The antennae are continuously moving so as to pick up stimuli coming from the outside world. These motile antennae actively search for external stimuli.

The sensory receptor cells of different organs are not absolutely alike. There are some substantial differences. For example, the visual cells contain a special substance, known as visual purple, which can change under the impact of light. This photochemical reaction makes the perception of light possible. There is no visual purple in the receptor cells of the other sense organs. The substances by which they perceive stimuli have not yet been known.

The great similarity in the structure of the various re­ceptor cells is hard to explain. Probably, the overall design scheme proved to be so practical that this prompted nature to make use of a standard model in constructing all the diverse sense organs.

Over millions and millions of years, the animal kingdom on our planet trod the long path of evolution from primitive unicellular creatures, which were hardly able to per­ceive external stimulation, to modern humans with their numerous, perfect and highly receptive sense organs. One might think that man could have nothing in common with an infusorium, but this is not so. The receptor cells of a man and a bird, a fish and an insect, a mollusk and any other animal perceive the outside world and respond to any stimulation from the environment using motile antennae which are very similar in design. Even unicellular organisms, such as the Euglena, use motile antennae. These antennae are so well designed that they have survived for thousands of mil­lions of years. The living organisms of the Earth have preserved their motile antennae from the very origin of life up to the present day.

They made the astonishing discovery that the receptor cells (the ones that perceive stimulation) in all the sense organs of all animals on the Earth are very similar in struc­ture. Each of them has a tiny mobile hair, or flagellum. These flagella are also very similar in diflerent receptor cells. Each flagellum contains two central fibrils surrounded by a ring of nine pairs of motile fibrils. The structure of the flagellum rarely varies.

The flagella in a receptor cell play the same part as the antennae in radio sets, and are therefore called recep­tor antennae. Through these antennae we perceive the outside world. The antennae in the receptor cells of the eye, for example, respond to light energy, or photons. The antennae of the organ of smell perceive the energy of the molecules of odoriferous substances. The antennae of the auditory cells respond to the energy of sound waves, or sounds.