Thesis (M.A.)--Boston University / The pericardial organs are neurosecretory structures situated within the pericardial cavity of stomatopod and decapod crustaceans. Their location in the pericardium is such that blood approaching the heart must flow directly over them. This arrangement facilitates the distribution of pericardial organ secretions to all parts of the organism. It is not known how many different neurohormones are secreted by the pericardial organs, but at least one of these has been shown to act as a strong cardioexcitor. Electron microscopy of pericardial organs has revealed the presence of membrane-bounded granules, approximately 1500 A in diameter, which are believed to contain the neurosecretory material. An attempt was made here to isolate the cardiaexcitor by means of differential centrifugation of crude 0.9 M sucrose homogenates of pericardial organs from two species of Cancer. Live Homarus heart served as bioassay material. Cardioexci tatory effects were found in material which sedimented below 17,000 x g. On two occasions another active sediment was found at 71,000 x g. The present data is in agreement with the hypothesis that the cardioexcitor is contained within neurosecretory granules. However, a definitive statement to this effect cannot be made, as sediments were not checked microscopically. Presumptive evidence is offered for the existence of a second type of neurosecretory granule.