O. L. Simmons — Development qf Lungs of Spiders. 127 



detail but think 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 pos- 

 terior surface of the appendages of the second abdominal 

 somite in the same manner as described by Kingsley ('85 and 

 '93) for the gills of Limulus. They have the 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 struc- 

 ture upon the posterior surface of the abdominal appendages. 

 These appendages sink in, without any inversion or other com- 

 plications, in exactly the manner theoretically deemed proba- 

 ble by Kinsgley, 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 horse shoe crab. 



II. The tracheae develop from the next pair (third abdomi- 

 nal somite) of limbs. In their earlier stages these appendages 

 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 College, Mass., May 25th, 1394. 



BIBLIOGRAPHY. 



'92. Bernard, H. M. The Apodidse. London, 1892. 



'93. Bernard, H. M. Notes on the Chernetidas. Journal Linnasan Society; Zool., 



vol. xxiv, 1893. 

 '72. Bertkau, Ph. Ueber die Respirationsorgane der Araneen. Archiv f. Natur- 



geschichte, xxxviii, 1872. 

 '86-87. Bruce, A. T. Observations on the Embryology of Insects and Arachnids. 



Baltimore, 1886-87. 

 '85. Kingsley, J. S. Notes on the Embryology of Limulus. Quarterly Journal 



of Microscopical Science, xxv, 1885. 

 '93. Kingsley, J. S. Embryology of Limulus. Journal of Morphology, viii, 



1893. 

 '90. Kishinouye, K. On the Development of Araneina. Journal of College of 



Science, Imperial Univ. Japan, iv, 1890. 

 '90. Lang, Arnold. Lehrbuch der vergleichenden Anatomie, II Heft, Jena, 1890. 

 '81 a . Lankester, E. Ray. Note on the Existence in the King Crab of Stigmata 



corresponding to the Respiratory Stigmata of the Pulmonate Arachnida, 



etc. Proceedings of the Royal Society, xxxii, 1881. 

 '81 b . Lankester, E. Ray. Limulus an Arachnid. Quarterly Journal of Micro- 

 scopical Science, xxi, 1881. 



Am. Jour. Sci.— Third Series, Vol. XLVIII, No. 284. — August, 1894. 

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