ARTHROPODA 47 



pods probably arose and have become differentiated by 

 specializing along particular lines. This does not mean 

 that modern arthropods have necessarily become better 

 than their remote ancestors. Specialization, in zoology, 

 means that an animal is far from primitive ancestral con- 

 ditions. Some specializations have been along progres- 

 sive lines, others have made arthropods simpler in struc- 

 ture. Modifications have not taken place hit or miss in 

 any direction, but have followed rather definite lines. The 

 metameres of the body have been grouped into three 

 regions the head, thorax, and abdomen. In some cases 

 the head and thorax have again fused so that there are 

 only two regions the cephalo thorax and abdomen. The 

 appendages have been greatly modified in many instances 

 one branch has been lost, so that an uniramous condition 

 has come about; often the segments are lost, combined, or 

 greatly changed in shape; sometimes the appendages are 

 wholly absent from the abdomen. In most arthropods 

 the ancestral number of mouth parts has been found to be 

 insufficient and the walking legs just behind have been 

 pressed into service. In one group five pairs of thoracic 

 legs have been turned forward to help the mouth and only 

 three are left for walking. The nervous system has shown 

 a tendency to fusion in some arthropods the ventral 

 ganglia are no longer distinct but have fused into one great 

 mass of nervous tissue. The sense organs (eyes, ears, etc.) 

 have become very complicated in some cases. The kidneys 

 seldom retained the primitive metameric arrangement but 

 have usually been restricted to a single segment of the 

 body, or wholly lost and replaced by a new type of excretory 

 organ which opens into the intestine. The arthropods \ 

 have, then, become specialized by losing parts of the body, ) 

 by adding new organs, by condensation and combination, / 

 and in other ways but all have retained the same general / 

 fundamental plan of structure, which shows that they are/ 

 related. ^ 



As the arthropod line developed, it early split into two 



