The brain is small and simple in some species, such as nematode worms ; in other species, including vertebrates, it is the most complex organ in the body.

The brain is small and simple in some species, such as the nematode worm, where the body plan is quite simple: a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement ( a ganglion ) for each body segment, with an especially large ganglion at the front, called the brain.

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