A comprehensive review has been made of crustacean hormones together with a summary of invertebrate prostaglandin studies. The review aims to provide a background to the study of cirripede hormones. The morphology of the central nervous system of Balanus balanoides and B.hameri is described and briefly compared to that of Conchoderma auritum thereby providing a framework for the neurosecretory studies which follow. The phenomenon of neurosecretion, especially pertaining to the supraoesophageal ganglion, is dealt with in detail for B.hameri, and also discussed for B. hameri and C.aur itum. 4 neurosecretory cell types have been recognized in B. hameri and their secretory product has been matched tentatively with elementary granule types present in the neuropile and nerves of the supraoesophageal ganglion. The origin of one type of elementary granule remains unknown. Sites at which exocytosis of granules occur include the neuropile and the nerve terminals beyond the median ocellus. The latter constitute the first known example of a neurohaemal area in the Entomostraca. Neurosecretory cells are also present in the median ocellus and the ultrastructure of this photoreceptive organ together with its relation to the supraoesophageal ganglion are described. Attempts to artificially induce egg laying in barnacles have failed. However, evidence is presented to support the contention of Barnes et a 1. (1977) that the stimulus to egg laying is derived from some factor present in the seminal plasma. The embryology of B. balanoides and the effect of 20-hydroxyecdysone on this process have been studied. There is no obvious effect from hormone concentrations of up to 1 ug/ml until egg stage 9, but beyond this stage the eggs undergo progressive cytolysis. Probable reasons for this effect are discussed. The hatching substance of B. balanoides has been extracted and partially purified. The compound is a prostaglandin and is released by adult tissues in response to feeding. The prostaglandin acts by inducing the release of dopamine within the embryo, which in turn stimulates embryonic muscle.