Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/302

Rh 282. AEACHNIDA [SCORPIONS. 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 estab lished to warrant it. Not that there is not a wide difference iu 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 in the second (Pycnogonidea), no traces have been yet found 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, &quot;Challenger&quot; 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 trachea), en tirely 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, pi. 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 scor pions 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-Scor- piones; 2. Scorpiones. Sub-Order I. Pseudo-Scorpiones. These are distinguished from the Scorpiones chiefly by the absence of a tail and by their mode of respiration (figs. 1 6, 17, 18). The body is of an oblong-oval form; the cephalo- thorax is coriaceous ; the abdomen sessile and, generally, com posed above of eleven, underneath of ten segmental rings or articulations, usually covered above and below with a double longitudinal series of transverse corneous plates, united later ally by a membrane, the folds of which correspond to the transverse plates. In the second sub-abdominal segmental plate, on each side of the median line, there is (in some species) a small spiracular opening, and in the lateral con necting membrane of the third segment, opposite to the end of the third sub-abdominal plate, on either side, is another similar opening. These four stigmata are the external aper tures to the tracheary system (fig. 18); behind and between the two first mentioned stigmata is a spinning organ, furnished with minute tubes for the emission of the silken threads ; and close to this spinner, at the fore margin of tho first abdominal segment, is the genital aperture. The legs are closely affixed by the coxal joints to the lower side of the cephalo-thorax, and sit closely to each other, having a very small longitudinal space between them analo gous to a sternal plate, of which, however, there is not even a rudimentary one, properly so-called. The number of joints in the legs is seven (Menge), but there appears to be a variation in this respect in some species. Each tarsus ends with two curved finger-like claws terminating with a short nail, and beneath them is another claw, modified at its extremity, gradually enlarging into a kind of conical form, probably acting as a sucker for the purpose of adhesion to smooth objects (fig. 18, d~). The tarsal claws spring from a small heel or claw-joint. Fig. 16. ,0 Fig. 17. Fio. 16. CheUfer tesamoidet, Savigny. a, cephalo-thorax; b, abdomen; r, e, fakes ; d, &amp;lt;l, eyes ; e, hinder segment of cephalo-thorax ; /, front ditto ; h, h, palpi. Fio. 17. The same; portion of under Bide, a, rudimentary sternum; 6, J maxillae; c. palpus; d, digital joint; e, movable fang; /, labium; o, o, falces. The palpi are five-jointed, long and strong, terminating with a digital joint of large size, more or less bulbous at its base, and drawn out at its extremity into a jaw-like claw, furnished with a hairs and bristles, and + / generally serrated along its inner edge. Against this claw another, arti culated to the extremity of the bulbous portion, acts in opposition like tho pincers of a crab. Sup porting the palpi on either side are the max illae, forming the lower side of the mouth, the upper side being formed Fio. 18. CheUfer cavcroides. Linn; portion of bv two didartvlo fnlpp=s under side- a a b 6 * orifices of the respir- uy IAVO Uiuaciyie ICCS, atory organs, c, genital aperture ; d, sucker- besides the movable jaw liketarealclaw; e, e, superior tarsal clivws. of the falces, there are connected with them some denti culate spines, or other spiny processes and bristles. Within the mouth is a tongue, which also serves as a sternal