CHAPTER XX. . 

 PHYLUM XI: ARTHROPODA. 



250. General Survey. The crayfish and grasshopper 

 belong to this group, and illustrate two of the four prin- 

 cipal divisions. The crayfish belongs to the Crustacea, 

 and the grasshopper to the Insecta. In addition to 

 these are the Arachnida or spiders, and the Myriapoda 

 which includes centipeds and other types with legs along 

 the entire body. 



The branch of arthropods embraces more than half the 

 species in the animal kingdom and many of these species 

 are very rich in individuals. They agree with annelids 

 in having a bilaterally symmetrical body made up of 

 segments, and a nervous system of numerous ganglia on 

 the ventral surface. The larvae (grubs and caterpillars) 

 are much like worms in appearance. They differ from 

 worms in the fact that the paired appendages which spring 

 from these segments are jointed. The epidermis of the 

 arthropods secretes a horn-like substance, which in some 

 cases becomes very thick and resistant, making an ex- 

 ternal skeleton for protection and the attachment of 

 muscles. If this skeleton remains thin at the joints the 

 animal is flexible; if not, the segments may be fused in 

 such a way as to become quite rigid, as in the crabs. In 

 some cases they are fused in the anterior part of the body 

 and flexible elsewhere. 



The great number of species referred to above points 

 to the fact that the arthropods are able to adjust them- 

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