XXI SYSTEMATIC POSITION Breau 
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 Pycnogons, 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 
body 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, Galeodes, 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, Verh. zool.- 
bot. Ges. Wien, xii., 1902, pp. 57-64 ; also Stebbing, in Anowledge, 1902. 
1 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. 
