503 INVERTEBRATE MORPHOLOGY. 



1. Subclass Apterygota. 



Tlie members of this subclass are all small and do not 

 possess wings, the absence of these structures being a primi- 

 tive feature and not due to degeneration resulting from para- 

 sitism or other causes. In some forms rudiments of abdomi- 

 nal limbs are present in the adults, and there is no meta- 

 morphosis in the post-embryonic development (primary 

 ametabolism). 



1. Order Thysanura. 



The Thysanura or Bristle-tails possess ten a,bdominal 

 segments, the terminal one bearing two- or three-jointed hair- 

 like processes, whence the name applied to the order. The 

 body in some forms (Lepisma) is covered 

 \ / with scalelike hairs giving it a silvery-gray 



\ / appearance, but in other cases these are 



\ § wanting. The antennae vary in length, but 



^^ are always simple cylindrical structures, 



S(^^^^ the terminal joint in some forms {Campodeay 



^^|||L^ bearing a peculiar bilobed structure sup- 

 ^IIl posed to be sensory, and the mouth-parts 



^^^^^*^Bj^j^ are adapted for biting purposes and are 

 P^ usually well developed. The first abdom- 



^3; inal segment in some forms bears a pair of 



^^ indistinctly-jointed appendages, probably 



^^ rudimentary limbs, and a number of the 



ff % succeeding segments in Gampodea bear spur- 



K % like processes, also supposed to be limbs 



ff % and recalling the spurs of Scolopendrella, 



M % especially as protrusible glandular struc- 



^ *' tures, comparable perhaps to crural glands, 



^^^" ,~'\~ '^"^P" ^" occur in association with them in some 



siap/iyhnua (after 



Lubbock from Hux- lOrmS. 



''"^) The nervous system shows but little 



concentration, eight abdominal ganglia occurring in Lepisma, 

 and eyes are usually present, being in some cases compound. 

 The stigmata vary in number, being usually ten, though in 

 Gampodea they are reduced to three, and the tracheae in this 



