126 GENERAL ERINCIPLES OE ZOOLOGY 



Antimeres and Metameres. — The s\mmclrical parts of an animal 

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

 adfacent 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 beliind the other, the i)icla))icirs. 



Metamerism or scginciitalion is spoken of when the body consists of 

 numerous segments or metameres (consult fig. 60). A'ery often it is 

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

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

 But tliis cxlcnial mdaincrism may be entirely lacking, and the metamerism 

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

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

 ous similar parts, the vertelinc, wlrich 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 tlie case of 

 the externally segmented earthworm also, the ganglia of the nervous 

 system, the vascular arches, the nephridia or segmental organs, the seta', 

 and the septa of the body cavity are repeated metamericallv. 



Homonomous and Heteronomous Metamerism. — The examples 

 mentioned are well adapted for illustrating Iioiiioiioiiioiis and liclcroiiomoiis 

 metamerism. The earthworm is homonomously metameric, because the 

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

 exist between them. jNIan and all vertebrates, on the contrary, are helero- 

 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 dift'erent 

 from those of the neck, the thorax, or the tail. A tlivision of labor has 

 taken place among the segments of an heteronomous animal. 



Heteronomy and Homonomy. — The distimiion between hcteronomy 

 and homonomy is of great physiological interest. Tlie 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 Lunibricidie are cut in two cacli part not only lives, but 

 it even regenerates the jxirt which is lacking; if, cm the nlhcr hand, the same 

 thing is done to a hcteronomously 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. \\'hile Crustacea, amphibia and reptiles 



