ORIGIN OF TRACHEA, 179 
carboniferous beds, we can pretty accurately determine the 
time of their origin. The development of the first Tracheate 
Insects out of gill-bearing Zoéa-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 trachez, 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 all living Trache- 
ate Insects, the Primeval 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 (Libellula)—in their earliest youth, as larve, 
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 shaped organs are 
met with as real water-breathing organs or gills, in many 
crabs and ringed worms, and, moreover, in the latter as real 
dorsal appendages or limbs. The “tracheate gills,” found in 
the larve of many primeval 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 
