Comparison of the mitochondrial gene arrangements of three sea spiders, Achelia bituberculata , Endeis spinosa and Nymphon gracile , and a horseshoe crab, Limulus polyphemus , which is considered to be an arthropod ground pattern. The mitochondrial gene arrangements in A. bituberculata (complete) and E. spinosa (partial) are the same as the arthropod ground pattern shown in L. ployphemus, except for the unexpected placement of trnQ (Q) in A. bituberculata. In N. gracile, however, the gene arrangement differs markedly from the other two pycnogonids. The two trnL1 and trnL2 genes are marked by L 1 and L 2, respectively. L 1 and L 2 in A. bituberculata and N. gracile have the orientation and position generally shown in the chelicerates and myriapods among the arthropods. Refer to the Fig. 1 legend for abbreviations of components.