//. TYPES IN DETAIL IVITH THEIR SUBORDINATE GROUPS. 3^ 



species, the " horse-shoe crabs." 

 These have a shortened abdomen, 

 consolidated into a single piece, 

 and bearing six plate-like append- 

 ages, of which all but the first bear 

 gills. The larvae pass through a 

 so-called "trilobite" stage, in 

 which they greatly resemble the 

 extinct group of Trilobitea. V. su- 

 pra, p. 26. This circumstance sug- 

 gests a possible relationship be- 

 tween these groups. x Limulus. 

 Sub-class II. ARACHHOIDEA TRACHEATA . . Mostly terrestrial, breathing 



either by gill-plates enclosed in 

 pneumatic chambers, the so-called 

 ' ' lungs, "or by tracheal tubes re- 

 sembling those of insects. First 

 two pairs of appendages serve as 

 mouth parts, and are often chelate. 

 Last four pairs serve as legs. 



Order i. Scorpionidea Elongated forms, with a broad 



prae-abdomen of seven somites and 

 a narrow tapering post-abdomen 

 or " tail " composed of six somites 

 and ending in a poisonous spine. 

 Second pair of appendages enor- 

 mously developed and chelate. 

 Four pairs of "lungs." 



X Scorpio. 



Order 2. Pseudoscorpionidea. .Tiny animals found under bark (and 



sometimes in houses) resembling 

 miniature scorpions, but without 

 the tail. They breathe by tracheal 

 tubes. ' X Chelifer. 



Order 3. Phalangida Entire body a shortened oval. 



Reduced abdomen closely applied 

 to cephalo-thorax, but distinct and 



