356 THE TRACHEAE SYSTEM OF THE IMAGO. 



capillaries, derived from the closed trunks, are exposed to the 

 aerating 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 tracheas proper 

 from the parablast. 



A second view originated from Butschli [128] ; he regarded 

 the tracheae as homologous with the sericteria and Malpighian 

 vessels, and with the segmental organs of Annelids. Semper 

 also considered the tracheas to be highly modified segmental 

 tubes [152]. 



Moseley* regarded the tracheas 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 

 Palmen [153J , whose work on the morphology of the tracheae 

 is exceedingly valuable. Palmen believed Kowalevski's account 

 of the development of the tracheas 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 t he- 

 insect becomes terrestrial they remain permanently open. 

 * Phil. Trans., 1874, vol. clxiv., p. 777. * 



