(15) ARACHNIDA - ORDER V: SCORPIONIDEA - INTRODUCTION
Order V. - SCORPIONIDEA.
In spite of differences in their internal anatomy, which appear to ally the Pseudo-Scorpionides more to the spiders than to the true scorpions, some leading features of external structure seem to stamp them at a glance as most nearly related to the latter. The impression inevitably created on a comparison of the true and false scorpions is that the latter are little scorpions without tails. Including them, therefore, here in the same ordinal group as the true scorpions, we may shortly characterize the order as follows: - Body ekongated-oval; cephalo-thorax of one undivided piece, hard, horny, and often tuberculated or granulose, while traces of thoracic segments soldered together are generally more or less perceptible. Abdomen united to the cephalo-thorax throughout its entire breadth, and composed of several segments formed by articulated transverse coriaceous plates on the upper and under sides, united by two lateral cartilaginous membranes. Falces didactyle; palpi terminating with a didactyle claw or pair of pincers; the coxal joints of the legs, fixed to the lower side of the thoracic segments, form a kind of sternum; the sternal plate, properly so-called, being either wanting or very rudimentary. Eyes various in number, two to twelve. So far the true and false scorpions run together but when compared more closely, differences, especially of internal structure, appear, which necessitate their subdivision into two groups or sub-orders. Menge and others, on account of these differences, separate the two groups more widely still; but one chief ground of this wider separation, the respiratory system, being in one tracheal, in the other (so-called) pulmonary, seems scarcely sufficiently well established to warrant it. Not that there is not a wide difference in their respective respiratory systems, but that the whole question of the respiration of Arachnids is hardly so settled as to justify any broad lines of classification being based upon it. In some groups of the first order, Acaridea, and also of a distinct respiratory system, though perhaps this arises from the minuteness of the creatures themselves and the extreme delicacy of the membranous walls of the air-vessels. This latter especially, as has been lately observed in regard to some obscure Myriapodous insects, - Peripatus (H.N. Moseley, "Challenger" expedition, 1874, Ann. And Mag. N.H., 4 ser. Vol. xiv. p. 225), - would make it almost impossible, by any means, to discover these tracheal tubes, when once the air had been expressed from them. Then, again, the so-called pulmonary organs of the true scorpions appear to be, simply, modified tracheae, entirely destitute of that which specially stamps the true lung, that is, the presence of blood-vessels bringing the vital fluid together, and carrying it through the respiratory organs for aeration. The Araneidea, moreover, have both the tracheal and so-called pulmonary systems combined; and also in this (more extensively worked than any other) order-Araneidea-the researches lately published on their organs of respiration by Philipp Bertkau )Arch. F. Nat., xxxviii 1872, Heft 2, pp. 208-233, pl. 7), show how much has yet to be learnt concerning this part of their internal structure. Considering, therefore, that the difference between the respiratory systems of the true and false scorpions should not separate them from the same ordinal group, these with other differences yet appear to necessitate a division within the order. We propose, therefore, to distinguish them under two sub-orders: - 1. Pseudo-Scorpiones; 2. Scorpiones.
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