OVER the years many data have accumulated on life histories and the larval forms of water-mites. This information, when collated, indicates a broad uniformity in the water-mites and, moreover, points up phyletic lines that have not been well established on other data. These broad phyletic lines above the family level are of utmost significance in classification. When the data reviewed here are greatly supplemented it will be possible to refine critically the classification. At present only the potential value of certain data can be pointed out. It is true that the differentiation and definition of species is still the most pressing task in acarine taxonomy, but it must be emphasized that the refinement of higher classification in this group will depend on an accurate understanding of the major phyletic lines considered below. Those unacquainted with the watermites are immediately overwhelmed by the marked differences in adult morphology of the more than forty families. Students of the group seem to learn the families as entities. There are recognizable groups of related families but as a general rule the families appear unique. Adult exoskeletal features give, at best, an inadequate picture of relationships above the family level. Adult structure need not be reviewed here; only a brief outline of the kinds of characters that assume particular importance in classification is relevant to the main discussion. Sclerites of the adult are used in the discrimination of species and the definition of families, and four main groups of structures are particularly significant: genital structures, body sclerites, mouthparts, and the legs and coxae. Both mouthparts and leg structures are conservative. Certain divergent families have peculiar mouthparts, but as a rule the trends seen in the structure of legs and mouthparts are difficult to categorize and of little value in family definitions. External genital structures and body sclerites are the main features considered in the separation of water-mite families. The striking discontinuities and the resultant uncertainty about relationships are better explained through an understanding of the manner in which the features of the exoskeleton may develop. Sclerites were formed as arthropods evolved from a soft-bodied form. Obviously, all arthropods acquired an exoskeleton at one time, but the order Acarina is exceptional in that only a few, small sclerites of the legs are homologous throughout the order, and the differences in other exoskeletal features are so great as to suggest that they evolved recently. In addition, the sclerites of mites are often the result of the appearance of entirely new sclerites and not just modifications of existing sclerites, as is the usual case in other arthropod orders. The development of body sclerites was a recent occurrence in water-mite evolution. Many families have a unique set of independently evolved body sclerites, and these sclerites commonly serve as insertions for the enlarged muscles needed for swimming. The presumed ancestral form, as well as certain extant primitive watermites, are without body sclerites. These animals possess a weak musculature and have no sclerotizations of the body wall. They are very close to terrestrial mites, and are poor swimmers. Terrestrial locomotion, or, more precisely, movement through the medium of air, requires less power than movement through water; hence, the heavier musculature and stronger supportive structures found in