34 INVERTEBRATE ZOOLOGY. 



Order 2. Xiphosiira (Limuloirlea) Mostly fossil, with three surviving 



species — Abdomen consolidated in recent 

 forms and bearing six plate-like append- 

 ages, of which the last five bear lamel- 

 late gills. A long terminal spine at end 

 of abdomen. The larvae pass through 

 a ' • trilobite " stage, showing three well 

 marked longitudinal areas— and without 

 the terminal spine. -^ Limnlns. 



Sub-class II. Arachnoidea Tracheata. ..Mostly terrestrial forms— breath- 

 ing either by gill plates hanging from the 

 roof of pneumatic chambers, or by tra- 

 cheal tubes. Both sorts communicate 

 with the exterior by paired stigmata sit- 

 uated on the ventral side of the abdomen. 

 1-6 i>air of eyes — generally simple. First 

 two pairs of appendages employed as 

 mouth iiarts, often chelate. Last four 

 serve as legs. 



Order 1. Scorpionidea Cephalo-thorax of one piece, broad prae- 



abdomen of seven segments and narrow, 

 elongated post-abdomen of five, ending 

 in a poisonous spine. Mandibles chelate. 

 Second pair of appendages enormously 

 developed and chelate. Four pairs of 

 respiratory chambers. -^ Scorpio. 



Order 2. Pseudoscorpionidea Cephalo-thorax of one piece. Abdomen 



11-jointed, broad and flat, without atten- 

 uated portion or sting. First and second 

 pairs of appendages as in Scorpionidea. 

 Breath by tracheal tubes, which open by 

 two pairs of stigmata on second and 

 third abdominal segments. Spinnerets 

 on the second abdominal segment. Small 

 animals found under bark, etc. 

 if Chelifer. 



