6 VERTEBRATE ZOOLOGY 



METAMERISM 



The segmental organization, metameric structure, is secondary in 

 importance only to the primary axiate organization and is undoubt- 

 edly one of the developmental consequences of the latter. The ver- 

 tebrates constitute but one of three great metameric groups, the other 

 two being the annelids and the arthropods. Annelids, the lowest of 

 metameric types, show metamerism in its most generalized condition. 

 In them segmentation involves equally both internal and external 

 structures, and there is little or no obliteration of primary metamerism 

 by fusion of groups of contiguous metameres into larger regions. 

 In the arthropods the external metamerism to a large extent retains its 

 primitive condition, especially in regions back of the head or cephalo- 

 thojax, but the primitive internal metamerism is obscured through 

 the almost complete obliteration of the ccelom by the system of ve- 

 nous sinuses. 



Metamerism in the vertebrates, especially in the higher groups, 

 is confined largely to certain internal structures and is scarcely at all 

 expressed in external characters. Even the internal metamerism is 

 to a large extent obscured by the compacting of the anterior meta- 

 meres into a head, and by the fusion of the original segmental body 

 cavities into a few large compound ccelomic chambers. The segmen- 

 tation is best expressed in the structures derived from the embryonic 

 mesoblast: the myotomes, the nephric tubules, the vertebral units, 

 and their skeletal accessories. The central nervous system also re- 

 tains its primitive metameric segmentation in post-cephalic regions, 

 but in the cephalic region the metameric elements are very difficult 

 to make out. There are believed to be primitively five (possibly six) 

 metameres, three or four in front of the otic vesicle and one behind it. 



Back of the head, however, the vertebrate is composed of a linear 

 series of body segments or metameres, each of which has, theoreti- 

 cally, its own complete set of body organs. There is, however, never 

 an ideal or complete development of all the characteristic segmental 

 organs in any one metamere. In the anterior metameres the organs 

 of lowest dynamic activity, such as those of excretion and reproduc- 

 tion, do not appear; and in the posterior metameres the organs of high- 

 est dynamic activity, such as the organs of special sense and the 

 higher coordinative faculties, do not appear. And there is a graded 

 series between these extremes. 



