AN INTRODUCTION TO THE METAZOA 91 



is true of the vertebrates, which have the vertebrae of the back- 

 bone, the ribs, and nerves metamerically arranged. The earth- 

 worm (Fig. 154) is a good illustration of both external and in- 

 ternal metamerism; the body consists of a great number of 

 similar segments, and the ganglia of the nerve cord, the cham- 

 bers of the body cavity and the excretory organs are segmentally 

 arranged. 



The earthworm may serve also as an example of an animal 

 with homonomous segmentation, since the metameres are similar. 

 The crayfish (Fig. 202), on the other hand, is a heteronomous 

 animal, since division of labor has resulted in the dissimilarity 

 of the metameres of different regions of the body. The verte- 

 brates, including man, are all heteronomous. 



Appendages. — - The external appendages of animals are out- 

 growths of the body, which are used for locomotion, obtaining 

 food, protection, respiration, and many other purposes. They 

 are greatly modified for their various functions, and these 

 modifications furnish excellent material for the study of homolo- 

 gous and analogous organs. For example, the fins of fishes, the 

 wings of birds, and the arms of man serve to distinguish their 

 bearers from one another; nevertheless, these structures are 

 homologous, since they are morphologically equivalent. 



