214 SCORPIONS. 



converted into large pincers ; while in spiders they are short, and resemble the 

 other limbs. In scorpions and the other groups, where these limbs form prehensile 

 weapons, they are called chelae ; whereas in the spiders and ticks, where they are 

 smaller and tactile in nature, they are known as palpi. Behind the palpi or chelae 

 come four pairs of limbs, acting as the locomotor organs. The palpi or chelae are 

 typically composed of six segments, and armed with a single claw, which, however, 

 may be fused with the terminal segment, as in scorpions. The legs seem primarily 

 to have been six-jointed ; and their segments, from base to apex, are respectively 

 termed coxa, trochanter, femur, patella, tibia, tarsus. One or more of these is, 

 however, almost invariably divided, so that the number rises to at least seven. 

 Thus, whereas in the scorpions the tibia, and in spiders the tarsus, is divided, in 

 other groups, like the Pedipalpi, the tarsus may be composed of a number of small 

 segments. Accordingly, six pairs of large appendages are attached to the forepart 

 of the body ; and since this part was supposed to represent the combined head and 

 thorax of insects, it is termed the cephalothorax. The abdomen may bear small, 

 dwarfed limbs, as in scorpions and spiders, but its limbs are never, either structurally 

 or functionally, like those of the cephalothorax. Although it may be undivided, 

 this part never contains more than twelve segments, and often much fewer. 

 Allowing twelve to the abdomen, and six to the cephalothorax, the body of the 

 more typical members of the class comprises eighteen segments. All Arachnoids 

 breathe air, either by means of short sacs or of long tracheal tubes communicat- 

 ing with the exterior by apertures (stigmata) on the lower surface of some 

 of the abdominal segments. The young which, save in scorpions, are born in 

 the egg stage, resemble their parents, and in the course of growth only undergo 

 a series of moults without metamorphosis. The class may be divided into eight 

 orders, the first of which includes 



The Scorpions, — Order Scorpiones. 



In this group all the typical eighteen segments of the body are developed, although 

 the last five are abruptly narrowed to form with the telson, or poison-sting, the 

 tail. The whole abdomen, including the tail, is distinctly jointed ; but the cephalo- 

 thorax is covered above with a single plate, or carapace, bearing the eyes. The 

 latter vary from six to ten, two being placed together in the middle, and the others 

 arranged at the sides of the forepart of the carapace. Of the appendages, the 

 four hinder pairs are similar, being tipped with a pair of claws, and used for 

 locomotion. The two front pairs, however, have been transformed into pincers or 

 nippers, the first pair, or mandibles, being small and three-jointed, and the second, 

 the chelas, or great pincers, of large size and six-jointed. The coxae of the four 

 pairs of legs are immovably united to form the floor of the cephalothorax, and 

 wedged in between those of the last two pairs there is a single sternal plate, the 

 shape of which is of considerable value as a character in the classification of these 

 animals. The breathing-organs consist of four pairs of sacs, — of which the 

 cavities are filled up with a number of fine plates, arranged like the leaves of a 

 book, — are placed upon the third, fourth, fifth, and sixth segments of the abdomen, 

 and their apertures open upon the sternal or ventral plates of these segments. 



