(K) 
W. !■'. r UK CELL. 
the liimeu permanently open and allow the air to circulate 
freely through it. 
The anterior (ventral) and posterior (dorsal) walls of the 
pedicel are close together and lined with a smooth, stout, 
chitinous membrane, but the two main tracheal trunks are 
now connected by a spined intertracheal canal of communi- 
cation, e.xactly resembling the similar canal already described 
for the lungs. 
I have no other stages between this and the adult form, 
the chitinous skeleton of which is drawn in fig. 31, and may 
be readily derived from the post-embryonic stages just 
described. In fact the only essential difference between the 
adult form and that after the second moult consists in the 
])resence in the former of a large number of fine tracheal 
tubules or secondary branchlets, which spring from the main 
trunks either singly or in clusters, particularly from the ends 
of the tendinal trunks and of the two branches of the lateral 
trunks. 
The entochoudrite {f. 9) of the earlier stages is now found 
attached to the apex of a main tendinal trunk (w. tr.), which 
is not continued beyond the entochondrite in this s])ecies 
except in the form of a bunch of fine tubules. 
In Attus, therefore, the two main tracheal trunks 
[m. tr., fig. 31) are serially homologous with the pair 
of entapophyses or ectodei'inal tendons (ec. f. 8, fig. 
20), to which the entochoudri tes of the ventral 
longitudinal muscles of the pulmonary and tracheal 
somites are attached, and are actually homologous 
w i t h t h e c o r r e s p o n d i n g e n t a p o p h y s e s of the second 
]) ulmonary segment of Tet ra p n e n m onous spiders. 
The lateral basal lobes (/. fr., fig. 31) of the ti achem 
are directly derived from the lateral proliferation 
of the earlier stages, and are serially homologous 
with the pulmonary sacs of the previous segment, 
and are to be considered as actually homologous 
with the pulmonary sacs of the second pair of lung- 
books o f T e t r a ]■) n e u m o n o u s spiders. 
