If you touch the hydra with a needle, then it will shrink into a lump. What is the name of this reaction? What processes does it consist of?

Reflex is a reaction of a living organism to an irritant, which takes place with the participation of the nervous system.
It consists of processes:
Receptor perception of irritation
– transmission of excitation (in hydra – from cell to cell, because the nervous system is diffuse)
– response (in hydra – compression in a lump, with the participation of epithelial – muscle cells).