Phylogenetic Uncertainty.— Invertebrate zoologists have long debated the relationships of biramous-limbed Crustacea to other groups of arthropods. Haeckel (1866) recognized two groups of arthropods on the basis of respiratory anatomy. The Carides included the crustaceans, trilobites, xiphosures, and eurypterids; and the Tracheata, included the arachnids, insects, and myriapods. Work on the onychophorans (Mosely, 1874) indicated a link between annelids and the terrestrial myriapod-insect line, but this left the origins of the aquatic carides unresolved. Lankester (1881) demonstrated that the xiphosures were allied to the arachnids rather than to the crustaceans. These and other studies led Haeckel (1896) to revise his position and propose two separate lines of arthropods: one of primarily aquatic groups with primitively biramous appendages (crustaceans, trilobites, and chelicerates); and the other of primarily terrestrial groups with uniramous limbs (onychophorans, myriapods, and insects). Haeckel's new arrangement emphasized the convergent origins of trachea in the arachnids and uniramians. Korschelt and Heider (1890) preferred a monophyletic scheme with the two evolutionary lines united by a pre-onychophoran, “protostracan,” ancestor. By modern standards, however, this might indicate that the arthropods are a morphological grade, rather than a true clade.