A Basic Introduction to Cells and the Biology of Living Things

In order for an animal to be alive, it must have both internal and external organs. At the most basic level, all animals are multicellular, meaning that they need both an internal body that stores food and an exoskeleton that protect this body from the environment. In some cases, animals are even capable of moving, breathe oxygen, can breed sexually, and eat. However, in other domains, animal life is made up of separate tissues and organs.


All animals do not share these characteristics. Some animals are eusocial, which means that they live in a group of hundreds or thousands of organisms that collectively care for their young. On the other hand, other animals are solitary and feed on their own.

The classification of the animal kingdom includes both humans and other vertebrates. Humans are classified as a mammal (although most people will immediately think of cats or dogs when thinking of the mammal category). Other vertebrates include fishes, amphibians, reptiles, crustaceans, insects, non-mammalian unicellular organisms such as algae, and social insects such as bees. There are genes that make both humans and all other vertebrates differ from the rest of the animal kingdom, called species-specific genes.

Genes produce differences between different types of mammals and insects. In a way, all animals are but simple versions of one another. Most animals belong to one of six major categories of mammals: eutherians (order eusocial), chordates, metatheriids, Prototheria, unicellular organisms, and metazoa. Insects belong to the same order as crustaceans and mollusks, which also belong to the class of unicellular organisms.

Every animal has certain characteristics that set it apart from other animals. When looking at animal taxonomy, whether it is an animal or a plant, consider what its body plan is like. Insects have bodies with head, thorax, abdomen, and wings. They have jointed limbs, jaws, tongue, throat, and even genital organs.

Animals and insects differ in cell division, both in size and number of cells. They also have genetic material that determines their shape and color. The numbers and types of cells in animals and in insects can be compared using specific cells. A comparison of DNA, however, is difficult because different animals and insects have very slight similarities. The letters on the DNA stand for pairs of identical pairs, so if a DNA sequence is repeated across two pieces of a pair of chromosomes it stands to reason that it is a variation.

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