Beneath the third, fourth, fifth, and sixth segments are the paired openings of the lung-sacs.
Metasoma.—The first segment is usually and the remainder are invariably enclosed in complete chitinous rings and show considerable variations in their comparative size and shape, and in the arrangement of the ridges and keels with which they are usually furnished. The post-anal segment is more or less globular at its base, constituting a “vesicle,” and terminates in a fine curved point, the “aculeus,” perforated for the passage of the delicate poison-duct. With the abdomen fully extended the point is directed downward, but in the attitude of attack or defence, when the “tail” is carried horizontally over the back, the sting points forward in the neighbourhood of the animal’s head.
Fig. [170].—A, Diagram of a Scorpion’s leg; 1, coxa; 2, trochanter; 3, femur; 4, patella; 5, tibia; 6, protarsus; 7, tarsus; p.s, pedal spur; t.s, tibial spur. B, Fourth tarsus of Palamnaeus swammerdami; l, lateral lobe. (After Pocock.)
Appendages.—The three-jointed chelicerae are powerful and chelate. The first joint is small, but the second is strongly developed and bears at its anterior end on the inner side a projection which forms the immovable finger of the chela. The third joint, or movable finger, is articulated on the outer side, and both fingers are armed with teeth whose arrangement is useful in distinguishing the species. The pedipalpi consist of six joints. The coxa is small and has an inwardly directed lamella which assists in feeding. The trochanter is also a small joint, bearing, normally at right angles to the longitudinal axis, the powerful humerus or femur. Then follows the brachium or tibia, again directed forward, and the last two joints form the chela or “hand,” the terminal joint or movable finger being on the outer side as in the chelicerae. In systematic determination special attention is given to the “hand.” In some forms the upper surface is uniformly rounded, while in others a “finger-keel” divides it into two flattish surfaces almost at right angles. The biting edges of the fingers are usually furnished with rows of minute teeth arranged characteristically in the different genera. The ambulatory legs are seven-jointed, though, unfortunately, authors are not agreed upon the nomenclature of the joints. Kraepelin[[242]] names them coxa, trochanter, femur, tibia, and three-jointed tarsus, and Simon[[243]] agrees with him. Pocock’s names[[244]] are coxa, trochanter, femur, patella, tibia, protarsus, and tarsus, and it is certainly convenient that each joint should have a separate name, but it must be borne in mind that the tibia of different authors is not always the same joint. Special attention must be directed to the three terminal joints, which furnish highly important characteristics. The tibia (in Pocock’s sense) is sometimes provided with a “tibial spur” at its lower distal extremity. From the soft arthrodial membrane between the protarsus and tarsus may proceed one or more dark-tipped claw-like spurs, the “pedal spurs.” The terminal joint (tarsus of Pocock) is variously furnished with hairs and teeth, and always ends in a pair of well-developed movable claws beneath which a much reduced and sometimes almost obsolete third claw is distinguishable. The tarsus generally projects in a “claw-lobe” over the base of the superior claws, and sometimes lateral lobes are present. The first and second coxae have triangular maxillary lobes directed towards the mouth. The third and fourth coxae are fused together on each side, and those on one side are separated from those on the other by the sternum. In other respects the four pairs of legs are usually similar.
Internal Anatomy.
The alimentary canal is a fairly uniform tube, nowhere greatly dilated. The very small mouth leads into a small suctorial chamber, and this is connected by a narrow oesophagus, which pierces the cerebral nerve-mass, with a slightly dilated portion which receives the ducts of the first pair of gastric glands, often called salivary glands. The succeeding portion in the prosoma receives four or five more pairs of ducts from the well-developed gastric glands. In the rapidly narrowing first metasomatic segment the intestine receives one or two pairs of Malpighian tubes, and thence proceeds to the anus, situated ventrally in the last segment.
The vascular system is of the usual Arachnid type, the heart being a seven-chambered dorsal longitudinal vessel lying in a pericardium, with which it communicates by seven pairs of valvular ostia. Lankester[[245]] has demonstrated several pairs of superficial lateral veins connecting two deep-seated ventral venous trunks with the pericardium. The lung-books are, so to speak, pushed in to dilatations of these trunks, so that some of the lateral veins carry blood newly aerated by the lung-books directly to the pericardium.
The nervous system is not greatly concentrated except in the prosoma, where there is a single ganglionic mass which innervates not only the whole prosoma but the mesosoma as far as the first and sometimes the second pair of lung-books. There are two mesosomatic ganglia, variously situated in different genera, and each metasomatic segment has its ganglion.
The generative organs are more or less embedded in the gastric glands. There are two testes, each composed of a pair of intercommunicating tubules, and connected by a common vas deferens with the generative aperture, which is furnished with a double protrusible intromittent organ. A pair of vesiculae seminales and a pair of accessory glands are also present. The female possesses a single ovary, consisting of a median and two lateral tubules, all connected by cross branches.