//. ACERATA: SCORPIONIDA. 



447 



like the Linguatulida and Acarina, where the young have but two 

 or three pairs of appendages, acquiring the others later. 



Legion I. Arthrogastrida. 



Arachnida in which the abdominal somites are distinct. 

 Order I. Scorpionida. 



The scorpions bear a superficial resemblance to crayfish and for 

 a long time were associated with them, since (fig. 402) they have 

 four pairs of walking feet (3-6), while the pedipalpi (2) are large 

 and bear pincers. The chelicerae are also chelate. The pedipalpi 

 and the two anterior pairs of legs have the basal joint expanded 

 for chewing. The peculiarities of the abdomen mark the group 

 off from all other arachnids. It consists of seven broader somites 

 attached by their whole width to the cephalothorax and behind 

 six narrower somites, forming a tail or postabdomen. The last 

 somite is bent ventrally in a sharp spine and contains two large 

 poison glands. It is the sting ' of the animal, which, in the case 

 of the small species, causes painful wounds in man ; and in the 

 large tropical species is, perhaps, fatal. Usually scorpions feed 

 upon insects, which they seize with the pincers, and, arching the 



FIG. 461. Under surface of scorpion, showing the combs and the outlines of the lung 

 sacs with their spiracles (orig.). 



tail over the back, kill with the sting. On the ventral surface of 

 the second abdominal somite (fig. 461) are a pair of appendages, 

 the combs or pectines; rods with teeth on one side of uncertain 

 function. They are clearly appendages with modified gill leaves, 

 and from their nearness to the sexual opening and their rich nerve 

 supply are supposed to be stimulating organs in copulation. The 



