CHAPTER XIII 

 PHYLUM ARTHROPODA 



INTRODUCTION AND CLASS CRUSTACEA 



The arthropods are segmented animals distinguishable from all 

 others in that they bear paired, jointed appendages and have a 

 chitinous exoskeleton. This is by far the largest phylum in the 

 animal kingdom, for it contains more than three times as many 

 species as all the remaining phyla put together. In spite of the 

 immense numbers, the group is clearly recognizable as a natural 

 unit. 



In many respects, the arthropods represent the highest 

 development found in the non-chordate animals. In earlier 

 chapters, it has been shown that the bodies of the higher seg- 

 mented worms are composed of a linear repetition of similar 

 rings or somites. The somites of these worms often bear appen- 

 dages, called parapodia, but these in their highest development 

 are mere flaplike folds of the body wall and are never jointed. 

 The tendency toward speciahzation of appendages has been 

 carried much further in the arthropods, for here they have become 

 more highly organized and they are definitely articulated. 

 Metamerism of the body also finds higher expression in the 

 arthropods, for in most arthropods the segments of the body 

 have undergone greater specialization and more distinct regional 

 differentiation than is encountered in any annelids. Since the 

 segments of the worms show so little differentiation, the metamer- 

 ism of the worms is said to be homonomous, while that of the 

 arthropods is very distinctly heteronomous. In most instances 

 at least head and trunk regions are recognizable and in many 

 cases three body divisions are distinctly set off as head, thorax, 

 and abdomen. 



The body covering or integument of arthropods is composed 

 of a chitinous material overlying the hypodermis. To permit 

 movement of the body, joints occur in this otherwise unwieldy 

 exoskeleton. These joints are termed sutures, and skeletal 

 areas bounded by sutures are designated as sclerites. Inorganic 



203 



