Neural System

The neural system found across the animal kingdom is built from highly specialised cells called neurons — cells adapted to detect a stimulus, receive it, and transmit it onward as an impulse.

How elaborate this system is varies a great deal across animal groups, becoming progressively more organised as we move to more complex animals:

• In simple, lower invertebrates the neural organisation is minimal. Hydra, for example, has no brain or ganglia at all — its neurons form a diffuse network spread through the body, often called a nerve net.
• In insects the system is considerably better organised: a brain is present, along with a number of ganglia (clusters of nerve cell bodies) and connecting neural tissue. …