EESPIBATORY ORGANS IN ARANEyE. 
35 
practically unclianged after the stage represented in fig. 17, 
we can disting-uish in the operculum of the adult spider (1) a 
nearly horizontal portion to which the ventral saccule is 
attached, and which belongs to the ventral surface of the 
abdomen, and (2) a strong-ly inclined portion on the lower 
part of the lateral surface of the abdomen. The horizontal 
part corresponds to the ventral wall of the embryonic appen- 
dage in hg. 3 {iv'.x'. in figs. 13c and 1 7), or the median wall of 
ail earlier stage (fig. 1), while the inclined portion, which forms 
much the greater part, is the distal and dorsal wall of the 
appendage, i . e. — the part x'.z'. in figs. 13c and 1 7, or the distal 
and lateral wall of an early stage (fig. 1). Anteriorl}’^ the 
operculum curves strongljr towards the median line, and this 
incurved part corresponds, of course, to the anterior wall of 
the embryonic appendage (fig. 18). All the surfaces pass over 
gradually into one another and cannot be sharply dis- 
tinguished. 
The lung-books of the young spider. — Not much remains to 
be added on the subsequent development. 
At the time of hatching the lung-book has much the appear- 
ance of fig. 18, except that the pulmonary .sac (now the ante- 
chamber) has much thinner walls, lined with chitin internally, 
and the dorsal saccules are long’er. Moreover, that portion of 
the epithelium of the pulmonary sac immediately adjoining 
the spiracle now forms a thin-walled, narrow, hollow neck or 
stalk (pedicel) connecting the ante-chamber proper with 
the edge of the spiracle. 
'I'his pedicel persists throughout all later stages, and its 
chitinous lining acts both as an air-passag’e to the ante- 
chamber and as a sort of ligament by means of which the 
lung-complex is firmly attached to the outer cuticula of the 
body. 
3’he dorsal horn of the ante-chamber preserves its charac- 
teristic curved form, and, as Bertkau (’72) showed long ago, 
continues to provide new lung-septa. According to W. 
Wagner (’88), the addition of new septa goes on until the age 
of sexual maturity is reached. In Attus floricola at the 
