336 ARACHNIDA— ARANEAE CHAP. 
the orifices on this sieve-lhke plate lead to a large number of 
small glands, the “ cribellum glands.” 
Respiratory Organs.—Spiders possess two kinds of breath- 
ing organs, very different in form, though essentially much 
alike. They are called respectively “ lung-books ” and “ tracheae.” 
The Theraphosae (and Hypochilus) have four lung-books, while all 
other spiders, except Mops, have two. Tracheae appear ‘to be 
present almost universally, but they have not been found in the 
Pholeidae. 
The pulmonary stigmata lead into chambers which extend 
forwards, and which are practically filled with horizontal shelves, 
so to speak, attached at the front and sides, but having their 
posterior edges free. These shelves are the leaves of the lung- 
book. Each leaf is hollow, and its cavity is continuous, anteriorly 
and laterally, with the blood-sinus into which the blood from the 
various parts of the Spider's body is poured. 
The minute structure of the leaf is curious. Its under sur- 
face is covered with smooth chitin, but from its upper surface 
rise vast numbers of minute chitinous points whose summits are 
connected to form a kind of trellis-work. The roof and floor of 
the flattened chamber within are connected at intervals by 
columns. The pulmonary chamber usually contains from fifteen 
to twenty of these leaves, and the two chambers are always 
connected internally between the stigmata. 
The tracheae are either two or four (Dysderidae, Oonopidae, 
Filistatidae) in number, and their stigmata may be separate or 
fused in the middle line. Each consists of a large trunk, pro- 
jecting forwards, and giving off tufts of small tubes which lose 
themselves among the organs of the abdomen, but do not ramify. 
In the tracheae of Argyroneta’ a lateral tuft is given off im- 
mediately after leaving the stigma, and another tuft proceeds 
from the anterior end. Histologically the main trunk of the 
trachea is precisely like the general chamber of the pulmonary 
sac, and differs greatly from the trachea of an insect. 
Cephalothoracic Glands.—In addition to the generative 
glands and the so-called “ liver” which occupy so large a portion 
of the abdomen, there are, in Spiders, certain glandular organs 
situated in the cephalothorax which call for some notice, These 
are the coxal glands and the poison-glands. 
1 M‘Leod, Bull. Ac. Belg. (3), iii., 1882, p. 779. 
