550 
W. F. rUECELL. 
more or less cylindrical tubes (say, by longitudinal division), 
accompanied by tlie disappearance of the bicellular columns 
of the septa. The trachea thus produced would be composed 
of an ante-chamber formed out of the pulmonary ante- 
chamber, with a bunch of tubules on its anterior surface, 
formed out of metamorphosed pulmonary saccules. This 
metamorphosis does not involve a reduction in the effective- 
ness of the respiratory organ, and I imagine the trachea so 
produced to be in no way inferior, but rather superior, to the 
lung-book. Accoi-dingly it would doubtless increase in size 
and tfike over the mnin respiratory functions, and this 
would be accompanied, in the case of the second respiratory 
segment, by a corresponding reduction in the number of the 
leaves of the anterior lung-books, in accordance with Lamy’s 
law of the inverse correlation between the size of the tracheae 
and the number of the lutig-leaves. 
(2) The saccules may have disappeared, leaving only the 
pulmonary sac or ante-chamber, which would then constitute 
a trachea, and may subsequently elongate or even acquire 
secondary branches. These latter, however, would not be 
homologous with pulmonary saccules. This method of origin 
really consists in a reduction in the effectiveness of tiie 
respiratory organ of the somite, and would be accompanied, 
in the case of the second respiratory segment, by- a corre- 
sponding increase in the number of the leaves of the anterior 
lung-books, which would then become the principal organ of 
respiration. This, then, would be exactly the opposite pro- 
cess to that which would have taken place in the first case. 
It appears to me very probable that both these methods of 
origin have actually occurred in the Aranem, the first method 
being applicable to the Dysderidte and their allies, and the 
second to the rest of the tracheate spiders. It will be con- 
venient to take these two sections of spiders in turn. 
(1) The Dysderidm, Oonopidm, and Caponiidm, 
being those forms with the tracheal spiracles far 
apart and not moved backwards, i. e. still nearly 
in their primitive positions. — If, after the metamor- 
