PHYLUM ARTHROPODA 



669 



tracheae. In the Spiders there are either four pixlmonary sacs 

 (Fig. 553), or two pulmonary sacs and a system of tracheae (Fig. 554). 



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Tracheae are present in the Phalangida and also in the majority 

 of the Acarida. In the Xiphosura the organs of respiration are 



