Nerve ring of the hypostome in hydra: is it an origin of the central nervous system of bilaterian animals?

Abstract

A hypothesis, 'the nerve ring in hydra shares a common origin with the central nervous system in bilaterian animals', is discussed in this review. The nerve ring of hydra is a ring of neurons whose neurites make a bundle running circumferentially around the hypostome just above the tentacle zone. This nervous structure has unique features in the hydra nervous system. It shows a tight association of neurons in contrast to the diffuse nerve net seen in other regions. It shows static developmental characters in contrast to the dynamic features of hydra nerve net present in other regions. Moreover, its structure and location are similar to the central nervous system (CNS) of other animals without a complex CNS such as nematodes and starfishes. Functions of the hydra nerve ring are also studied to test the hypothesis. The identified function is a crumpling of the tentacles, corresponding to the function of the inner nerve ring of hydrozoan jellyfish. The jellyfish nerve ring is considered to be a primitive central nervous system of radiates. Considering all the information available, the hypothesis is highly possible.