2 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



symmetrical, the other types of symmetry will not be considered here. For them consult H, 



pages 123-25. 



Bilateral symmetry.— The parts of a bilaterally symmetrical animal are arranged 

 symmetrically with reference to three axes, the longitudinal, transverse, and sagittal axes; 

 the two ends of the sagittal axis in any given cross-section are unlike. There is but one plane 

 of symmetry in such an animal— that plane which passes through the longitudinal and sagittal 

 axes— namely, the median sagittal plane. It divides the animal into approximately identical 

 right and left halves, which are mirror images of each other. The structures of vertebrates 

 are either cut in half by the median sagittal plane, in which case they are spoken of as unpaired 

 structures, or they are placed symmetrically on each side of this plane, equidistant from it, 

 in which case they are paired structures. The digestive tract is the only system which does not 

 exhibit a symmetrical relation to the median plane in the adult, although it, too, is bilaterally 

 symmetrical in early embryonic stages. 



D. METAMERISM OR SEGMENTATION 



Segmentation or metamerism is that structural condition occurring in certain groups of 

 animals in which all or most of the paired parts or structures are repeated at regular intervals 

 along the anteroposterior axis. The body of such animals consequently is composed of a 

 longitudinal series of divisions or elements, in each of which all or most of the systems of the 

 body are represented, either by entire paired organs or structures or by a portion of the median 

 unpaired structures. Each such division or element of the body is called a metamere, somite, 

 or segment. The anterior and posterior boundaries of each segment may or may not be 

 marked externally by a constriction of the body wall. In the former case the animal is said 

 to exhibit both external and internal metamerism; in the latter case internal metamerism alone 

 is present. 



In an ideal segmented animal all of the segments are identical with each other in all of 

 the details of structure. No such animal exists because both the head and the terminal 

 segments must of necessity differ, if only slightly, from the other segments, but the ringed 

 worms, such as Nereis and the earthworm, closely approach the ideal. Such segmented animals 

 in which the various segments are nearly alike are said to possess homonomous segmentation. 

 The majority of segmented animals, however, display heteronomous segmentation, in which the 

 various segments have become unlike each other in many respects. 



The segmented groups of animals are the annelids, the arthropods, and the vertebrates 

 and their relatives. In the evolution of segmented animals there has been a continuous pro- 

 gression from the homonomous to the extreme heteronomous condition. Homonomous 

 segmentation represents a primitive and generalized type of structure in which the various 

 segments are more or less independent of each other and each is capable of performing all 

 of the necessary functions. But with the evolution of heteronomy, the segments become 

 unlike and there is a division of labor among them, some portions of the body elaborating 

 certain functions and others other functions. Each segment is then no longer capable of 

 performing all of the functions, but is dependent upon its fellow-segments with a resulting 

 unification and organization which is lacking in the homonomously segmented forms. 



The heteronomous condition is derived from the homonomous through a number of 

 different processes, such as loss of segments, fusion of adjacent segments, enlargement or 

 reduction of segments, loss of organs or parts from some segments with their retention in other 

 segments, structural changes among the repeated organs or parts so that those of different seg- 

 ments become unlike, etc. 



The vertebrates are heteronomously segmented animals with internal segmentation only. 

 The embryos of vertebrates much more nearly approach the homonomous condition, an 



