The Internal Anatomy of Spiders 



near its base. Their position is indicated externally on the 

 living animal as pale trapezoidal spots with rounded angles (Fig. 

 153). The first pair open, one on each side, in the epigastric 

 furrow; when there is a second pair they open a short distance 

 back of the first pair. 



From the anterior wall of each sac there project a series of 

 horizontal, leaf-like folds, which has suggested the name of book- 



1 



Fig. IS3- 



VENTRAL ASPECT 



OF THE ABDOMEN OF 



A TARANTULA 



Fig. 154- DIAGRAM OF A BOOK-LUNG 



1 , lung-slit 



2, space filled with blood 



3, leaves of the book -lung 



lungs (Fig. 143). But as these leaves are also attached to the 

 lateral walls of the sac, the lung has been more aptly compared 

 to that form of a letter-file that consists of an envelope divided by 



manypartitions into numerous 

 compartments (Lang '91). 



Each leaf of the book- 

 lung consists of a flattened, 

 sac-like fold of the body-wall, 

 and hence of two lamellae. 

 These lamellae are connected 

 at frequent intervals by ver- 

 tical supports; and the leaves 

 are kept apart, so that the air 

 can circulate between them, 

 by a palisade of vertical, 

 knobbed spines on the upper surface of each (Fig. 154). At 

 and near the free edge of the leaves, these spines are longer, and, 

 instead of being knobbed, divide at the tip into several branches, 

 which anastomose with branches of adjacent spines (Fig. 155). 



146 



Fig. 1 ss. THE TIPS OF TWO LEAVES 

 OF A BOOK-LUNG 

 i,i, palisade of spines on dorsal lamella 



2, vertical support between lamellae contracted 



3, vertical support stretched 



4, blood-cavity 



5, globule of blood (after Bcrteaux) 



