The word “animal” has many meanings, but most commonly refers to a class of living things with no appendages or a nervous system. Humans are mammal; birds, insects, and fish are animals. All other vertebrates are classifications of animals. Animals have lived in all parts of the earth. They have died out in many places due to destruction or habitat loss, but there are many vegetarians around who would kill animals if they were on a menu.
The term “animal” is usually used in a non-scientific way to refer to some form of living being. Animals reproduce by asexual reproduction, where all reproduction occurs without the involvement of any male or female. With a few exceptions, animals eat vegetation, breathe oxygen, can move, reproduce sexually, and consume other animals as food.
The classes of animals are broadly divided into two major categories: Protista (plant-like) and Eutheria (animal-like). Within each class of animals are the classification of kingdoms. Kingdom animalia consists of more than seventy different kinds of animals, including all fishes and most crustaceans. This kingdom contains representatives of all animal groups.
Eutheria consists of animals with a central nervous system, such as whales and dolphins. It also includes reptiles, birds, mammals, and porpoises. A variety of eutherian animals reproduce by a process called sexual reproduction, through which one reproduction event may occur over the rest of an animal’s life. Some eutherians reproduce asexually, by breeding together to form a species. Some others reproduce asexually by means of a process called cloning, where one individual’s reproductive cells are injected into an egg that has already been fertilized.
The eutherians are very diverse in their arrangement of cells and their ability to form organs. Many are without a cell wall, while many others have both a cell wall and a tail. However, all animals with a cell wall live only a single life cycle, regardless of whether they reproduce or not. Animals that reproduce through sexual reproduction have a living tissue called a reproductive organ, while an animal that reproduces by cloning has no cells of its own. Therefore, all animals are either asexually reproducing (sexually reproducing) or non-reproducing (naturally reproducing).
Insects are among the most studied animals in all animal research. Insects have very detailed body plans with complex structures, which has made it simple for scientists to study them. They can be further classified into about twelve kingdoms of insects, all of which have specific names. For instance, there is the family of insects known as mosquitoes, whose members are also called flies. All the insects in this family are found around the world. Some of the insects belong to the insect kingdom, but are classified as belonging to another animal kingdom.