The word cell refers to several types of organisms. Cells such as paramecia, dinoflagellates, diatoms, and spirochetes are self-maintaining organisms; cells such as lymphocytes, erythrocytes, muscle cells, nerve cells, cardiac muscle, and chloroplasts are more specialized cells that are a part of higher multicellular organisms. Regardless of size or whether the cell is a complete organism or just part of an organism, all cells have certain structural components in common. All cells have some type of outer cell boundary that permits some materials to leave and enter the cell and a cell interior composed of a water-rich, fluid material called cytoplasm that contains hereditary material in the form of deoxyribonucleic acid (DNA).