EESPIKATORY ORGANS IN ARANE/E. 
81 
the Dysderid®, in which the entire tracheal system is 
probably derived from lung-books, we find the tubules arising 
in a dense cluster from the apex of the elongate trunks 
and from a small basal branch on the posterior side, but 
none from the anterior or under surface of the trunks. 
Further, in all tracheae of the Age! ena-type, which is that of 
the majority of the Dipneumonous families, the lateral trunks 
have no secondary branchlets at all. 
On the other hand we find these tubules at various places 
on the tendinal trunks in the Attidas amd other groups (fig. 
31), which shows that the tubules may arise anywhere on a 
tracheal trunk, when required, and quite independently of 
the pulmonary saccules, since in this case they could not have 
been derived from the latter. In Attus floricola there is 
no embryological evidence that the tubules of the lateral 
tracheal branches have anything to do with pulmonary 
saccules, for whereas these latter commence to form in the 
pulmonary segment at an early embryonic stage the tubules 
do not appear until long after the young spider has been 
hatched. It is, however, conceivable that the earlier lung- 
saccules may have been entirely suppressed in the tracheal 
.segment, so that only the post-embryonic lung-saccules 
reappear as secondary tracheal tubules in certain cases, and 
the possibility of the anterior terminal tubules of the Dysde- 
ridte and those of the lateral lobes of the Attidas being of 
this nature must be borne in mind. 
Bertkau (’72, ’78) attempted to utilise the presence or 
absence of secondary tubules as the basis of a system of 
classification, but Lamy (:02) has shown that this character 
has little value for this purpose, since within the same 
family some forms may be provided with tracheal tubules, 
while closely related forms are entirely Avithout them. 
The origin of the lung-books in Arachnids. — A fourth question 
in connection with this subject is Avhether the lung-books of 
nary saccules, since the ante-clianiber is doubtless that of the pair of 
lung-books which the tracheae have replaced. I examined sections of 
C. spiralifera. 
VOT.. o4, PART 1. 
•NEAV SERIES. 
6 
