proteinaceous in nature, with several of them apparently polypeptides of relatively low molecular weight. Some of the reported pigmentary-effector hormones are recognizable as distinct molecular species on the basis of chromatographic behavior and molecular size. The diabetogenic hormone is a much larger molecule and is a protein. The properties of the molt-inhibiting hormone of the eyestalk, although still too little known for specific classification, indicate that it belongs to one of these two categories. Tests of functional specificity and structural studies, which in most cases depend on further chemical purification, seem attainable in the near future. The subsequent comparative studies that may become feasible will be of physiological and biochemical interest. Modern crustacean may be considered to have started with the discovery (Perkins, 1928; Koller, 1928) that integumentary chromatophores are influ? enced by a blood-borne substance originat? ing in the eyestalk. These studies in physio? logical color change were considerably ex? tended by Brown (1935) who presented experimental evidence for the existence of a number of physiologically-different chromatophorotropins. Another pigmentaryeffector hormone was demonstrated with the discovery (Kleinholz, 1936) that photomechanical movements of retinal pigments in crustaceans are regulated by eyestalk extracts. The diversity of physiological processes in crustaceans that has since been reported under endocrine control has been reviewed recently by Barrington (1964), Charinaux-Cotton and Kleinholz (1964), and Fingerman (1965). The best-known physiological effects of eyestalk hormones are shown diagrammatically in Figure 1. One consequence of this hormonal abundance, reminiscent of the variety of con? trolling hormones found in the anterior pituitary gland of vertebrates, is the ques? tion of specificity of physiological effect. That is, does one eyestalk hormone have one specific role only, and hence does the multiplicity of reported endocrine effects imply a multiplicity of different hormones, or, are a few substances produced in the neurosecretory system of the eyestalk, each of which may have more than one physio? logical action? This very question was being argued some thirty years ago for the eyestalk chromatophorotropins. The prob? lem was later further complicated by the discovery of other neurosecretory struc? tures, outside the eyestalk, extracts of which also showed chromatophorotropic activity. Barrington's (1964) comment It should be remembered, however, that the indi? vidual substances (the color change hor? mones) have not yet been characterized, either structurally or functionally, with the rigorous precision that we encounter in many fields of vertebrate endocrinology is even more pertinent with the larger num? ber of endocrine effects now known.