286 THE APODID^ partii 



The origin of the book-leaf trachecX from the gills 

 of the Xiphosuridae, fascinating as it is, breaks clown 

 when carried into detail. It is easier to believe that 

 the lung-books are only a specially concentrated 

 arrangement of the tracheal tubes, no more extra- 

 ordinary than the other extreme, viz. the diffuse 

 arrangement found in the Hexapoda. We find 

 almost every form of tracheal arrangement between 

 these two extremes within the division of the 

 Tracheata, and further both tubular and book-leaf 

 tracheae within the Arachnida. We think that the 

 evidence in favour of the new classification, to be 

 drawn from the form of the tracheae, is not con- 

 vincing. 



The most probable origin of the tracheae appears 

 to us to be that which refers them back to dermal 

 glands. The original Tracheatan-Annelid on first 

 migrating on to the land probably respired through 

 the whole skin. The increase of surface afforded by 

 the ducts of the dermal^ glands would very naturally 

 be taken advantage of. The walls of these ducts being 

 internal, their surfaces would be selected and special- 

 ised until they undertook the whole respiration. That 

 this was the origin of the tracheae is rendered very pro- 

 bable by the fact that the openings of the tracheal tubes 

 in Peripatus are, in some species at least, scattered 

 irregularly over the whole body. This derivation of the 

 tracheae from dermal glands receives some support also 



^ If these include the coxal glands, it may throw light upon the 

 developmental relations between the book-leaf trachere of the Arachnida 

 and their rudimentary abdominal limbs. 



