646 ZOOLOGY SECT. 



ORDER 7. ACARIDA. 



Arachnida in which the body exhibits no division into regions. 

 The mouth-parts are adapted either for biting or piercing and 

 sucking. The organs of respiration, when present, are in the 

 form of tracheae. 



This order includes the Mites and Ticks (Figs. 556 and 557). 



ORDER 8. XIPHOSURA. 



Arachnida in which the body consists of a cephalothorax covered 

 over by a broad carapace, and an abdomen of seven firmly united 

 segments, with a long narrow tail-piece or telson. The cephalothorax 

 bears a pair of short chelate appendages and five pairs of legs. The 

 abdomen bears in front a pair of united plate-like appendages, 

 forming the operculum, followed by five pairs of flat appendages 

 overlapped by the operculum. The organs of respiration are lamelli- 

 fofm gills attached to the abdominal appendages. 



This order includes the King-crabs (Limulus, Figs. 558 and 

 559). 



ORDER 9. EURYPTERIDA. 



Arachnida with a relatively small cephalothorax, followed by 

 twelve free segments and a terminal, elongated, narrow telson. 

 There are a pair of pre-oral leg-like or chelate appendages and 

 four more leg-like appendages on the cephalothorax, the last ex- 

 panded to form swimming paddles. A broad operculum is situated 

 immediately behind the cephalothorax. There are pairs of lamellate 

 appendages on certain of the anterior free segments. The exo- 



skeleton is characteristically sculptured. 

 This order includes only a number of 



extinct (Palaeozoic) forms of large size 



(Fig. 560). 



3. GENERAL ORGANISATION. 



The external form in the Scorpionida 

 has already been sufficiently described. 

 Most nearly related to that order in this 

 respect are the Pseudoscorpionida or 

 Book-scorpions and their allies. In these 

 (Fig. 550) there is an unsegmented 

 cephalothorax, or the carapace is crossed 

 'ii by two transverse grooves which may 

 Lan8 s m( *i cate segmental divisions. There is a 

 broad abdomen consisting of eleven or 

 twelve segments ; the post-abdomen is not represented, nor the 

 caudal sting. The chelicerae are small ; the pedipalpi are large, and 

 resemble those of the Scorpions in their chelate form. Spinning 

 glands are present. 



