RESPIRATORY ORGANS IN AUANE.E. 
17 
provisional appendage {ah. app. 1-4) in successive stages of 
growth, that of the eleventh segment being the smallest and 
that of the eighth the largest. 
The segments are marked off from each other by distinct 
transverse grooves, which are shallow, except immediately 
behind the appendages, where they are considerably deepened 
(f/r.), and where the ectoderm forms a distinct post-appen- 
dicular fold, projecting at right angles, or nearly so, to 
the general surface into the body. The posterior wall of this 
fold is comparatively thin, like the adjacent epithelium of 
the following segment, but the anterior wall is much thicker, 
being, in fact, a direct continuation and a part of the posterior 
wall of an appendage, as I shall presently show. 
A similar post-appendicular infolding (as distinct from the 
pulmonary .sac to be described later) appears to be also found 
in Limulus (Kingsley, ’85). In the older spider-embryo 
those of the posterior pairs of appendages serve as places 
of attachment for the ventral longitudinal muscles of the 
abdomen. 
The deep infoldings behind the first pair of abdominal 
appendages extend from the medial end of the hind margin 
of each a])pendage nearly, but not quite, up to the extreme 
lateral end, and, moreover, the lateral part of the infolding 
{t/r., fig. 7a) is always .slightl}', but distinctly, deeper than its 
medial part hg. 7). I’liese two figures represent the 
appendages just before the earliest appearance of the rudi- 
ments of the lung-books. 
V. The Deveeoi'Siext ok the Lung-Kooks. 
Stage with two pulmonary furrows (stage 2, SL 2). — 'I’he 
appendages of the pulmonary or eighth post-oral segment 
undergo considerable changes in passing from the stage just 
described to the next one, which I shall term the “stage with 
two pulmonary furrows.” Fig. 1 is a transverse section of 
this stage, and shows that the appendages are still near 
together, although the reversion has commenced. This stage 
VOL. 54, PART 1. — NEW SERIES. 2 
