The sense organs are in no way responsible for the sensations because the power of sensation is res­tricted only in the central nervous system, these serve merely as avenues of approach to central nervous system.

Sense organs are of common occurrence from invertebrates to vertebrates. In protozoans they are represented by pigment spots which are sensitive to light.

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In arthropods sensilla represent the simple types of sense organs, they contain only a few nerve cells and occur in hairs or setae, scales, cones, tubes, slits, or the like.

Some sensilla serve as mechanoreceptors which respond to tactile stimuli, to sound, to other vibrations or to pressure changes.

Another type of sense organs that are relatively simple, though they are complex in higher arthropods and molluscs, are the statocysts of lower inverte­brates.

The simpler statocysts are multicellular structures that are innervated by the dendritic ends of bipolar sensory neurons.

They are located in various regions of the body and contain a mineral concre­tion. As an organism moves, the concretions follow the course of the movement and signal information to integration centres through the nerve endings that they conic in contact with.

In vertebrates the sense organs are of very complex nature and all degrees of structural complexity are found in sense organs throughout the vertebrates and invertebrates.

The sense organs are generally destined to receive only one kind of stimulus and not any other and thus they are adequate for the stimulus.

Any change in the environment capable of causing an active response is referred to as stimulus.

Various changes having direct influence:

The various changes that have direct influence on the variously located sense organs or receptors are the following:

External changes:

The external changes of biological im­portance are variations in the wave length and intensity of light, variations in molecular shapes which are detected as smells or tastes, and mechanical or pressure changes detected as hearing or touch.

Internal changes:

It is important that the body monitors the changes that are taking place within it so that homeostatic adjustment can be made.

Chemical and osmotic changes, temperature fluctua­tions, mechanical changes and gas tensions are some of the important pieces of information that are fed into the central nervous system by the internal sense organs.

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