1 08 Invertebrata. 



c. With a short up-turned abdomen : 



Crabs = Sub-order III. Brachyura. 

 B. Some thoracic limbs ambulatory, thus mak- 

 ing twelve, fourteen, or sixteen pairs of 

 walking limbs = Order II. Stomapoda. 

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

 sessile eyes = Sub-class I. 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 with a variable 

 number of rings. 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 

 antennae developed as such, but the mandibles are 

 always present, and, in scorpions, the maxillary palps 

 form pincers or claws like those of a crab ; such claws 

 are called chelicera. The second pair of maxillary 

 palps form the first pair of walking limbs, while the 

 first and second pairs of thoracic limbs, as seen in the 

 true insects, are developed as the second and third 

 pairs of legs, and the third pair of thoracic limbs is 

 absent ; at the base of the abdomen is a curious pair 



