(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 coordinated 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. The Animal Series.--We have found that a one-celled animal can perform certain functions in a rather crude manner. Man can perform these same functions in an extremely efficient manner. Division of labor is well worked out, extreme complexity of structure is seen. Between these two extremes are a great many groups of animals which can be arranged more or less as a series, showing the gradual evolution or development of life on the earth. It will be the purpose of the following pages to show the chief characteristics of the great groups of the animal kingdom. [Illustration: The glasslike skeleton of a _radiolarian_, a protozoan. (From model at American Museum of Natural History.)] I. Protozoa.--Animals composed of a single cell, reproducing by cell division. The following are the principal classes of Protozoa, examples of which we may have seen or read about:-- CLASS I. _Rhizopoda_ (Greek for _root-footed_). Having no fixed form, with pseudopodia. Either naked as _Amoeba_ or building limy (_Foraminifera_) or glasslike skeletons (_Radiolaria_). CLASS II. _Infusoria (in infusions)._ Usually active ciliated Protozoa. Examples, _Paramoecium_, _Vorticella_. CLASS III. _Sporozoa (spore animals)._ Parasitic and usually nonactive. Example, _Plasmodium malariae_. [Illustration: A horny fiber sponge. Notice that it is a colony. One fourth natural size.] II. Sponges.--Because the body contains many pores through which water bearing food particles enters, these animals are called _Porifera_. They are classed according to the skeleton they possess into limy, glasslike, and horny fiber sponges. The latter are the sponges of commerce. With but few exceptions sponges live in salt water and are never free swimming.