THE FIRST AIR-BREATHERS. 139 



altogether in their details of structure ; and all three seem to 

 have been placed on the earth about the same time. They are : 

 First, the Myriapods, or Gallyworms and Centipedes ; secondly, 

 the Insects ; and thirdly, the Arachnidans, or Spiders and 

 Scorpions. 



In the Myriapods a system of air-tubes, kept open by elastic 

 spiral fibres, penetrates the body by lateral pores, thus retaining 

 the resemblance to the lateral respiration of the Crustaceans 

 and worms. In the Insects, where this type of structure rises 

 to its highest mechanical perfection, and where the animal 

 is enabled to be not merely an air-breather, but a flier, the 

 same system of lateral pores and internal air-tubes is adopted, 

 and is so extended and ramified as to give a very perfect respi- 

 ration. In the Spiders and Scorpions the system is the same, 

 except that in the latter and a part of the former the whole or 

 a part of the tracheal system becomes expanded into air- 

 chambers simulating true lungs. 



Among the Vertebra):es, the fishes are bieathers by gills 

 attached to arches at the sides of the neck. But already in 

 the Devonian we have reason to believe that there were fishes 

 having the swimming-bladder opening into the back of the 

 mouth to receive air, and divided into chambers, so as to con- 

 stitute an imperfect lung. And here we have not, as in tlie 

 lower types, an adaptation of the old water-breathing organs, 

 but an entirely new apparatus. In the next grade of Verte- 

 brates we find, as in the Frogs, Water-lizards, etc., that the 

 young are aquatic and breathe by gills, while the adults ac- 

 quire lungs, sometimes retaining their gills also, but in the higher 

 forms parting with them. Thus in the vertebrates alone we 

 have true lungs, distinct structurally from gills ; and these 

 lungs attain to their highest perfection in the "birds and 

 mammals. 



The oldest air-breathers at present known are insects allied 

 to the modern May-flies. They were discovered by the late 

 lamented Prof. C. F. Hartt in the plant-bearing shales of the 



