But while in such unicellular creatures all functions are properties
of the individual cell, among higher forms _systems_ take the place of
the protoplasm of the single cell. There is a circulatory system, a
respiratory system, etc.; but we must once more point out that such
systems are made up of cells, so that every function of the highest
animal may be finally reduced to what takes place in the unicellular
animal. A circulatory system consists of a heart and blood-vessels,
all filled with blood, which latter is "the life," as was known from
the earliest times; yet this same blood is of no more use for the
nourishment of the body while it is contained in those tubes which
constitute the blood-vessels than is bread locked up in a pantry to a
hungry boy. That which really provides the nutriment for the body is a
fluid derived from the blood, a something like the liquid part of
blood and known as _lymph_. This latter is to the cells of any tissue,
as a muscle, as is the water filled with the food on which an amoeba
lives. In like manner, in spite of the complicated apparatus which
supplies oxygen and removes carbon dioxide, the respiratory system,
respiration is finally the work of the cell, as in amoeba; a
muscle-cell respires exactly as does the one-celled animal.