GENERAL MORPHOLOGY. 357 
These facts led Palmén to the conclusion that the spiracles 
are virtually, although not functionally, present in the larva, 
and that the closed system is a modification of the open 
system. This view does not necessarily follow, however, from 
the facts, for if the trachez are developed from parablast, the 
existence of cell-strings uniting the tracheal trunks and the 
skin might be expected; and the manner in which ecdysis 
takes place may be regarded as an adaptive modification. 
The ecdysis of the intima of the trachez in the fly nymph 
occurs, nearly inthe same way, as in larve with closed trachez ; 
new spiracles are formed, which are quite independent of the 
larval spiracles, whilst the vestibule of the spiracle and the valves 
are developed from the integument, and not from the cell-string, 
through which the intima of the tracheze of the pronymph is 
withdrawn (see Pl. XXII., Figs. 6 and 7). 
Palmén further regarded the longitudinal trunks as originating 
by the anastomosis of trachee springing directly from the 
spiracular sacs. In the higher insects this does not appear 
to be their origin, and the existence of these vessels is so 
constant in all insects, whilst the position and number of the 
spiracles is so variable, that it can hardly be doubted, I think, 
that they originated in Tracheata in which the spiracular vessels 
did not exist. 
The only instance in which the longitudinal trunks are 
absent in the less highly differentiated Tracheata are the highly 
abnormal Protracheate, Peripatus, and in some Thysanuridz. 
In Peripatus the tracheal vessels are so irregular that they 
cannot be taken as the original form of the very constant 
tracheal system of the Myriapoda and Insecta, and it appears 
equally probable that the genus Peripatus is a highly degene- 
rate form. With regard to the condition in some Thysanuride, 
described by Palmén, it is quite as possible that the tracheal 
system has undergone retrograde metamorphosis. The Spring- 
tails cannot be regarded as generalised Orthoptera, and there 
is no evidence that retrograde follow the same lines as 
evolutionary modifications. 
It appears to me most probable that the tracheal system is 
