26o 



MANUAL OF ZOOLOGY 



In the Mites and Ticks (Fig. 155) no distinction into 

 regions is recognisable, but there are the same series of 

 paired appendages as in the scorpions and spiders. The 

 chelicerae and pedipalpi, and also the legs, differ somewhat 

 in form in different groups, in accordance with differences 

 in mode of life. Organs of respiration, when present, take 

 the form of tracheae. The cattle tick, Ixodes bovis, is the 

 medium of conveyance of the Texas cattle disease. 



Fig. 155. — Cattle Tick Above, nat. size, 

 side view ; below, dorsal view, en- 

 larged. (From Packard.) 



Fig. 156. — Itch mite (Sarcoptes scabi- 

 aei). (After Leuckart.) 



The Xiphosura or King-crabs, an order comprising the 

 single genus Limulus, differ widely from the scorpions and 

 spiders. 1 Limulus (Fig. 157) is a marine arthropod breath- 



1 The Xiphosura, with their fossil allies, the Eurypterida, are by some 

 authors regarded as forming a distinct class, by others they are included 

 among the Crustacea. It seems better to refer them to a distinct class, the 

 Merostomata. — AMERICAN EDITOR. 



