ARACHNIDA 



447 



development, the rest of the Arachnida have developed the typical 

 arthropod tracheal system. The spiders, at least, have passed through 

 a primitive lung-book stage from which they have not all emerged. 

 In fact they show all the stages of replacement of lung books by 

 tracheae, which actually arise as diverticula of the lung itself. Thus 

 we have the following stages in the spiders : 



(i) Two pairs of lung books and no tracheae in the families 

 Atypidae, Liphistiidae and Aviculariidae. 



Fig. 331. Respiratory organs of spiders. After MacLeod. A, Horizontal 

 section through the opisthosoma of Argyroneta. i, stigma opening into a 

 cavity from which arise bundles of 2, terminal and 3, lateral tracheae; 

 4, lung book with leaflets in section. B, Longitudinal section through lung 

 book of a spider, i, pneumostome or stigma; 2, free edge of leaflet; 3, air 

 space between leaflets ; 4, blood space within leaflet. 



(2) An anterior pair of lung books and a posterior pair of stigmata, 

 opening into tracheae, in the majority of families. 



{2 a) An anterior pair of lung books, the posterior pair of stigmata 

 and tracheae having entirely disappeared, in the family Pholcidae. 



(3) Two pairs of stigmata, both opening into tracheae, in the family 

 Caponiidae. 



These form a complete series. The adherents of the theory that 

 lung books have given rise to tracheae claim that, on the whole, those 

 spiders which have two pairs of lung books are the most primitive 



