SYNOPSIS. In order for photoperiodic phenomena to be expressed by any animal, the organism must possess some sort of photoreceptor. This may be in the form of an eye or an extraocular receptor, and the invertebrate phyla illustrate fantastic diversity in design of either receptor type. While all the major invertebrate phyla possess photoreceptor organs of one type or another, the best-studied groups are those with highly complex eyes. These include the crustaceans, the molluscs, and some polychaete annelids. Many species in these groups possess eyes having extreme sensitivity, good spatial resolution, and in some cases multiple spectral channels. In a few taxa, the eyes are known to provide input to circadian oscillators, which suggests that they may also be employed for measurement of photoperiod. Extraocular photoreceptors include dermal and ganglionic sense organs, which also feed into circadian systems in numerous invertebrates, from cnidarians to molluscs and arthropods.