126 GENERAL PRINCIPLES OF ZOOLOGY 



Antimeres and Metameres. The symmetrical parts of an animal 

 are called antimcrcs; each antimere has organs which occur likewise in its 

 adjacent antimere. The right arm of man is the antimere of the left, 

 the right eye of the left, etc. Frequently there is also a repetition of 

 organs in the direction of the long axis. Thus the body is made up not 

 only of symmetrical parts, the antimeres, but also of similar parts placed 

 one behind the other, the metameres. 



Metamerism or segmentation is spoken of when the body consists of 

 numerous segments or metameres (consult fig. 60). Very often it is 

 recognizable externally when, for instance, the limits of the segments 

 are marked on the surface by constrictions (arthropods and annelids). 

 But this external metamerism may be entirely lacking, and the metamerism 

 find expression only internally in the serial succession of organs. Man, 

 for example, is segmented only internally; in his skeleton there are numer- 

 ous similar parts, the vertebras, which follow one another in the long axis. 

 In fishes 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 arches, the nephridia or segmental organs, the setae, 

 and the septa of the body cavity are repeated metamerically. 



Homonomous and Heteronomous Metamerism. The examples 

 mentioned are well adapted for illustrating homonomous and heteronomous 

 metamerism. The earthworm is homonomously metameric, because the 

 single segments are much alike in structure, and only slight differences 

 exist between them. Man and all vertebrates, on the contrary, are hetero- 

 nomously metameric, because the successive segments, in spite of many 

 points of agreement, 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. 



Heteronomy and Homonomy. The distinction between heteronomy 

 and homonomy is of great physiological interest. 1 he more different the 

 segments of an animal become the more dependent they are upon each other; 

 so much has the whole become unified that the single parts can live only while 

 the continuity is maintained. On the contrary, if the connection between the 

 parts be less intimate, they are more similar, and the more able to exist after 

 separation from one another. This is well shown in instances of mutilation. 

 When many species of Lumbricicke are cut in two each part not only lives, but 

 it even regenerates the part which is lacking; if, on the other hand, the same 

 thing is done to a heteronomously segmented animal, either death immediately 

 ensues, as in the case of the higher vertebrates, or the parts live for a short time a 

 hopeless existence, as can be seen in the case of frogs, snakes, insects, etc. There 

 is always a certain capacity for regeneration, which is the more restricted, 

 the more complete the organization. While Crustacea, amphibia and reptiles 



