Amoebas excrete particles of waste from anywhere on their surface, and most rid themselves of excess water by active pumping from an organelle known as the contractile vacuole. Amoebas have no mouth or anus, and they can also take in food particles anywhere on their cellular membranes.
Amoebas are capable of changing shape, a feature which they use for several purposes. They capture food by enveloping, shaping their bodies to surround it, wrapping it in a cell membrane that they absorb as a vacuole. They move by stretching out a portion of their bodies, known as a pseudopod, and flowing into it. They have a variety of diets, with some being predators of other protists and bacteria. Others consume organic debris from waste and dead organisms. Still others are parasites of animals, and can cause disease.
Amoebas have two major stages, the active amoeba form and a cyst form. Amoebas change to a cyst form in adverse conditions, such as when food is scarce or when the cell could not otherwise survive. The cell contracts and exudes a secondary, protective membrane. It is in this form that parasitic amoebas transfer from the environment to a host, or from host to host. Once conditions improve, the secondary membrane ruptures and the amoeba emerges.