PHYLUM V. 



ARTHROPOD A. (Crustaceans, Arachnids, Myriapods, 



AND Insects.) 



Animals which are externally segmented and have segmented ex- 

 tremities (Fig. 542, A). 



External Structure.— The segmentation of arthropods is heterono- 

 mous, the somites or body segments being unequal in size, and in most 

 cases the body is made up of three divisions, the head, the thorax, and 

 the abdomen. Fusion occurs very frequently between contiguous somites 

 so that their boundaries are obliterated: the somites of the head are 

 always thus fused. The appendages or extremities are elongated, seg- 

 mented projections of the ventral body wall, there being typically a 

 single pair on each somite except on the terminal one. In many 

 cases where the segmentation has disappeared secondarily the number 

 of pairs of appendages gives a clue to the number of somites originally 

 present. 



The appendages are primarily locomotoiy and sensory organs, but 

 we find them performing many other functions. The first pair forms 

 the antennae or feelers, where these are present, and their function is 

 usually purely sensory. One or more pairs form the jaws, which have 

 thus a right and left position. The appendages of the middle and hinder 

 part of the body usually preserve their locomotory function and form 

 the walking or swimming legs. These, however, often serve also other 

 purposes, as for respiration and the transportation of eggs or young, or 

 as spinnerets in spiders, and as stings, anal feelers, and ovipositors 

 among the insects. 



The following scheme taken from Korschelt & Heider's Textbook of 

 Embryology illustrates the homologies of the anterior pairs of appendages 

 in the principal groups of Arthropoda: 



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