famous sea creatures as penguins, sharks, turtles, and rays.
The main article will focus on animals. Animals are categorized in two broad ways. The first is by strict morphological classification. In this case, all members of a group are lumped together under the same name, which helps simplify the taxonomic process. Another method of classification, though it relies more on genetic evidence than actual morphological similarities, consists of assigning an animal species to a group based on similarities of its genome.
The second type of classification is based more on similarities of behavior. Animal behavior is frequently described using a combination of morphological and behavioral attributes. Thus, Crocodiles may be classified as reptiles and amphibians, but they have elk-like antlers that function as eyes, and their hooves (both feet and legs) are well developed. All living creatures can be grouped into one of three broad categories: land animals, aquatic animals, and insects. Land animals are mammals, birds, and reptiles such as lizards, chameleons, bees, fish, and others.
Aquatic animals have bodies and heads that connect to the water. They include all classes and varieties of the fish family, as well as all the sub-classes of each of those families. Insects have insectoid bodies and head arrangements, while the classifications of land-mammal and mammal are generally accepted as referring respectively to reptile and to mammal. Finally, all land animals share a body plan (the basic outline of their body) that has evolved over millions of years and which is highly simplified today. The detailed body plans of all these classifications are depicted in diagrams called phylogenetics, where each branch of the tree is assigned a single genetic form.
As to the origin of species, whether they depend upon animal exploitation, or adapt to their environment on their own; the fossil record provides many examples. In most cases, there is nothing that can be directly traced to an animal source, although fossils can be found in soils, rocks, and sediments. In fact, modern humans are indirectly responsible for much of what goes on, as we eat far more animal products than we did a few generations ago.