The Open Door Web Site : Biology : Communication Inside the Organism

An animal often needs to react quickly to the sensory information it receives. In animals with a simple body structure special cells, called nerve cells, connect the sensory cells directly to the muscle cells. This forms a simple nerve net, as found in the jellyfish.

One-celled animals, such as Ameoba and Euglena, are able to detect and react to the stimuli of light, chemicals and touch. In one-celled organisms it is the whole cell which acts as a sensory receptor.

Drawing of Hydra to show simple nerve net © Shirley Burchill

Jellyfish, such as hydra, have a simple nerve-net which connects with the stinging cells in the tentacles.

In larger animals whose bodies are composed of thousands of cells, such as jellyfish, stimuli are detected by specialized cells. This means that the animal has developed cells whose only function is to detect stimuli.

Sea anemone and clown fish, Bristol Zoo, UK © Shirley Burchill

Jellyfish have stinging cells which are sensitive to the touch of prey. The stinging cells may also react to the chemicals on the bodies of the prey. When the trigger of the stinging cell is touched, a poisonous thread shoots out from inside the cell. Hundreds of stinging cells may be released to paralyse and capture a single prey.

Once the jellyfish has captured its prey, the prey needs to be placed in the mouth and from there into the body of the jellyfish, so that it can be digested. It is one thing to capture a prey but, unless the muscle cells of the tentacles are "informed" of its capture, the prey will stay attached to the tentacles until it rots away. The jellyfish has a simple nervous system made of specialized nerve cells. These nerve cells are linked together to form a nerve chain. The nerve cell at one end of the chain is in contact with the stinging cell. The nerve cell at the other end of the chain connects with a muscle cell. In this way, each stinging cell is indirectly connected to a muscle cell. Therefore, when the stinging cells capture a prey, the nerve net informs the muscle cells. The muscle cells then contract so that the tentacles move the prey to the jellyfish's mouth.

In large, complicated animals, such as the earthworms and insects, large groups of nerve cells have developed close together.

These collections of nerve cells at the anterior (front) end of the earthworm and in the head of the insect represent the "brain" of these animals.

The brain co-ordinates the information coming into it from the sensory cells. It then sends information to the muscle cells and other tissues through a network of nerve cells.

The brain receives the information from the different sensory cells and sends out information to the muscle cells and other tissues. The larger and more complicated animals are, the more the brain is developed in order to cope with the extra co-ordination required.