KEY TO CLASSES AND ORDERS OF ANIMALS 27 



15(16, 17) Animals marine; legs long and slender. Animals not 

 microscopic. (Pratt, 458.) Order Pycnogonida 



16(15, 17) Animals marine or fresh water, always microscopic. 

 (Pratt, 457.) Order Tardigradi 



17(15, 16) Animals usually terrestrial, may be in fresh water. 18 



18(19) Abdomen joined to the thorax by a short narrow 

 stalk. The spiders. (Pratt, 413; Comstock, 39.) 



Order Araneida 



19(18) x\bdomen fused to the cephalo thorax. The ticks and 

 mites. (Pratt, 436; Comstock, 81.) 



Order Acarina 



Class MYRIAPODA 



1(2) Nearly all the visible segments of the trunk with two 

 pairs of appendages each. Antennae usually of seven 

 segments. The common millipeds. (Pratt, 467.) 



Order Chilognatha or Diplopoda 



2(1) Not more than one pair of appendages on each visible 

 segment. 3 



3(6) More than twelve pairs of legs; the first pairs of trunk 

 appendages transformed into maxillipeds and bearing 

 poison claws. 4 



4(5) Dorsal plates corresponding in number to the segments 

 and pairs of legs, body elongated. Tarsi not divided 

 into a number of small segments. The common centi- 

 pedes. (Pratt, 473.) Order Chilopoda 



5(4) Eight dorsal plates and fifteen pairs of very long legs. 

 Body shorter than antennae and posterior pair of 

 legs. Tarsi divided into a number of short segments. 

 (Pratt; 477-) Order Schizotarsia 



6(3) Not more than twelve pairs of legs. 7 



