Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/304

Rh 284 AKACHNIDA [SCORPIONS. marked groove always divides the caput longitudinally, dividing also the tubercle on which the central pair of eyes is placed. This groove possibly represents the union of the two segments of a kind of duplex caput analogous to the duplex generative system noted poslea (p. 285). The abdomen is sessile, i.e., united to the thorax throughout its whole breadth, and composed of twelve segments, of which the five posterior ones form a tail ; these latter are segments in the strictest sense, while those of the abdomen proper are, more correctly speaking, articulations covered above and below with transverse horny plates. Articulated to the fifth joint of the tail is a bulb terminating with a sharp curved sting. Poison secreted in the bulb is injected into wounds through two minute perforations near the extremity of the sting. The legs are not very robust, nor long, nor very unequal in length ; each consists of seven joints, of which the basal ones (coxae), are fixed to the under side of the cephalo-thorax, and act partly as a sternum ; the only portion which can be said to represent a real sternal plate being a very small sub-triangular, or in some genera, quad rangular piece immediately behind the first segment of the abdomen between the extremities of the coxae of the fourth pair of legs (fig. 20, k). These latter are soldered to those of the third pair, the junction being visible as a more or less distinct groove. The coxse of the first and second pairs have angular pieces attached to them, used in manducation, and hence called supernumerary maxillae (fig. 20, o, o, b, b); but while the pieces of the first pair might perhaps be so named, those of the second pair, closing together as they do on their inner margins, rather form a kind of sub-triangular sternal appendage representing the labium, of which there is none properly so-called. Each tarsus ends with two rather long curved superior, and one short straight tooth-like inferior, claws. The first and second segments of the abdomen underneath are almost rudimentary; between them and the anterior edge of the first segment is situated the external aperture to the organs of generation, the horny plate pro tecting it being divided longitudinally by a more or less distinct groove. From the second segment, and articulated to it, spring two very conspicuous, curious, comb-like appen dages (fig. 20, h, h), composed of a longitudinal shaft of several distinct joints, with a number of slightly curved, bluntish tooth-like processes, fitting closely together, and articulated to their hinder edge, nearly at right angles, like the teeth of a comb. The shafts and teeth of these appen dages vary in the details of their form and structure ; their use has not yet been certainly ascertained, but their position points to some connection with the process of generation, which is also the opinion of L. Dufour and others ; while they have also been thought to be intended to brush away obstructions from the spiracular orifices and other parts ; but as in no case are they long enough to reach more than the first pair (out of four) of these orifices, and every con sideration of structure is against their being used to clean other parts, this conjectured use is scarcely probable. The spiracular orifices are situated on the under side of the abdomen in four pairs one pair in a transverse line on each of the third, fourth, fifth, and sixth segmental plates, and are in general easily seen. The eyes (six, eight, ten, or twelve in number) are placed on the fore part and upper side of the cephalo-thorax, two, generally large ones, in a transverse line near the middle of the upper part of the caput, the rest (much smaller and varying in size in different genera and species) in two symmetrical groups, one on each side near its fore corners. The falces form, as in other Arachnids, the upper side of the mouth; they are strong, cylindrical, and didactyle, the outer jaw-like terminal claw being articulated in oppo sition to the inner one; both are toothed, the denticulations varying in number size and form. Below the falces (and forming with the before-mentioned cox;J. appendages a complete lower boundary to the mouth) are two strong maxillae, from each of which (as a basal joint) springs a long strong palpus of four joints, the last (digital) joint being more or less bulbiform at its base and didactyle at its extremity. The outer claw is movable, and (like the fixed one) serrate or denticulate on its inner edge. The size and form of this didactyle joint vary in different species and genera ; in some it is of enormous size, and its denticulations very strong, while in others it is scarcely larger than the joint which precedes it. It is the possession, the position, and the office of the exact counterpart of this palpus, which, apart from other considerations, so plainly stamp the affinity of the two sub-orders of the Scorpionidea, in spite of the differences of their respective internal anatomy. Within the mouth parts above noted is a pointed tongue (languette) tipped with hairs. INTERNAL STRUCTURE. The Muscular System of scor pions is similar to that of other Arachnids. It is well detailed by L. Dufour (Savants etrangers, xiv. pp. 609- 611), who divides the muscles into thoracic, abdominal, and caudal; those of the abdomen being also subdivided into tegumentary, perforunt, and cardiac. The Organs of Digestion consist of a straight narrow intestine, with little or no dilatation, running from the mouth to the anus, which has its external opening at the lower extremity of the fifth or last segment of the tail. At the junction of the stomachal and intestinal portions the biliary vessels are inserted, and from each side of the former (stomachal part) there issue five narrow caeca run ning into a mass of fatty matter on either side. The Respiratory Organs, or pulmo-branchiae, have their ex ternal orifices or stigmata (as before mentioned) in four pairs on the ventral surface of the abdomen; the pulmo-branchiae, of which one is connected with each stigmatic opening, are hollow sacs, the walls of which are folded into delicate laminae, each being duplex, 1 and all lying one upon an other like the leaves of a book; the air is admitted to these through the external openings, which are closed by movable horny lips as in insects. (Jones, Animal Kingdom, 2d ed., 416.) According, however, to L. Dufour (I.e., p. 617), the lips of the stigmata are immovable, and have a varied direc tion in different species. The air is admitted to the puhno- branchiae by the action of a linear opening in a supple membrane or diaphragm within the space between the lips and the laminae (L. Dufour, I.e.] Organs of Circulation. These, according to Newport (Phi los. Trans. 1843, pp. 286-298), consist of an elongated, dorsal muscular vessel (or heart) divided into eight cham bers, separated from each other by valves, and with auri cular openings or valvular orifices, at the division of each chamber, on its dorsal surface. From both sides, and at each end of the dorsal vessel, arteries convey the vital fluid for distribution to different parts of the body. This dorsal vessel apparently acts in a manner analogous to that of the heart in vertebrate animals. The vital fluid is conveyed back to the &quot; heart &quot; from the pulmo-branchia., where it has been oxygenated by means of a great number of slender canals, formed by the collection together of vessels that have their origin at the hinder part of the inner side of the pulmo-branchiae. These canals pass round the sides of the body in the hinder part of each segment, and pour their contents into the dorsal vessel through the valvular orifices above mentioned. The conveyance of the fluid from the general circulation to the pulmo-branchise is effected by means of an intricate system of anastomos ing pulmonic and capillary vessels, situated chiefly below the nervous chord on the ventral surface of the body. 1 L. Dufour, I.e.