368 Introduction to Animal Morphology. 



protected by circles of hairs, and sometimes have 

 valves moved by muscles. The tracheae branch and 

 end in a network around the viscera ; sometimes large 

 branches coalesce, making longitudinal or transverse 

 air channels through the whole body. In Arachnida 

 the tracheae are usually incomplete, and only in limited 

 regions of the body. Their branches are usually fas- 

 ciculate. Trombidium has one pair, as has Phalan- 

 gium, in which they are freely branched. In Galeodes 

 there are three pair. In the short dilated tracheae of 

 some, the lining membrane is dilated into many (60- 

 100 in Scorpions) book-leaf-like plates, whose inter- 

 spaces receive air from the stigmata ; these are called 

 tracheal-lungs, but they are only closely compressed 

 tracheae with the lumina of their tubes flattened. Many 

 intervening gradational forms exist. Of these, one or 

 two (Mygale) pair i-xist in Spiders; two pair in Phry- 

 nid;i> ; four pair in Scorpions, in the four anterior ab- 

 dominal segments. In others a pair of tracheae, with 

 no spiral thread nor branches, co-exist. 



Beside the poison gland of the antennary jaws, 

 and the lateral glands of Adenopleura, there is a pair 

 of tail glands in the Scorpion, opening at the base of 

 the tail claw. The spinning gland of the spider con- 

 sists of several pair of abdominal glands, opening on 

 the flat spinning field, or an eminence below the anus, 

 by ducts which can be opened or closed at will, these 

 secrete a material which, when extended and exposed 

 to the air, becomes chitinoid. There may be two 

 Mygale), three (most genera), four(Drassus,Clubione) 

 pair of spinning warts, on each of which many (400 in 

 Tegenaria, 100 in Segestria, 1000 in Epeira) ducts 

 open, hence each thread consists of hundreds of 



