The relation of the Leptostraca to other groups of Crustacea has long been a problem of interest, although the alliance with the Eumalacostraca has been amply justified (Claus, 1872, 1888, Calm an, 1909). Claus discussed the essentially Malacostracan form of the appendages of Nebalia , and the mode of feeding by the aid of these appendages has been shown (Cannon, 1927) to be a specialized modification of the type shown by the simpler Malacostraca (Cannon and Manton, 1927, Some of the apparent differences between the Leptostraca and the Eumalacostraca have recently been shown to be differences of degree rather than of kind. Thus the seventh abdominal segment of Nebalia is found also in the embryo mysid, but is partially or completely fused with the sixth segment in the adult Lophogaster and Hemimysis respectively (Ma n to n, 1928,a and b). Thus the basal number of abdominal segments in the Eumalacostraca as well as the Leptostraca may be seven. The presence of a large caudal furca in Nebalia may also be a difference of degree, if this furca is homologous with the embryonic furca of a mysid (Manton, 1928, a). Other differences, such as the presence of a fully formed carapace adductor muscle, of cephalic liver lobes, etc., require further investigation. Of the resemblances between the Leptostraca and Eumalacostraca, the mode of development of the former is said to resemble that of the Mysidacea, but the embryology of no Leptostracan has been adequately followed, and the recent work on mysid development has considerably modified many previous views on this subject.