(2) The organs of digestion: the food tube and collections of cells which form the glands connected with it. The enzymes in the fluids secreted by the latter change the foods from a solid form (usually insoluble) to that of a fluid. Such fluid may then pass by osmosis, through the walls of the food tube into the blood.
(3) The organs of circulation: the tubes through which the blood, bearing its organic foods and oxygen, reaches the tissues of the body. In simple animals, as the sponge and hydra, no such organs are needed, the fluid food passing from cell to cell by osmosis.
(4) The organs of respiration: the organs in which the blood receives oxygen and gives up carbon dioxide. The outer layer of the body serves this purpose in very simple animals; gills or lungs are developed in more complex animals.
(5) The organs of excretion: such as the kidneys and skin, which pass off nitrogenous and other waste matters from the body.
(6) The organs of locomotion: muscles and their attachments and connectives; namely, tendons, ligaments, and bones.
(7) The organs of nervous control: the central nervous system, which has control of coördinated movement. This consists of scattered cells in low forms of life; such cells are collected into groups and connected with each other in higher animals.
(8) The organs of sense: collections of cells having to do with the reception and transmission of sight, hearing, smell, taste, touch, pressure, and temperature sensations.
(9) The organs of reproduction: the sperm and egg-forming organs.
Almost all animals have the functions mentioned above. In most, the various organs mentioned are more or less developed, although in the simpler forms of animal life some of the organs mentioned above are either very poorly developed or entirely lacking. But in the so-called "higher" animals each of the above-named functions is assigned to a certain organ or group of organs. The work is done better and more quickly than in the "lower" animals. Division of labor is thus a guide in helping us to determine the place of animals in the groups that exist on the earth.
