PROMORPHOLOGY. 

 D 



137 



FlG. 89. Cross-section of a fish passing through the fore limbs. DP, sagittal axis; 

 RL, transverse axis; a, dorsal aorta; c, body cavity; d, gut; eft, notochord; #, 

 shoulder-girdle ; h, heart ; w, muscles ; ?i, anteri9r end of the kidneys ; p, peri- 

 cardium ; oo, neural arch ; lib, haemal arch ; r, spinal cord. 



Antimeres and Metameres. The symmetrical parts of an 

 animal are called antimeres; each antimere has organs which occur 

 likewise in its adjacent antimere. The right arm of man corre- 

 sponds to the left, the right eye to the left, etc; the same organs- 

 are repeated in the direction of the transverse axis. Frequently, 

 however, the repetition of organs occurs not only in the direction 

 of the transverse axis, but also 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. 



Internal and External Metamerism. Metamerism or segmen- 

 tation is spoken of when the body of an animal consists of 

 numerous segments or metameres (consult fig. 59). Very often 

 it is recognizable externally when, for instance, the limits of t he- 

 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, in metameric or segmental arrange- 

 ment. Man, for example, is segmented only internally; in his- 

 skeleton there are numerous similar parts, the vertebrae., which 



