EBSPIRATOKr ORGANS IN ARANE^. 
65 
the thicker anterior wall of the trachea is equivalent to the 
anterior wall of the pulmonary sac together with the lung- 
saccules produced by the latter. 
Three forms of this type of trachea are mentioned by 
Bertkau (’72), namely, those having : (1) the two median 
trunks united at base to a short common tube, as in the 
Theridiidse and some Argiopidae (Bertkau and Lamy) ; 
(2) a medial and lateral trunk united at base in pairs on 
each side to form two short common trunks, as in Tegeuaria 
(fig. 21), this being the usual form, according to Bertkau 
and Lamy; and (3) the four trunks springing separately 
from the pedicel, as in some Argiopidae, e.g. Linyphia 
(fig. 25). 
The resemblance between this third form of 
tracheal system (fig. 25), in which the lateral trunks 
at first take a lateral course before running for- 
wards, and tlie pulmonary system of an Attus (fig. 20) 
is very striking, and clearly shows the homology 
of the medial trunks {vi. tr.) with the eut apophyses 
(ec. t. 8), and of the lateral trunks (/. tr.) with the 
pulmonary sacs {pulm. ».). The parallel between the 
spinous intertracheal canal of communication (connecting 
the median trunks with one another and with the lateral 
trunks at their base) and the interpulmonary canal of com- 
munication (connecting the entapophyses with one another 
and with the pulmonary sacs) is complete, as may be seen by 
comparing fig. 24 with fig. 23 (sagittal sections through the 
lateral region of the canal [can.'] along the lines indicated in 
figs. 25 and 20), and fig. 26 with fig. 23b (median sections 
along the lines indicated in figs. 25 and 20). From these 
figures it will also appear perfectly clear that the 
medial trunks of the tracheal system cannot be con- 
sidered as branches of the lateral trunks any more 
than the entapophyses of the pulmonary segment 
are branches of the pulmonary sac. 
In the Agelena-type of tracheie the medial trunks 
generally take a fairly straight course as far as the euto- 
VOL. .54, PART 1. NEW SERIES. 
o 
