⭐ Cnidaria (e.g. Hydra) body form


Introduction
The basic nervous system
Nerve nets
Hydra


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Introduction

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Hydra and jellyfish are multicellular organisms that are radially symmetrical. This means they look identical when split at any point around a circle, looking downwards onto their body plan.

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The basic nervous system

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They have very basic nervous systems and digestion, and the aqueous medium they reside in supports their bodies.

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They don’t have a centralised system with a brain or a spinal cord. Their neurones are simply spread across their body and interconnected, producing signals that are not traceable depending on the location of the stimulus.

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This means that if there is food or danger on their left side, they wouldn’t know, and instead respond overall. They move accordingly towards food or away from a negative stimulus, as a whole and without regard for the stimulus location.

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Nerve nets

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The sea star, however, does have nerve nets in each arm, connected centrally by a ring, so it can have more specific responses compared to a full-body net such as that of hydra.

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