#General Characters of Bacteria.#--A _bacterium_ consists of a finely granular mass of protoplasm, enclosed in a thin gelatinous envelope. Many forms are motile--some in virtue of fine thread-like flagella, and others through contractility of the protoplasm. The great majority multiply by simple fission, each parent cell giving rise to two daughter cells, and this process goes on with extraordinary rapidity. Other varieties, particularly bacilli, are propagated by the formation of _spores_. A spore is a minute mass of protoplasm surrounded by a dense, tough membrane, developed in the interior of the parent cell. Spores are remarkable for their tenacity of life, and for the resistance they offer to the action of heat and chemical germicides. Bacteria are most conveniently classified according to their shape. Thus we recognise (1) those that are globular--_cocci_; (2) those that resemble a rod--_bacilli_; (3) the spiral or wavy forms--_spirilla_. _Cocci_ or _micrococci_ are minute round bodies, averaging about 1 µ in diameter. The great majority are non-motile. They multiply by fission; and when they divide in such a way that the resulting cells remain in pairs, are called _diplococci_, of which the bacteria of gonorrhœa and pneumonia are examples (Fig. 5). When they divide irregularly, and form grape-like bunches, they are known as _staphylococci_, and to this variety the commonest pyogenic or pus-forming organisms belong (Fig. 2). When division takes place only in one axis, so that long chains are formed, the term _streptococcus_ is applied (Fig. 3). Streptococci are met with in erysipelas and various other inflammatory and suppurative processes of a spreading character. _Bacilli_ are rod-shaped bacteria, usually at least twice as long as they are broad (Fig. 4). Some multiply by fission, others by sporulation. Some forms are motile, others are non-motile. Tuberculosis, tetanus, anthrax, and many other surgical diseases are due to different forms of bacilli. _Spirilla_ are long, slender, thread-like cells, more or less spiral or wavy. Some move by a screw-like contraction of the protoplasm, some by flagellæ. The spirochæte associated with syphilis (Fig. 36) is the most important member of this group.