ORIGIN OF TRACHEA. I 79 



carboniferous beds, we can pretty accurately determine the 

 time of their origin. The development of the first Tracheate 

 Insects out of gill-bearing Zoea-crabs, must have taken place 

 between the end of the Silurian and the beginning of the 

 coal period, that is, in the Devonian period. 



Gegenbaur, in his excellent " Outlines of Comparative 

 Anatomy," ^^ has lately endeavoured to explain the origin 

 of the Tracheata by an ingenious hypothesis. The system 

 of trachese, or air pipes, and the modifications of organiz- 

 ation dependent upon it, distinguish Flies, Centipedes, 

 and Spiders so much from other animals, that the concep- 

 tion of its first origin presents no inconsiderable difficulties 

 to phylogeny. According to Gegenbaur, of aU living Trache- 

 ate Insects, the Primaeval Flies, or Archiptera, are most 

 closel allied to the common primary form of the Tra- 

 cheata. These insects — among which we may especially 

 mention the delicate Day flies (Ephemera), and the agile 

 dragon-flies (LibeHula) — in their earliest youth, as larvae, 

 frequently possess external tracheate gills which lie in two 

 rows on the back of the body, and are shaped like a leaf or 

 paint-brush. Similar leaf or paint-brush shajDed organs are 

 met with as real water-breathing organs or giUs, in many 

 crabs and ringed worms, and, moreover, in the latter as real 

 dorsal appendages or limbs. The " tracheate gills," found in 

 the larvcB of many primaeval winged insects, must in all 

 probability be explained as " dorsal limbs," and as having 

 developed out of the corresponding appendages of the Anne- 

 lida, or possibly as having really arisen out of similar parts 

 in Crustacea long since extinct. The present tracheal 

 respiration of the Tracheata developed at a later period out 

 of respiration through " tracheate gills." The tracheate gills 



