Final answer:
Hydra respond to a tap on their substrate through a thigmonastic response by retracting their tentacles as a protective measure.
Step-by-step explanation:
Hydra, a type of freshwater polyp, respond to a tap on their substrate through a thigmonastic response. Thigmonastic responses are touch responses that are independent of the direction of stimulus. In the case of Hydra, when they are tapped, their tentacles retract, protecting their vulnerable body parts from potential harm or disturbance.
This response is a protective mechanism that allows Hydra to remain safe when there is a sudden disturbance in their environment. By retracting their tentacles, they minimize the chances of injury or disruption.
In summary, when Hydra are tapped on their substrate, they exhibit a thigmonastic response by retracting their tentacles as a protective measure.