Describe the range of variation in the nervous systems of invertebrates

Although most invertebrates have a nervous system, the complexity of the system varies from one organism to another.
Some invertebrates such as the hydra and starfish have a simplified system of nerves, which is called a nerve net. It consists of just nerves, capable of sense and touch only. Meanwhile, other invertebrates such as the planarian have a more developed nervous system, complete with a brain and nerve cord.