x PHYLUM ARTHROPODA 259 



the web is composed. The fine threads of viscid secretion 

 issuing from the ducts harden on exposure to the air, and 

 are worked up into the web by means of the posterior legs. 

 There are six or eight eyes on the carapace. The organs of 

 respiration are either four pulmonary sacs similar to those 

 of the Scorpion, or two pulmonary sacs and a system of 

 tracheae resembling those of Insects. 



In the Mites and Ticks (Acarida) (Fig. 144), no distinc- 

 tion into regions is recognisable, but there are the same series 



Fie. 144. Itch mite (Sarcoptes scabiaei). (After Leuckart.) 



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 Xiphosura or King-Crabs, an order comprising the 

 single genus Limulus, differ widely from the Scorpions and 



