An endeavour to show that tlie trachese arose Irom setiparous sacs. 521 



over the coxal glands, because of their doubtful origin, the fact that 

 in some cases we have tracheœ on the cephalothoracic segments, 

 is of prime importance for our argument. Their bearing on the sub- 

 ject I shall endeavour presently to point out. 



Following the row on to the abdomen , we have book-leaf 

 tracheöe, tubular tracheae and spinning glands. My assumption that 

 these are all homologous structures simplifies matters very much. 



Taking the tracheae first, it is a matter of inditference whether they 

 develop into concentrated lungs specialised for the aeration of a 

 blood stream, as is the case in the Scutigeridœ, or into long tubular 

 tracheae for the aeration of the tissues directly. Both forms are de- 

 ducible from the same structure. 



This derivation of both forms of tracheae from setiparous (acicular) 

 glands at once explains the presence of tracheae on the céphalothorax 

 of the Arachnida, this being one of the principle difficulties in the 

 way of those who would deduce the Arachnidan abdominal trachea? 

 from embedded gills. This latter, at least as explained by Macleod 

 and more recently by Laurie ^), is one of those fascinating but seduc- 

 tive hypotheses which at first sight seem convincing, but which 

 have to be given up in the face of the great morphological difficulties 

 which they involve. It compels us, for instance, to assume that the 

 cephalothoracic tracheae have had an entirely different origin, so that, 

 as I have pointed out elsewhere ^ ), it is necessary to assume that the 

 same structures, tubular tracheio, have had two independent origins 

 in the same animal! For example, in the Solpugidce, the tubular 

 tracheae of the abdomen are considered to be modified lung books, 

 but the tubular trache» of the thorax are a new formation. As far 

 as I can discover from the examination of series of sections of 

 Galeodes, there is absolutely no difference between the tracheie which 

 open through the large stigmata of the thorax and those opening 

 through the more insignificant stigmata in the abdomen. It is difficult 

 to believe that they had a separate origin. 



The embedded gill theory must, I think, definitely give way 

 before some simpler theory, such as that here put forward. The 

 development of book-leaf trachete from ectodermal invaginations such 



1) Laurie, Respiratory organs of Scorpio and Slimonia, in: Zool. 

 Anz., No. 386, 1892. 



2) Some observations on the relations of the Acarids to the 

 Arachnida, in : J ournal of the Linnean Soc. Zoology, vol. 24. 



