Organs are internal structures that carry out specific functions. An organ is composed of an outer covering, or epithelium, and one or more other kinds of tissues. Indeed, most organs include all four tissue types. The gut (digestive tract) is a good example (Figure 39.3). Its inner surface is lined with a sheet of columnar epithelial cells. Different types of epithelial cells in this lining secrete hormones or digestive juices or absorb nutrients from the gut. Beneath the epithelial lining is a layer of connective tissue called the mucosa. Within this connective tissue are blood vessels, neurons, and glands (clusters of secretory epithelial cells). Concentric layers of smooth muscle tissue enable the gut to contract to mix food with digestive juices. A network of neurons between the muscle layers controls these movements.

An individual organ is usually part of an organ system, a group of organs that work together to carry out certain functions. The stomach, small intestine, liver, and pancreas, for example, are parts of the digestive system. The structure of an animal involves an organizational hierarchy. Cells form tissues, tissues form organs, organs work together as organ systems, and all work together as a multicellular organism.