Studies on the characters of the gnathosoma, among other details, establish the Histiostomatidae to be a monophyletic group and the Guanolichidae to be its sister group. A synapomorphy of Guanolichidae and Histiostomatidae is the reduction of the digitus mobilis of the chelicerae to small remnants. For the first time a reconstruction of histological sections of internal structures of a histiostomatid gnathosoma, namely of Histiostoma palustre Wirth, 2003, is presented. The coxal endites are laterally connected with the pedipalps and close the ventral gnathosoma completely. An apomorphy of the Histiostomatidae and an important structure for comparative studies is the palparmembrane which is shaped distally by the coxal endites.