336 ARACHNIDA — ARANEAE chap. 



the orifices on this sieve-like plate lead t(j 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 Ili/pochilus) have four lung-books, while all 

 other spiders, except JVoj^s, have two. Tracheae appear to be 

 present almost universally, but they have not been fovmd in the 

 Pholcidae. 



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. Belcj. (3), iii., 1882, p. 779. 



