The echinoderm axial organ is located centrally, at the meeting point of the animals' "circulatory" systems. This, and its unusual histology, have prompted the implication of a number of different functions for the organ, including circulation, excretion and host defense. None of these have been definitively proven. Claims that starfish axial organ cells respond to antigen in an adaptive manner, resembling the vertebrate antibody response, also remain unconfirmed. The purpose of the work on Strongylocentrotus purpuratus presented in this thesis was to independently test the hypothesis that the cells of the axial organ respond to antigenic challenge. To determine if the cells are modified quantitatively following antigen encounter, the cell types of the organ were first classified according to morphological characteristics. Cell subpopulations were then monitored quantitatively after in vivo exposure to several types of antigens. Axial organ cells were also analyzed for qualitative changes; cells from antigen exposed urchins were assayed for immunocytoadherence to antigen-coated sheep erythrocytes. The coelomic fluids from the same animals were tested for antigen-specific binding molecules using ELISA assays. Immunocytoadherence and ELISA assays were validated with cells and sera from trout similarly exposed to antigen and control treatments. Eight subpopulations of axial organ cells were identified. Four of the cells types resemble cells found in the coelomic fluid. A possible precursor cell to the red spherule cell was found to be a member of the axial organ cell population. Changes in the axial organ cell subpopulations after antigen exposure were not significant, regardless of the type of antigen, exposure method or sample time. Cells of the axial organs from TNP-exposed urchins did not bind TNP-SRBC's, and the hemagglutination observed was neither TNP-inhibitable, nor dependent on previous expoure to the TNP antigen. Coelomic fluids from several urchins bound TNP to a greater extent than BSA, but the frequency of binding among urchins was not dependent on their prior treatment. Titrational analysis of one set of urchin coelomic fluids against the hapten, TNP, the carrier, LPS, or an unrelated protein, BSA, revealed identical binding curves with each molecule. This, in addition to a higher titer of binding molecules in the control animals compared to antigen-injected animals, led to the conclusion that the response was nonspecific. The work presented in this thesis does not support the hypotheses that the urchin axial organ cells respond to antigenic challenge either by a modification in cell types present in the organ, or by the production of cell surface or secreted antigen-specific binding molecules. / Graduation date: 1993