CHAPTER 16 



OJI 



Sfr* ^ir* 



Phylum Arthropoda. 



Crayfish, Crabs, 



Barnacles, Water 



Fleas, Sow Bugs, 



and Others 



HE arthropods are joint-footed animals. 

 To this phylum belong the lobsters, crabs, 

 water fleas, barnacles, millipedes, centi- 

 pedes, scorpions, spiders, mites, and insects 

 (Fig. 109). An arthropod is bilaterally 

 symmetrical, and consists of a longitudinal 

 series of segments; on all or some is a pair 

 of appendages. An animal of this phylum is 

 covered with a hardened exoskeleton, con- 

 taining chitin which is flexible at intervals 

 to provide movable joints. It possesses a 

 nervous system of the annelid type and has 

 a coelom which is small or absent in the 

 adult; the body cavity is a hemocoel filled 

 with blood. 



The arthropods comprise about 78 per 

 cent of all known species of animals (Fig. 

 1). They are the dominant animals on the 

 earth, if numbers of different species are 

 accepted as criteria of dominance. The va- 

 riety of the multitudes of arthropods seems 

 infinite, but the fundamental plan of struc- 

 ture is the same. The common cravfish ex- 

 hibits to excellent advantage the character- 

 istics of the class Crustacea as well as of 

 arthropods in general. The segmented ap- 

 pendages of the crayfish are particularly 

 interesting since they seem to have devel- 

 oped from a common type but have become 

 greatly modified for the performance of 

 various functions. Many arthropods, includ- 

 ing the crayfish, possess compound eyes— a 

 type of visual organ very different from 

 those of other invertebrates and vertebrates. 

 Other biological phenomena exhibited by 

 the crayfish and worthy of special mention 

 are the power of regeneration, autotomy, 

 habit formation, and superficial cleavage of 

 the fertilized egg. Many other Crustacea are 

 of great biological interest and of economic 

 importance. 



CAMBARUS-A CRAYFISH 



The crayfish (crawfish) is found in fresh- 

 water lakes, streams, ponds, and swamps 

 over most of the world. The genus Cam- 

 barus is common in the central and eastern 



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