I o 8 /;/ vertebra ta. 



c. With a short up-turned post-abdomen : 



Crabs = Sub-order III. Brachyura. 

 B. Some thoracic limbs ambulatory, thus mak- 

 ing twelve, fourteen, or sixteen walking 

 limbs=Order II. Stomapoda. 



G. Free, with a cephalo-thorax, twenty segments and 

 sessile eyes = Sub-class VII. Edriophthalmia. 



CHAPTER XX. 



SPIDERS AND MITES. 



CLASS II. Arachnoidea. These are terrestrial air- 

 breathing creatures in which the segments that com- 

 pose the head and thorax are united to form a single 

 cephalo-thorax, but their articulated limbs are to some 

 extent represented, and of these, four pairs are usually 

 used in walking. There is an abdomen and very rarely 

 a post-abdomen. Whenever eyes are present they are 

 not compound bundles of crystal rods covered by a 

 common cornea, as in crustaceans, but they consist of 

 separate transparent cones surrounded with pigment 

 and always few in number. There are never any 

 mandibles developed as such, but the mandibular 

 palps are often present, and sometimes, as in scor- ' 

 pions, they form pincers or claws like those of a crab. 

 The antennae are modified into jaws, called chelicera ; 

 the two pairs of maxillary palps form the first 

 and second pairs of walking limbs, while the first and 

 second pairs of thoracic limbs are developed as the 

 third and fourth pairs of legs, and the third pair of 

 thoracic limbs is absent, unless a curious pair of 



