Development of the Lungs of Spiders. 219 



the tracheaj of the adult. At the inner end of tlie tracheal 

 trunk thus formed the nuclei are arranged in a radiating or 

 busli-likc manner, apparently indicating that here is the place 

 where tlie trunk is about to divide into the tracheal twigs ; 

 but r have not been able to trace any tracheal lumina between 

 these cells. I have not followed the later history of the 

 tracheal system with any detail, but tliink that the foregoing 

 is sufficient to justify my thesis that the tracheae and the lungs 

 are to be regarded as homologous structures. 



Conclusions. 



From the preceding it will be seen that : — 



I. The lungs of the spider arise as infoldings upon the 

 posterior surface of the appendages of the second abdominal 

 somite in the same manner as described by Kingsley (1885 

 and 1893) for the gills of Limulus. They, have tlie same 

 growing-point at the base of the appendage and form the lung- 

 leaves in exactly the same way that the gill-leaves arise. In 

 other words, the lung-book of the Spider (and presumably of 

 all Arachnids which possess one) arises at first as an external 

 structure upon the posterior surface of the abdominal append- 

 ages. These appendages sink in without any inversion or 

 other complications, in exactly the manner theoretically 

 deemed probable by Kingsley, so that there can no longer be 

 any doubt as to the exact homology existing between the 

 lungs of the spider and the first pair of gills in the horseshoe- 

 crab. 



II. The tracheje develop from the next pair (third abdo- 

 minal somite) of limbs. In their earlier stages these append- 

 ages show on their posterior surface a folding similar to that 

 on the preceding members. From this it follows that the 

 lung-book condition is the primitive, the tracheae of the 

 Arachnids being derived from it. And with these facts there 

 is left no ground for those who regard the " Tracheata " as a 

 natural group of the animal kingdom. 



Tufts CoUego, Mass., 

 May 25, 1894. 



Bibliography. 



(1892) Behxard, H. M— ' Tlie Apodidc-e.' Londou, 1892. 



(1893) Bkrnaed, H, M. — " iS^otes on the Chernetida).'' Journ. Linn. 



Soc, Zool. vol. xxiv., 1893. 

 (1872) Behtkau, Ph. — " Ueber die Respirationsorgane der Araneeu." 



Archiv f. Naturgeschichte, xxxviii., 1872. 

 (1886-87) BnucE, A. T. — ' Observations on the Embiyology of Insects 



and Arachnids.' Baltimore, 1886-87. 



