II. ACERATA: SCORPIONIDA 



391 



In development the arachnidan tracheae arise in connection with the ab- 

 dominal appendages, as do the lungs. (In the Solpugida; and some mites 

 cephalothoracic trachea? occur.) This shows that the arachnidan tracheae 

 are entirely different in origin from the trachea? of insects. 



The gonads (only the Tardigrades are hermaphroditic) are abdominal 

 in position and open by paired ducts (sometimes with a single mouth) on 

 the first abdominal somite. In most cases the animals are oviparous, 

 but the scorpions and many mites bear living young. In many instances 

 the mothers care for their eggs and young, the scorpions carrying their 

 families on their bodies. Only rarely is there a metamorphosis, and 

 then in the aberrant forms 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 (figs. 366, 421) they have four pairs 

 of walking feet (3-6), while the pedipalpi (2) are large and bear pincers. 



FIG. 421. Under surface of scorpion, showing the combs and the outlines of the 



lung sacs with their spiracles (orig.). 



The chelicera3 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 cephalo- 

 thorax and six narrower somites behind, forming a postabdomen. The 

 last somite is produced into a sharp spine and contains two large poison 



