356 THE TRACHEAL SYSTEM OF THE IMAGO. 
capillaries, derived from the closed trunks, are exposed to the 
aérating influence of the surrounding water. 
More recent researches render it probable that the earliest 
Tracheata were terrestrial and not aquatic as Gegenbaur 
supposed; but even if this is true, the tracheal gills of larval 
Termites, already mentioned (p. 160), show that Gegen- 
baur’s hypothesis does not necessarily break down, and 
his view is consonant with the origin of the trachez proper 
from the parablast. 
A second view originated from Biitschli [128]; he regarded 
the tracheze as homologous with the sericteria and Malpighian 
vessels, and with the segmental organs of Annelids. Semper 
also considered the trachez to be highly modified segmental 
tubes [152]. 
Moseley* regarded the trachez as highly modified cutaneous 
glands, and this view has of late been favourably received ; it 
is, however, quite inconsistent with their developmental history, 
if, as can now scarcely be doubted, they arise from the para- 
blast. 
The most serious opponent of Gegenbaur’s hypothesis is 
Palmén [153], whose work on the morphology of the trachez 
is exceedingly valuable. Palmén believed Kowalevski’s account 
of the development of the tracheze to be correct, and held that 
the open, and not the closed, tracheal system is the primitive 
form. He discovered that the great longitudinal trunks of 
those larvae, which have a closed tracheal system, are con- 
nected with the integument, at the points at which spiracles 
are subsequently developed, by delicate cellular strings; and 
that a solid rod of chitin is developed in the axis of each 
cell-string, which is continuous on the one hand with the 
integument, and on the other with the intima of the tracheal 
trunk; at each ecdysis the tracheal intima is drawn through 
the cell-string, which becomes for the time pervious, and is 
shed by the opening which afterwards becomes the spiracle. 
After the larval ecdysis the cell-cords close; but when the 
insect becomes terrestrial they remain permanently open. 
* Phil. Trans., 1874, vol. clxiv., p. 777. 
