KESPIBATORY OEGANS IN AEANEiE. 
53 
eiitapopliyses (ec. t. 8, fig. 36) were similar in both pulmonary 
segments. 
The only other order possessing* the interpulmonary folds 
is the Pedi palpi, in which these folds are very well deve- 
loped in both pulmonary segments and much resembles 
that of Dipneumonous spiders (see Tarnani, ’89 and :04, and 
Piirner, ;04). 
VIII. ’1’he Development of the Tkachea: and the Entapo- 
PHYSES OF THE TkaCHEAL SEGMENT. 
'I'he tracheal appendages are, as nearly as possible, the 
exact counterparts of those of the pulmonary segment in the 
earliest stages, up to, say, the period when the pulmonary 
furi-ows begin to appear (compare ah. aj>p. 1 and 2 in fig. 4). 
'I'lie post-appendicular groove {(jr.) extends along the whole 
posterior side of the appendage (except, perhaps, as in 
ajjpendage 1, at the extreme lateral part), but it does not 
appear to be deeper laterally than medially. 
In the stage with two pulmonary furrows (figs. 1, 5 and 
5a), however, after the simultaneous subsidence of the epithe- 
lium lying between consecutive abdominal appendages we 
find that the post-appendicular groove is not almost obliterated 
in its medial half {tr. *■., fig. 5), differing in this respect from 
the corresponding groove of the pulmonary segment [yr., fig. 
5). On the contrary the infolding containing the groove has 
increased in depth along its whole extent, and continues to 
deepen in the following stages in such a way that its blind 
bottom is directed slantingly forwards {fr. .v., fig. 6a). This 
in-folding is the tracheal sac. 
If we examine a reconstruction of the appendage from the 
inner surface (fig. 27) at this stage (when about five pulmonary 
furrows are present and the mesodermal entochondrites 
begin to be formed), we find a broad transverse ridge [tr. s.) 
projecting into the body and nearly co-extensive with the 
posterior side of the base of the appendage. This ridge is 
the ectodermal in-folding which forms the tracheal sac. The 
