An invertebrate marine mollusk that has been subjected to extensive studies of cellular mechanisms of learning and memory because changes in its behavior may be linked to physiological and biochemical changes in specific neurons. Its neurons are 10 times the size of mammalian neurons, and its nervous system is composed of a limited number of neurons that may be traceable. Eric Kandel, M.D., won the Nobel Prize in Medicine in 2000 for his work in this area.