What Is an Animal?

Outline ■ How are animal bodies typically organized? ■ What are four major functions of animals? ■ How are animals classified?

■ How are animal bodies typically organized? Animals are composed of many cells, the basic units of structure and function in living things. The cells of most animals are organized into higher levels of structure, including tissues, organs, and systems.

Tissues, Organs, Systems A tissue is a group of similar cells that perform a specific function. Tissues may combine to form an organ. An organ is a group of different tissues that performs a more complex function than each tissue could perform alone. Organs combine to form systems.

Functions of Animals to obtain food and oxygen keep internal conditions stable move, and reproduce.

Adaptations Structures or behaviors that allow animals to perform these basic functions in their environments are called adaptations.

Reproduction Most animals produce by sexual reproduction The joining of an egg cell and a sperm cell is called fertilization. Some animals reproduce by asexual reproduction

Classification of Animals Biologists have grouped animals into phyla. Animals are classified based on how they are related to other animals. These relationships are determined by the animal’s body structure, the way the animal develops, and its DNA.

Classification of Animals One important structural characteristic used to classify animals is whether or not the animal has a backbone. Animals without backbones make up most of the animal species on Earth. These animals are called INVERTEBRATES. Animals with backbones are called VERTEBRATES.