Elementary Zoology. 



spiders. The head is fused with the thorax to form a cephalo- 

 thorax. There are four pairs of ambulatory limbs. 



Class I. Xiphoswa. — This class contains the king crabs 

 (Limulus), by some placed with the 

 Crustacea, as a group Gigantostraca. 

 There are really five pairs of ambulatory 

 limbs. The abdominal appendages bear 

 the gills, which are composed of a large 

 number of thin plates, like the leaves of 

 a book. Allied to the king crab are the 

 extinct Trilobites. 



Class 2. Scorpionida. — The scorpions 

 have also five pairs of walking limbs; 

 the abdominal appendages of Limtdus 

 are wanting, but instead there are invagi- 

 nated lung-books. 

 Class 3. Pseudoscopionida. — These are minute creatures, 

 with large claws like a scorpion, but breathing by means of 



Fig. 91. — Obisiuvt irotnbidio- 

 ides, (From Claus-Sedg- 

 wiclc's '■ Zoology.") 

 Magnified. 



Fig. 92. — Phryttus reniformis, (From same source.) 

 Ktt pedipalpi ; Gb^ flagelliform anterior leg. 



tracheae. The little-book scorpion (Fig. 91), is a common 

 example of this class. 



