Nerve cord

A rod-shaped body contains a digestive system running from the mouth at one end to the anus at the other. Alongside the digestive system is a nerve cord with a brain at the end, near to the mouth.
Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front

In some bilaterian animals, the nerve cord, which runs from the head to the tail, forms a main part of the central nervous system. Depending on their location they are classified as:

Some bilaterians have no nerve cords while others have more than one nerve cord; for example, both nerve cords are present in hemichordates. The two types of cords need not be singular: Platyhelminthes have two ventral nerve cords. Different lineages evolved nerve cord structures independently from an ancestral nerve net using a similar set of transcription factors across the lineages.[1]

References

  1. ↑ Martín-Durán, José M.; Pang, Kevin; Børve, Aina; Lê, Henrike Semmler; Furu, Anlaug; Cannon, Johanna Taylor; Jondelius, Ulf; Hejnol, Andreas (January 2018). "Convergent evolution of bilaterian nerve cords". Nature. 553 (7686): 45–50. doi:10.1038/nature25030. PMC 5756474.
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