THE PHYLOGENY OE THE TEACHEHI IN AEANEiE. 541 
in any spider. It furnishes the abdomen, cephalothorax and 
appendages Avith a very great number of fine air-tubes, but 
only a portion of these are represented in fig. 9 in which the 
terminal parts of the numerous bi-anching tubules, often 
measuring only 3 fx in diameter, have not been drawn in. 
The pedicel of the posterior tracheae opens into a capacious 
tracheal trunk (p. 645, text-fig. 16, c.tr.) lined with spines, 
which runs forwards at an inclination of about 45° for a very 
short distance only before dividing into two sub-cylindrical 
branches of equal length and thickness, which may be called 
the cephalothoracic trunks (fig. 9, ceplh.tr.), and run 
horizontally forwards into the cephalothorax, becoming 
thinner anteriorly. They remain in contact one above the 
other but the upper one a little more to the lateral side, and 
are somewhat flattened along- the place of contact. Soon 
after passing through the pedicel of the abdomen they each 
break up into a bunch of fine tubules which then spread in 
various directions. Those of the right upper trunk are drawn 
in fig. 9, from Avhich it will be seen that most of the tubules 
of the right trunk cross over to the left side and, generally 
remaining unbranched, enter the coxse of the left appen- 
dages, whereas only the posterior appendages of the right 
side receive tubules from this trunk. Several of the tubules 
give off dendritic branches {d.hr.), which ramify Avithin the 
cephalothorax and supply its upper part Avith air. While the 
inferior cephalothoracic trunks remain unbranched except at 
apex, the two upper ones each give off a small branch (fig-. 9, 
hr.) from the upper side near the middle. This branch is 
arborescent in form and divides into a number of tubules 
Avhich supply the region above the anterior pair of tracheae. 
In addition to the tAvo anterior trunks each of the short 
main trunks also gives off from its posterior side at base a 
cylindrical posterior trunk of half the diameter of either of 
the anterior ones. This trunk, which may be called the 
abdominal trunk, runs first in an upward and medial 
direction, and then curves and runs horizontally toAvards the 
