12 
W. F. PUKCEM,, 
(2) A short, flattened cdiamber (vestibule, rest.), leading 
lorwards and upwards from the spiracle into tlie body and 
giving off at its anterior or deepest part — 
(3) A pair of medial [vi.tr.) and a pair of lateral 
tracheal trunks [l.tr.), which may again give rise to 
tracheal branches (tv., fig. 21), the finest of these being the 
tracheal tubules [tr.tnh., fig. 31). The trunks and 
branches are lined with anastomosing spines (more rarely 
with spiral threads oidy), but the fine tubules have only 
spiral threads. 
'I'he anterior or deepest part of the cavity of the vestibule 
is always widened to form a transverse canal of communi- 
cation [can.) or passage with hooped spines, connecting the 
cavities of the tracheal trunks. The remaining or smooth 
portion of the vestibule forms a stalk or pedicel [yed.) to 
the whole tracheal system, and is supported cu each side by 
chitinous thickening or rod [rd.). 
111. lllSTOKICAL (DeVELOPMKNt). 
Development of the lung-books in Arachnida. — Metschnikoff 
(’71) gives an account of the development of the lung-books 
in scorpions, and observes that they arise as ectodermal 
invaginations just behind the four posterior pairs of abdo- 
minal ajipendages, which latter subsequently atrophy. Towards 
the end of the embryonic period the folds in the pulmonary 
sacs appear. 
Salensky (’71) was the first to study their development in 
Aianeas, and believed that the lung-books were formed 
by the invagination of the abdominal appendages (teste 
Jaworowski, ’94, p. 55). 
Bertkau (’72) showed that in the young spider, after the 
completion of the embryonic period, the lung-books continue 
to develop, new leaves being added at the growing dorso- 
lateral end, each new leaflet arising next to the one previously 
formed. 
Locy (’86) gives a detailed description of the later stages 
