The Aplysia withdrawing it's gills is demonstrating a gill and siphon withdrawal reflex (GSWR) phenomenon
Explanation:
The hill and siphon withdrawal reflex (GSWR) is involuntary and a defensive reflex. This reflex causes delicate siphon and gill to be retracted when an animal is disturbed.
A two-component reflex is triggered when weak or moderate stimulus is applied to a siphon or the mantle shelf. These two components consist of two reflex acts, the siphon-withdrawal reflex and the gill-withdrawal reflex. Together they often form a reflex pattern with short latency that protects the animals gill and siphon to potentially threatening stimuli.
An habituation in an Aplysia californica for example is which is an Aplysia gills is when a stimulus is presented continously to an animal and there is a progressive decrease in response to that particular stimulus.
