430 SCIENCE PROGRESS 



and Peripatus.* It is a particularly suggestive circumstance 

 that in the dipneumonic Araneida tubular and book-leaf 

 tracheae occur simultaneously. 



Since it has been shown by Brauer - that the lung-books of 

 Scorpions are derived from gills borne on mesosomatic 

 appendages,^ there is clearly no difficulty in assuming that 

 the gills of the hypothetical fresh-water Trilobite could similarly 

 have been transformed into lung-books sunk in the body and 

 communicating with the air by stigmata ; and once this step 

 was accomplished the further transformation into tracheae could 

 follow as indicated in the foregoing paragraph. A very primi- 

 tive condition of tracheae exists in Campodea, in which three pairs 

 of spiracles are situated in the thoracic region, and the tracheae 

 of each stigma keep distinct, so that there are six separate small 

 tracheal systems, three on each side of the body. In Machilis 

 also the nine pairs of tracheal systems keep distinct from each 

 other. 



Other instances of the application of this theory to explain the 

 sudden development of new groups in the organic world might 

 be readily multiplied ; but the foregoing examples may perhaps 

 be considered sufficiently striking to obviate the undue expan- 

 sion of this paper by entering into further details at the present 

 juncture. 



* Peripattis can hardly be regarded any longer as a member of the ancestral 

 stock of Myriapods and Insects, but as a highly specialised side-branch of the 

 Annelidan ancestor of Arthropods. 



^ Zeitschr. Wiss. Zool. lix. 1895, p. 351. 



^ Dr. F. W. V\y:ct\\{Quari . Journ. Micr. Sci. liv. pt. i, 1909), in a memoir on the 

 development and origin of the respiratory organs in Aranea" finds that in spiders 

 also the lung-books are derived from gill-books similar to those of Limulus, for the 

 first leaves of the lung-books appear on the free posterior side of the provisional 

 abdominal appendages, quite outside of the pulmonary invaginations. 



