138 GENERAL PRINCIPLES OF ZOOLOGY. 



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 the different forms, the 

 homonomous and the Jieteronomous, of metamerism. The earth- 

 worm is homonomously metameric, because the single 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 segments, in spite of many points of agree- 

 ment with one another, 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 heter- 

 onomy and homonomy is of great physiological interest. The 

 more different the segments of an animal become the more 

 dependent they are upon one another in order to be able to func- 

 tion normally; so much has the whole become unified that the 

 single parts can live only while the continuity is maintained. On 

 the contrary, if the connexion between the parts be less intimate, 

 they are more similar, and the more able to exist after separation 

 from one another. This is most beautifully shown in instances of 

 mutilation. It has been observed that when many species of 

 Lumbricidae are cut in two each part not only lives on by itself, 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. 

 In metamerism a phenomenon is repeated which obtains widely in 

 the animal kingdom? and contributes towards its higher develop- 

 ment; first there is a reduplication of parts, then a division of 

 labor, so that the final result is a whole composed of many parts, 

 yet uniformly organized. 



