This book deals primarily with the principal extracellular macromole­ cules of animal connective tissues. It attempts to answer some general questions about the biological organization of the tissues: What is the nature of this organization at various dimensional levels? What functions does the organization serve? How has it evolved? I have given major emphasis to the structures and properties of the macromolecular components of extracellular matrices from a wide range of invertebrates and vertebrates. In doing so, however, I have treated cursorily many important aspects of connective tissue biology that appeared to be only indirectly relevant to the principal questions asked. On the other hand, I have not hesitated to search for broad biological principles outside the prevailing conceptual boundaries of animal connective tissues and the lower molecular dimensional levels. The numerous speculations presented will, I hope, stimulate the reader to further thought and investigation. Acknowledgements My previously unpublished data that have been included in this volume could not have been obtained without the generous gifts of specimens and other assistance by a number of individuals and institutions. These are Dr. G. BERENSON of Louisiana State University, Drs. W. P. BRAKER and D. ZUMWALD of the Shedd Aquarium, Dr. S. H. CHUANG of the Univer­ sity of Singapore, Dr. L. G. CLARK and Mr. U. M. VARELA-DIAZ of the University of Pennsylvania, the Government of Nicaragua, Dr. E. CLARK of Cape Haze Marine Laboratories, Mr. C. E.