A worm may not look like much. Compared to other animals, such as cats, a worm really isn't that complicated -- it doesn't even have any bones. Yet, for such a simple organism, the worm still evolved methods to sense various stimuli in its environment in order to survive.
Worms such as earthworms have tissue at their heads that is sensitive to light. These are called photoreceptors, and in some worms they are actual spots like eyes. They allow the earthworm to sense when it's too bright above the ground to come out, as strong lights negatively impact them. They live in the dark, underground, but they can and do like low lights.
The Nervous System
Worms have a simple nervous system that consists of one major central system and several secondary nerves. The main system is a nerve cord that runs down the length of its body at its ventral (bottom) side and is attached to the rudimentary brain (cerebral ganglia) in its head. Each segment of the worm has a ball of nerves in it attached to the main cord, and these nerve balls (ganglia) stretch out miniscule fingers of nerve endings in the segment. These nerves help control motor functions and other parts of the worm's body and behave like our own nervous system: they are also linked to receptors on the skin that sense heat, cold, vibrations and other forms of touch. The nerve cord also makes the worm's muscles contract to rapidly shrink away from danger.
A worm's skin is made to stay moist and cool. It's covered with tiny hairs called setae, which help to anchor it inside the earth and keep it still. The epidermis also has single-celled sense organs all over it to cover a myriad of stimuli. If the worm's skin senses itself growing too dry, the worm will produce body fluids and mucus to remedy the situation. The skin has various loose-ended nerves within it that aid in sensing nearby movement -- moderate stimuli tend to remain ignored, while strong stimuli (stepping close to it) cause the worm to react.
At the head of the worm, above the mouth, is the tip of its body. This tip is not really a nose, as worms don't have them, but it is an important part of the sensory structures. This area is called the prostomium. It's filled with nerve endings that lead directly back to the brain. The prostomium also has chemoreceptors, small receptors that sense chemicals. Worms eat dirt and remove nutrients from that dirt; as a result, they require some way to sense the organic content, acidity, texture and calcium content of the earth in order to survive.
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