164 GENERAL PRINCIPLES OF ZOOLOGY, 



body is made up, not only of symmetrical parts, the anti- 

 meres, but also of similar parts placed one behind the 

 other, the mctaincrcs. This leads us to the consideration 

 of metamerism or segmentation. 



Internal and External Metamerism. Metamerism 

 or segmentation is spoken of when the body of an animal 

 consists of numerous segments or metameres (consult 

 Fig. 56). Very often it is recognizable externally when, 

 for instance, the limits of the segments are marked on the 

 surface by constrictions (artJiropods and annelids}. But 

 this "external metamerism" may be entirely lacking, and 

 the metamerism find expression only internally in the serial 

 succession of organs, in metamcric or scgmental arrange- 

 ment. Man, for example, is segmented only internally ; 

 in his skeleton there are numerous similar pieces, the 

 vertebrae, which follow one another in the long axis. In 

 fish the musculature also is made up of numerous muscle 

 segments, as any one can readily see by examining a cooked 

 fish. In the case of the externally segmented earthworm 

 also, the ganglia of the nervous system, the vascular 

 pulsating " hearts," the kidney tubules or segmental organs, 

 the setae, and the dissepiments of the body-cavity are re- 

 peated in each segment. 



Homonomous and Heteronomous Metamerism. 

 The examples mentioned are well adapted for elucidating the 

 character of the different forms of metamerism, the homon- 

 omous and the heteronomous metamerism. The earthworm 

 is homonomously metameric, because the simple segments 

 are much alike in structure, and only slight differences 

 exist between the anterior, the posterior, and the genital 

 segments. Man and all vertebrates, on the contrary, are 

 heteronomously metameric, because the successive seg- 

 ments, in spite of many points of agreement with one an- 

 other, have become very unlike. The segments of the 

 head have an importance, for the organism as a whole, quite 

 different from those of the neck, the thorax, or the tail. 

 A division of labor has taken place among the segments of 

 an heteronomous animal. 



