My first degree was in psychology, so when I began working with crustaceans, I was somewhat surprised to learn that Sigmund Freud worked on crayfish nervous systems early in his career, before producing the theories on the unconscious for which he would become famous. And the crayfish work was not trivial, either.
When I mentioned this to Jennifer Mather, my undergraduate supervisor, she immediately informed me that Jean Piaget, who work kick-started the field of developmental psychology, did many experiments with snails, particularly Lymnaea stagnalis, which is still widely used in neurobiology research. She like to get a rise out of her psychology colleagues by saying that they'd lost a damn fine malacologist when Piaget started studying psychology.
Today I learned, from the ABC Radio National show Ockham's Razor, that a third highly influential psychologist also worked with invertebrates: Alfred Binet. Binet's doctoral dissertation was titled, A contribution to the study of the subintestinal nervous system of insects. But if Binet's name sounds familiar, it's probably because you heard it as the second half of the name of the Stanford-Binet test. "Stanford" was a late addition to the test. Binet, with co-author Theodore Simon, developed it first, and published it as "New Methods for the diagnosis of the intellectual level of subnormals" in 1905. It was effectively the first IQ test. This was perhaps as influential as any invention (idea? concept?) in psychology. Despite horrendous misuse of IQ tests, I'm among those who count it as an important scientific advance.
So three of the most influential psychologists of all time started their careers studying inveterate behaviour and nervous systems. Coincidence? Probably. But a sweet and interesting one nevertheless.