SYSTEMATIC POSITION 527 



them by, and to inspect, in brief, the case as it stands at present. 

 The obvious features in which a Pycnogon resembles a Spider or 

 other typical Arachnid, are the possession of four pairs of walking 

 legs, and the pre-oral position and chelate form of the first pair 

 of appendages ; we may perhaps also add, as a more general 

 feature of resemblance, the imperfect subservience of limbs to the 

 mouth as compared with any of the Crustacea. The resemblance 

 would still be striking, in spite of the presence of an additional 

 pair of legs in a few I'ycnogons, were it not for the presence of 

 the third pair of appendages or ovigerous legs of the Pycnogon, 

 whose intercalation spoils the apparent harmony. We are 

 neither at liberty to suppose, with Claus, that these members, 

 so important in the larva, have been interpolated, as it were, 

 anew in the Pycnogon ; nor that they have arisen by subdivision 

 of the second pair, as Schimkewitsch is inclined to suppose ; nor 

 that they have dropped out of the series in the Arachnid, whose 

 Ijody presents no trace of them in embryo or adult. In a word, 

 their presence precludes us from assuming a direct homology 

 between the apparently similar limbs of the two groups,^ and at 

 best leaves it only open to us to compare the last legs of the 

 Pycnogon with the first abdominal, or genital, appendages of the 

 Scorpion and the Spider. On the other hand, if we admit the 

 seventh (as we must admit the occasional eighth) pair of 

 appendages of Pycnogons to be unrepresented in the prosoma of 

 the Arachnids, then, in the cephalothorax of the former, with 

 its four pairs of appendages, we may find the homologue of the 

 more or less free and separate part of the cephalothorax in 

 Koenenia, Galcodes, and the Tartaridae. There is a resemblance 

 between the two groups in the presence of intestinal diverticula 

 that run towards or into the limbs, as in Spiders and some Mites, 

 and there are certain histological and embryological resemblances 

 that have been in part referred to above ; but these, such as they 

 are, are not adequate guides to morphological classification. We 

 must bear in mind that such resemblances as the Pycnogons 



poden," Biol. Centralbl., Bd. xviii., 1898, pp. 603-609 ; Meisenheimer, Vcrh. zooL- 

 hut. Ges. TVien, xii., 1902, pp. 57-64 ; also Stebbiiig, in Knowledge, 1902. 



^ The chelate form of the foremost appendages is of little moment. A chela 

 consists merely of a more or less mobile terminal joint flexing on a more or less 

 protuberant penultimate one, and in the Scorpions, in Limulus, throughout the 

 Crustacea, and even in Insects (cf. vol. vi. p. 554), we see such a structure arising 

 independently on very diverse appendages. 



