The various organs of living things are grouped so as to form the several
organic systems. There are eight of these, and each performs a group of
related tasks which are necessary for complete life. The alimentary system
concerns itself with three things: it gets food into the body, or ingests;
it transforms the insoluble foods by the intricate chemical processes of
digestion; and it absorbs or takes into itself the transformed food
substances, which are then passed on to the other parts of the body. It is
hardly necessary to point out that the ingestive structures for taking
food and preparing it mechanically lie at and near the mouth, while the
digesting parts, like the stomach, come next, because chemical
transformation is the next thing to be done; while finally the absorbing
portions of the tract, or the intestines, come last. The second group of
organs, like gills and lungs, supplies the oxygen, which is as necessary
for life as food itself; this respiratory system also provides for the
passage from the body of certain of the waste gases, like carbonic acid
gas and water vapor. The excretory system of kidneys and similar
structures collects the ash-waste produced by the burning tissues, and
discharges this from the whole mechanism, like the ash hoist of a
steamship. The circulatory system, made up of smaller and larger vessels,
with or without a heart, transports and propels the blood through the
body, carrying the absorbed foods, the supplies of oxygen, and the waste
substances of various kinds. All of these four systems are concerned with
"commissary" problems, so to speak, which every individual must solve for
and by itself.

Another group of systems is concerned with wider relations of the
individual and its activities. For example, the motor system accomplishes
the movements of the various organs within the body, and it also enables
the organism to move about; thus it provides for motion and locomotion.