KESPIKATOEY ORGANS IN ARANEiE. 
83 
common origin has no doubt weighed considerably against the 
acceptance of the branchial origin of lung-books, but this 
should not be the case in Anew of the undoubted diphyletic 
origin of the trachem in Aranese, which, I think, I have suffi- 
ciently demonstrated. Further, in one and the same spider 
both parts of the tracheae, although of different origins, have 
exactly the same histological structure, hence similarity 
of structure in the fully developed tracheae does 
not mean similarity of origin. I mention this here 
expressly, since this similarity of structure has been used as 
Text-fig. 6. Text-pig. 7. 
Text-figs (! and 7. — Appendages of the genital segment and a 
pair of abdominal branchiate appendages of Limiilns, seen 
from behind (after Ray Lankester). ;/. o., genital openings; 
hr., gill-book ; ec. t.. external opening of an ectodermal tendon. 
an argument in favour of the monophyletic origin of all 
tracheie. 
The appearance in spiders of trachea) as newly acquired 
organs derived from two separate and distinct sources simul- 
taneously with the occurrence of other Avell-developed organs 
of respiration clearly shows how readily trachea) may be 
acquired.^ Why, then, should they not have originated 
' Pocock ('93), who was of opinion that tracheal tubes replaced lung- 
books at least twice in the gi-oup Aracbnida, viz. in the Dipneu- 
mones and in the Pseudoscorpiones, remarks (p. 17); “ The fact 
that these tubes have been developed tivice in the same group bears 
veiy strong evidence as to their efficacy as breathing organs. They 
