1 68 NATURAL SCIENCE. March, 1895. 



der Landtiere " (Leipzig, 1891), namely, that Limulus has been derived 

 from tracheate ancestors, living on the land. He further brings 

 forward reasons for supposing a similar origin for the Crustaceans. 



I need not again dwell at length upon the morphological and 

 palaeontological facts which seem to forbid us to accept this theory. 

 The comparative study of living forms shows that the scorpions and 

 scorpion-spiders (Pedipalpi), which breathe by lung-books only, are 

 more primitive than the spiders ; and that those spiders which 

 breathe by lung-books only are lower in the scale than those in which 

 the hinder pair of lung-books are replaced by tracheal tubes. These 

 conclusions are confirmed by the study of fossil forms. In phylo- 

 genetic speculations, due weight must be accorded to evidence from 

 all quarters ; and, while M. Jaworowski's researches deserve careful 

 consideration, the embryological fact which they establish need not 

 lead to the abandonment of the position required by the comparative 

 study of living and fossil arachnids. The admission of the corre- 

 spondence between the gill of Limulus and the lung of the spider 

 leaves us free to accept Mr. Simmons' interpretation of his observa- 

 tions, if some reasonable explanation can be found for the evanescent 

 tracheal tubes which M. Jaworowski describes. It seems possible 

 that, in the passage from an aquatic to a terrestrial life, before the 

 gill-bearing limbs became converted into lung-books, air tubes in con- 

 nection with these limbs would have been of great service to the 

 primitive arachnids. Such a stage is somewhat similar to what we 

 find at present in the land-isopods. As the limbs sank into the 

 abdomen, and the plates became adapted for breathing air, the 

 necessity for the tubes would pass away, and they would, in course of 

 time, become lost. Their appearance in the embryonic stage of 

 certain spiders is what might be expected upon such a hypothesis. 



Geo. H. Carpenter. 



