136 



GENEUAL riilNCIPLES OF ZOOLOGY. 



striicted. Great groups of animals, as most echiiioderms and coelenter- 

 atcs, are more or less completely radially symmetrical. 



4 and 5. Tlie 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 cousidera- 

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

 from tliat in the transverse direction — if organs He in the former which 



Fig. fi8.— Cross-section of an actinian {Adarnsia diayjJtana). AB. directive septa, 

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

 symmetry of the hody, while the second stands perpendicular to it : 7-11'. cir- 

 cles of paired septa of first to fourth order. 



are lacking in the latter or the reverse. There are then, if we take into 

 consideration the dissimilarity of the axes, two possiljle 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 hiradially syinmetrical animals are the 

 cteuophores, actinians (tigs. 87, 88), and corals. 



Bilateral Symmetry. — If now ^ve further supyiose that the ends of the 

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

 from tho.se of the other, we then reach the most usual foi'in, hi/atenil 

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

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

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

 divided synimetrieally 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. 



