136 



GENERAL PRINCIPLES OF ZOOLOGY. 



structed. Great groups of animals, as most eohinoderms and ccelenter- 

 ates, are more or less completely radially symmetrical. 



4 and 5. The next two fundamental forms have in common the fact 

 that three unequal axes perpendicular to each other are distinguishable ; 

 these may be designated as the main axis, the transverse axis, and the 

 sagittal axis : this is the case if, leaving the main axis out of considera- 

 tion, an arrangement of organs occur different in the sagittal direction 

 from that in the transverse direction if organs lie in the former which 



B 



FIG. 88. Cross-section of an actinian (Adamsia diaphana). AB, directive septa, 

 which are at the same time ends of the sagittal axis, which marks one plane or 

 symmetry of the body, while the second stands perpendicular to it ; I-IV, cir- 

 cles of paired septa of first to fourth order. 



are lacking in the latter or the reverse. There are then, if w r e take into 

 consideration the dissimilarity of the axes, two possible planes of sym- 

 metry : the animal can be symmetrically divided, (1) if the division passes 

 through the main and transverse axes, (2) if it passes through the main 

 and the sagittal axes. Such biradially symmetrical animals are the 

 ctenophores, actinians (figs. 87, 88), and corals. 



Bilateral Symmetry. If now we further suppose that the ends of the 

 sagittal axes become unlike, that at one end are organs quite different 

 from those of the other, we then reach the most usual form, bilateral 

 symmetry. The dissimilar ends of the sagittal axes are called ' dorsal r 

 and * ventral,' and further the terms ' right ' and * left ' are given to the 

 ends of the transverse axis ; a bilaterally symmetrical animal can be 

 divided symmetrically into aright and a left half by one plane, the median, 

 passing in the direction of the longitudinal sagittal axis ; a frontal sec- 

 tion (a section through the longitudinal and transverse axes) always gives 

 dissimilar parts, dorsal and ventral sides. 



