Aplysia are used extensively in neuroscience research. All Aplysia have exactly the same neuron configuration, making it easy to replicate experiments. Also the axon of an Aplysia neuron is fairly large (up to 1 mm in diameter), big enough to be seen under a light microscope making it considerably easier to implant electrodes.
Being a mollusk, Aplysia has a very basic neural structure making it an excellent model for studies of simple learning and memory. Experimenting on Aplysia, researchers have demonstrated that presynaptic inhibition is responsible for habituation while presynaptic excitation is responsible for sensitization.