At a recent meeting of the Society for Invertebrate Pathology, Dr. K. Kanungo of Western Connecticut State University organized a special symposium on the topic of invertebrate circulatory systems with emphasis on hemocytes and their role in internal defense. The contents of this volume of Comparative Pathobiology represent the proceedings of that gathering. As the Editor of the Journal of Invertebrate Pathology, I am intimately aware of the current popularity of research pertaining to invertebrate immune mechanisms of which hemocytes and associated molecules play a central role. Consequently, the contributions in cluded herein should prove to be of interest to a variety of investi gators including invertebrate zoologists, comparative immunolo gists, physiologists, and cell biologists. As has been stated in previous volumes of this series, Comparative Pathobiology is meant for the publication of proceedings of symposia devoted to some aspect of that broad spectrum of modern biology known as pathobiology. Those interested in having antici pated contributions considered for publication should contact me.