228 MANUAL OF ZOOLOGY. 



composed of twelve somites, but there is no evident line of de- 

 marcation between this region and the cephalothorax. The 

 thoracic segments carry four pairs of ambulatory feet. The 



Fig. 76. Scorpion (reduced). 



maxillary palpi are greatly developed, and constitute strong 

 nipping claws, or "chelae" (figs. 74, 76). The mandibles (an- 

 tennae) also form claws, or "chelicerae." The respiratory organs 

 are in the form of pulmonary sacs, four on each side, opening 

 upon the under surface of the abdomen by as many stigmata, 

 each of which is surrounded by a raised margin, or " peritrema" 

 (fig. 74, 3). 



The Scorpions are mostly inhabitants of warm regions, and 

 their sting, though much exaggerated, is of a very severe 

 nature. 



Family 2. Thelyphonida . The members of this family in 

 external appearance closely resemble the true Spiders, from 

 which they are separated by the possession of a segmented 

 abdomen and long, spinose palpi, and by the absence of spin- 

 nerets. They are distinguished from the Scorpionida by the 

 amalgamation of the head and thorax into a single mass, which 

 is clearly separated from the abdomen by a slight constriction, 

 as well as by the fact that the maxillary palpi terminate in 

 movable claws instead of chelae. Further, the extremity of 

 the abdomen is not furnished with a terminal hook, or "sting." 



ORDER II. ARANEIDA or DIMEROSOMATA. This order 

 includes the true Spiders, which are characterised by the amal- 

 gamation of the cephalic and thoracic segments into a single 

 mass, and by the generally soft, unsegmented abdomen, attached 

 to the cephalothorax by a constricted portion, or peduncle. 

 Respiration is effected by pulmonary sacs usually in combina- 

 tion with tracheae. (Hence the name Pulmotrachearia, some- 

 times applied to the order.) The number of the pulmonary 



