Let us begin with the _Protozoa_, or one-celled forms--the lowest form of animal life. The lowest form of this lowest class is that remarkable creature that we have mentioned in previous lessons--the _Moneron_. This creature lives in water, the natural element in which organic life is believed to have had its beginning. It is a very tiny, shapeless, colorless, slimy, sticky mass--something like a tiny drop of glue--alike all over and in its mass, and without organs or parts of any kind. Some have claimed that below the field of the microscope there may be something like elementary organs in the Moneron, but so far as the human eye may discover there is no evidence of anything of the kind. It has no organs or parts with which to perform particular functions, as is the case with the higher forms of life. These functions, as you know, may be classed into three groups, _i.e._, nutrition, reproduction, and relation--that is, the function of feeding, the function of reproducing its kind, and the function of receiving and responding to the impressions of the outside world. All of these three classes of functions the Moneron performs--but _with any part of its body, or with all of it_. Every part, or the whole, of the Moneron absorbs food and oxygen--it is all mouth and lungs. Every part, or the whole, digests the food--it is all stomach. Every part, or the whole, performs the reproductive function--it is all reproductive organism. Every part of it senses the impressions from outside, and responds to it--it is all organs of sense, and organs of motion. It envelops its prey as a drop of glue surrounds a particle of sand, and then absorbs the substance of the prey into its own substance. It moves by prolonging any part of itself outward in a sort of tail-like appendage, which it uses as a "foot," or "finger" with which to propel itself; draw itself to, or push itself away from an object. This prolongation is called a _pseudopod_, or "false-foot." When it gets through using the "false-foot" for the particular purpose, it simply draws back into itself that portion which had been protruded for the purpose.