AEACHNIDA. 



/.'. Postanal sclerite absent ; posterior 

 abdominal somites not narrowed to 

 form a movable stalk or tail. 

 a'. A deep constriction separating the 

 cephalothorax and abdomen, these 

 regions united by a narrow waist 

 or pedicel ; a pair of breathing- 

 organs, almost invariably in the 

 form of lung-sacs, forming a trans- 

 verse line with the median genera- 

 tive aperture * ; sternal area of 

 cephalothorax large, usually sub- 

 circular. 

 a 3 . Appendages of 2ud pair spiny, 



prehensile and subchelate, unlike 



the legs ; those of 3rd pair (1st 



pair of legs) very long, antenni- 



form, distally many - jointed : 



abdomen distinctly segmented, 



without spinning mainillse ; no 



poison-gland in mandible AMBLYPYGI. 



b z . Appendages of 2nd pair pedi- 



form, not prehensile and not 



subchelate, modified in male as 



reproductive organ : appendages 



of 3rd pair like the following 



pairs and constituting the first 



pair of walking-legs : abdomen 



usually unsegmented. always 



furnished with appendages in the 



formofspinningmamilke; poisou- 



gland in mandible ANANE^J. 



/>\ No deep constriction forming a 

 narrow waist between cephalo- 

 thorax and abdomen ; breathing- 

 organs always in the form of 

 tubular tracheae, the first pair on 

 the abdomen, never situated in a 

 transverse line with the generative 

 orifice; coxa; of cephalo-thoracic 

 limbs in contact or separated 

 by a forward prolongation of the 

 anterior abdominal sternite bearing 

 the generative orifice. 

 a*. Body elongate; abdomen consist- 

 ing of from 10-12 somites ; 



generative aperture situated well 



behind the coxa) of the last pai:- 



* Except in the spiders of the genus Tetragnatha. 



