5l8 THE INVERTEBRATA 



lung books of the scorpion there are also parallel leaflets, which are 

 sunk into pits with a confined opening (pneumostome). The air 

 circulates between these leaflets, but there is no evidence that air is 

 actively pumped in and out of the lung. Gaseous exchange then 

 appears to be entirely due to diflFusion. In spiders, however, a 

 complicated system of muscles has been described which bring about 

 expiration by compressing the lung. Inspiration follows by the 

 elasticity of the chitin lining. 



Fig- 353- Longitudinal section through the opisthosoma oi Limulus, showing 

 four of the five gill books. From Shipley and MacBride. i, operculum; 

 2, second gill book; 3, muscle which moves the gills up and down; 4, blood 

 vessels; 5, muscle which raises the operculum. 



Fig- 354- Diagram of respiratory organs of the Arachnida, After Kingsley. 

 A, Two segments with appendages (gill books), bearing leaflets on their pos- 

 terior face as in Limulus. B, Appendages partly (right) and wholly (left) 

 withdrawn into pits of the ectoderm so that the flat appendage forms the 

 floor of the pit and the leaflets are internal, a. anterior. 



It is generally supposed that the lung books of scorpions are derived 

 from gill books by the withdrawal of the leaflets into special pouches, 

 the lungs (Fig. 354). The appendages or plates disappear or form the 

 floor of the lung and the leaflets appear as folds of the lining. Lung 

 books, according to this view, are organs which, originally intended 

 for aquatic use, have been slightly adapted for terrestrial life, but while 

 the scorpions in their long history have shown no capacity for further 



