STUDIES ON THE POSTEMBRYONIC DEVELOPMENT OF HYALELLA AZTECA (SAUSSURE) 1 SISTER FRANCIS SOLANO GEISLER, S. S. J. (Catholic University of America. Washington, D. C.) Hyalella asteca is a common and widely distributed fresh-water amphipod crus-tacean. The genus Hyalella is the only one of the family Talitridae occurring in the fresh waters of America. The species H. asteca described by Saussure in 1858 from Vera Cruz, Mexico, where it was originally found among the ruins of the Aztec Indians, is widespread in North and South America, and has been cited in the literature more often under the synonyms Hyalella knickerbockeri (Bate) or H. dentata Smith. Holmes (1902, 1903) made observations on H. dentata Smith to determine the mode of sex recognition, food habits, thigmotaxis, phototaxis, and reaction to pres-sure. Weckel (1907) discussed the synonymy of this species and gave a minute description of its characteristic external features. Embody (1911), interested in propagating amphipods as food for fishes, made a study of the distribution, food, and reproductive capacity of four common fresh-water amphipods, including Hyalella knickerbockeri. He states that this amphipod breeds for 152 days during the warmer months of the year and averages about 18 eggs (15 times in 152 days) per brood. Jackson (1912) investigated the distribution and habits of this species, also its color, size, moulting, effects of starvation on moulting, breeding, locomotion, and enemies. In his paper there is no mention of temperature, and the age of the ani-mals was judged by their size. In 1915 Phipps made an experimental study of the behavior of amphipods with respect to light intensity, direction of rays and metabo-lism. Hyalella knickerbockeri was one of the animals included in his study. Gaylor (1922) reported on the life history and productivity of this species, and recently, the effects of population density upon its growth, reproduction and sur-vival were studied by Wilder (1940). In the above-mentioned investigations of Hyalella azteca no account exists of the morphological changes that occur in its postembryonic development, a necessary preliminary to experimental studies for which this animal seems suitable. In fact, few amphipods have been so studied. The work of Sexton (1924) on Gainmarus 1 A contribution from the Department of Biology, the Catholic University of America, Washington, D. C. This paper, prepared under the direction of Dr. Edward G. Reinhard, is based on the author's dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The writer wishes to express her gratitude to Dr. Reinhard who suggested the problem and gave constant assistance and encouragement throughout the work. To Dr. T. von Brand and Dr. W. G. Lynn thanks are due for helpful suggestions. The assistance of Mr. Clarence R. Shoemaker, Associate Curator of Marine Invertebrates, U. S. National Museum, who graciously placed at the author's disposal his fund of information on amphipods, is likewise gratefully acknowledged.