. ARACHNIDA. 633 



In most of the spiders (such as Segestria, Dysdera and Ar- 

 gyroneta) there are both a tracheary system and hmgs. The 

 two stigmata, from which these tracheae lead, open near the 

 puhnonary opening. In two other genera, Salticus and Micro* 

 phantes, tliere are two stigmata situated at tlie posterior end of 

 the abdomen. Siebold calls attention to a tracheary system 

 in many Araneaj opening by a transverse fissure placed neai 

 the spinnerets. From this opening a main trunk leads in, 

 soon dividing into four simple tracheae, which are not round as 

 usual, "but are flattened, riband-like, and without the trace 

 of a spiral filament ; these extend, with a gradual attenuation, 

 to the base of the abdomen. . . . The air received into these 

 organs is separated into as fine portions as that of the lungs.* 



The so called lungs of the spiders are little round sacs open- 

 ing by transverse fissures on the nnder side of the base of the 

 abdomen. The inner surface is divided into thin lamelloe, 

 connected together like the leaves of a book. Each of tliese 

 is formed by a membranous fold, between the two leaves of 

 which the air enters from the general cavity of the lung, and 

 is divided into very minute portions. No traces of blood ves- 

 sels have been found in these pulmonary lamelhB." (Siebold.) 



Among the organs of special secretion the poison and silk 

 glands require description. There are two poison glands 

 emptying into the throat, and thence opening out through hol- 

 lows in the jaws. (Plate 10, fig. 3, a, h.) In the scorpion the 

 poison gland is lodged in the last abdominal segment at the 

 base of the sting. 



The silk, as contained in the glands, is a viscid transparent 

 fluid, which on exposure to tlie air hardens into silk ; it is drawn 

 out by the legs through three, rarely two pairs of spinnerets. 

 There are usually five of these glands lodged in the abdomen, 

 and the "threads probably have diflferent qualities, according 

 to the glands from Avhich they are secreted." (Siebold.) 



"To form the thread this licpjid is drawn through the tubes^ 



♦According to Dr. Burnett, Bhincliard regards these anomalous tracheae as 

 only elongated pulmonary sacs. Leuckart, however, considers tliat these organs 

 are only a sort of trachea? deprived of tlie usual si)iral lilament to keep their walls 

 from collapsing, and he considers that the pulmonary sacs of the spider are sim 

 ply modified trachea;. — Dr. JF. I. Burnett's Translation of Siebolds's Anatomy of 

 the Invertebrata, 



