The purpose of this book is to summarize in a convenient form the present knowledge about the Gulf of Mexico. Such a summary is needed in connection with a large number of new investigations which are now being conducted in the Gulf of Mexico by Federal and State organizations and private institutions. It is hoped that the background information presented here will be useful to the investigators engaged in the new research projects and will save their time and effort. Scientific data concerning the Gulf of Mexico have been accumulating since the first explorations in the sixteenth century. They are scattered in thousands of technical publications, some of them rare and not readily available to persons in the Gulf States. For the purpose of this book the Gulf of Mexico is defined as a partially landlocked body of water indenting the southeastern periphery of the North American Continent. Its eastern boundary was drawn from Cabo Catoche Florida. This boundary does not constitute a natural barrier; it was arbitrarily determined because of the necessity of restricting the scope of the project. Inland the area under consideration extends to the limits of tidal waters. The book comprises a number of articles each written by a recognized authority in his field; these are arranged, with minor exceptions, in a taxonomic order following a list of phyla, classes, and orders prepared in 1936 for the American Association for the Advancement of Science and published by Duke University Press. This plan was carried out with the following exceptions: the sections on Rotatoria and Branchipoda were omitted because of the inability to find anyone willing to review these two groups; and, for the sake of convenience, the articles on parasitic worms are assembled in a single chapter. A pertinent bibliography is given at the end of each section.