Page "Nervous system" Paragraph 2

Nervous systems are found in most multicellular animals, but vary greatly in complexity.

Sponges have no nervous system, although they have homologs of many genes that play crucial roles in nervous system function, and are capable of several whole-body responses, including a primitive form of locomotion.

Placozoans and mesozoans — other simple animals that are not classified as part of the subkingdom Eumetazoa — also have no nervous system.

In Radiata ( radially symmetric animals such as jellyfish ) the nervous system consists of a simple nerve net.

Bilateria, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, one central cord ( or two running in parallel ), and nerves.

The size of the bilaterian nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans.

Neuroscience is the study of the nervous system.