The bodies of all living organisms, including plants, fungi, and animals, adhere to a distinct hierarchical organization, in which small, relatively simple structures combine to form larger, more complex structures. The structures can include the following:

Atoms: The smallest units that possess the characteristics of an element. Hydrogen is an example of an atom.
Molecules: Structures made up of two or more atoms. DNA is an example of a molecule.
Organelles: Groups of molecules organized to perform specific cellular functions. Mitochondria are organelles.
Cells: The smallest units that can carry out all of life’s processes. A single cell can be a complete organism, as in bacteria, or a part of a larger organism, as in urinary epithelium cells.
Tissues: Groups of cells organized to carry out a particular function in a multicellular organism, such as the epithelial tissue, which provides covering or lining to internal and external structures.
Organs: Groups of tissues organized to carry out a particular function. The urinary bladder, which is composed of urinary epithelial and muscle tissue, is an organ.
Organ systems: Groups of organs designed to carry out a particular task. The urinary system, which includes the urinary bladder, the kidney, and the ureter, is an organ system.

All animal bodies are organized to perform the same fundamental tasks: respiration and circulation, digestion, waste removal, structural support, movement, defense against disease, response to internal and external conditions, coordination of body activities, and reproduction. Physiology is the branch of biology that studies these fundamental biological tasks, and the processes by which animals carry them out.

Structural Organization among Invertebrates

Among the members of the animal kingdom, the body plans of invertebrates are very simple compared with vertebrates. The more primitive invertebrates, such as sponges and jellyfish, do not have organs at all. The table on the following page briefly outlines the levels of organization in the most common invertebrate groups.

Sponges	Several cell types but possessing no organized tissues or organs
Cnidarians	Cells organized into specialized tissues but no true organs
Ctenophores	Cells organized into specialized tissues but no true organs
Flatworms	The simplest animals, in which tissues are formed into simple organs
Nematodes	Tissues formed into simple organs
Mollusks	Tissues formed into specialized organs, as well as some basic organ systems, such as an excretory system
Annelids	Tissues formed into specialized organs, as well as some basic organ systems
Arthropods	Tissues formed into specialized organs, as well as some basic organ systems
Lophophorates	Tissues formed into specialized organs; appear to be related to the mollusks, annelids, and nematodes
Echinoderms	Tissues formed into specialized organs, as well as some basic organ systems