EESPIKATORY ORGANS IN ARANE.E. 
79 
pair of lung-books of Tetrapneumouous forms or whether the 
reverse was the case. 
That the lateral trache® are serially homologous with the 
pulmonary sacs of the preceding somite and, therefore, homo- 
logous with the same part of the lung-books of the ninth somite 
in Tetrapneumonous spiders, cannot, I think, be disputed, 
although the embryology of the latter group is not yet 
known. 
Ill deriving the lung-books from tracheae the simplest theory 
and the one that has been usually adopted by those who 
favoured this view, is to consider the pulmonary sac or ante- 
chamber to represent the main trunk of a trachea and the 
saccules merely modified lateral branches arranged in a single 
row and flattened by mutual pressure. 
A very serious objection to this view lies in the appearance 
of the two oldest pulmonary saccules on the embryonic ap- 
pendages q uite outside of the pulmonary sac. These 
two saccules cannot be branches of the main trunk, and in 
order to account for their presence we should have to assume 
that they themselves at one time each represented a separate 
tracheal trunk. This, however, could hardly have been the 
case, since all the saccules are formed in the embryo in exactly 
the same manner (apart from their position out of or within 
the sac) and should, therefore, have exactly the same phy- 
logenetic origin. 
Another view based by Jaworowski (^94) on embryological 
grounds and adopted by Bernard (’96, p. 375) on theoretical 
ones is to the effect that the lung-books arose by horizontal 
folds in the basal part of a vertical tracheal trunk. Hei’e 
also the appearance of the two oldest saccules, entirely out- 
side of the pulmonary sac, is too strong an argument against 
our acceptance of this theory, which, moreover, Jaworowski 
lias failed to prove embryologically, as I have already pointed 
out on a previous page (p. 33). 
In fact the only way we can derive the saccules of lung- 
books from tracheal tubes which appears to me at all feasible 
is to assume that an ancestral form of the Araneae possessed 
