162 



GENERAL PRINCIPLES OF ZOOLOGY. 



Great groups of animals, as most echinoderms and ccelenter- 

 ates, are radially symmetrical. 



4 and 5. The next two fundamental forms have in com- 

 mon the fact that three unequal axes perpendicular to each 

 other are distinguishable ; these may be designated as the 

 main axis, the cross or transverse axis, and the sagittal axis : 

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

 tion, there obtains in the sagittal direction an arrangement 

 of organs different from that in the transverse direction if 

 -organs lie in the former, which in the latter are lacking 



A 



I\ 



FIG. 85. Cross-section of an actinian (Aciatnsiti diuphana.}. AB, directive partitions, which 

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

 body, while the second stands perpendicular to it; /-//'', circles of paired septa of first to 

 fourth order. 



or reversed. There are then, so long as we take into 

 consideration the dissimilarity of the axes, two possible 

 planes of symmetry : the animal can be symmetrically 

 divided, (i) 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 

 ctenopliores, actiniaiis (Figs. 84, 85), and corals. 



Bilateral Symmetry. If now we further suppose that 

 the ends of the sagittal axes become unlike, that at one 

 end lie organs quite different from those at the other, we 



