The yellow string is one of the arm nerves of a cat
One-celled creatures don't have neurons, because they can communicate within themselves through the use of enzymes, but all multicellular animals do have neurons. The simplest and earliest example of neurons is in hydras, which evolved around 600 million years ago. In hydras, neurons are not organized into nerves, but just spread randomly throughout the body in a nerve net. Beginning with flatworms, about 550 million years ago, the neurons were twisted together into long nerves to reach every part of the body.
A neuron is a specialized kind of cell.
Nervous system and senses
To find out more about cells, check out these books from Amazon.com or from your library: