This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
Amoeba is the name of a family of animals which are microscopically minute, and inhabit both salt and fresh water. They consist of a mass of protoplasm unlimited by any envelope, containing granules, and usually a clear, rounded, firmer body, the nucleus, with a still denser speck in its interior, the nucleolus. This mass of protoplasm moves about by throwing out temporary processes in different directions, and changing its form by virtue of its contractility. In fact, the powers of assimilation, reproduction, irritability, and contractility, appear all to be present in" one common mass. There are other families in which such a mass as constitutes the amoeba is surrounded by a membranous covering or a hard shell.