THE CLASSIFICATION OF ANIMALS is Still Very much a field in which discovery and revision are continuing, even after two hundred years of study. The importance of classification in biology increases every year, because the experimental and practical fields find increasing need for accurate identification of animals and for understanding of comparative relationships. At least one outstanding biologist has opposed pubUcation of this new classification on the ground that it would be accepted as final, the classification, and would tend to make students think that all higher classification is finished. The intention of the compiler is just the opposite. Just as this classification is different in detail from all previous ones, so will future editions be still different, as we learn more about the comparative features of animals. It is anticipated that every new edition will spur students of the individual groups to propose improvements. It is therefore planned to issue corrected editions whenever appropriate. The very appearance of these subsequent editions will emphasize the growth of understanding of animal groups. Only one ostensibly complete classification of animals, living and fossil, has been published in recent years. That classification, by A. S. Pearse of Duke University, is a good one, based on the views of many specialists. Certain mechanical faults make it less usable than it should be, and the need for revision gave the original impetus to preparation of the present classification. Because Pearse did not usually indicate the source of his arrangements, he is not here cited as an authority. Nevertheless, the two classifications are basically very similar. No other single classification has been found that agrees so closely with the conclusions of the present study. It should be emphasized that, within certain limits, this classification is not a simple compilation of the views of specific workers. In nearly all details, choices have been made between conflicting schemes of various authors, not on the basis of the reputation of those authors but on my judgment of the soundness of their supporting arguments or on my analysis of the data they present. In none of the larger groups has the work of any single author been accepted without modification. Several considerations have influenced the decisions embodied in this classification. First, a false picture is given by a simplified classification, because the existing diversity is one of the principal features of the animal kingdom. Therefore, no groups should be combined merely for the sake of simplicity. Second, although the previous item would seem to require coverage of the groupings at all possible levels, to show the extreme range of division and subdivision, this is not in fact possible. Not only are there many conflicting groupings at certain levels, such as of phyla or orders, but there is no practical way to show these groupings in a general classification. It is a compromise that is believed to be effective to subdivide the phyla only into classes, subclasses, and orders. Other possible groupings, such as subphyla and superorders are referred to in the notes. Third, two groups which are so distinct at any level that they cannot be described in common terms must be separated at that level. (For example, Pterobranchia and Enteropneusta; see the Notes on the Taxa.) Fourth, groups which cannot be distinguished at any particular level by the type of characters used for their neighbors must be combined at that level. (For example, the sometime classes of Nematoda...