1. Both their book lungs and the anterior median eyes. 2. The bottom side of the abdomen is usually brown with yellow book lungs . 3. Book lungs are not related to the lungs of modern land-dwelling vertebrates.4. The unfolded " pages " ( plates ) of the book lung are filled with hemolymph. 5. See branchiostegal lung and book lung . 6. Recent Mesothelae are characterized by the narrow mygalomorph spiders, they have two pairs of book lungs . 7. The respiratory system consists only of book lungs , which could help explain why they are relatively inactive. 8. In a very few species the book lungs have developed deep channels, apparently signs of evolution into tracheae. 9. Unlike almost all other araneomorph or " true " spiders, members of the family have four book lungs . 10. Like the " primitive " suborder of spiders Mesothelae, they have two pairs of book lungs , and downward pointing chelicerae.
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