The purpose of this review is to highlight the role of pituitary adenylate cyclase-activating polypeptide
(PACAP) in a range of physiological and behavioral processes of gastropod molluscs, Helix and Lymnaea. Since its discovery
in 1989 PACAP has become increasingly recognized for its important and diversified roles in the central and peripheral
nervous system and in several peripheral organs of a variety of vertebrate and invertebrate species. Twenty-two
years after its discovery, PACAP is now one of the most extensively studied of the neuropeptides. This review surveys the
importance of PACAP and PACAP-like peptides in invertebrates, focusing mainly on the gastropod molluscs. The relevance
of studies on lower vertebrates and invertebrates, which do not have a pituitary gland, is to contribute to the unraveling
of fundamental effects of PACAP or PACAP-like peptides and to provide a comparative view.