ab. o. 



FIG. 47. Diagram of Medusa, illustrating radial symmetry. A, viewed from the 

 oral end of the principal axis; B, a section along the principal axis and through one of 

 the subordinate axes ad 1 : o, ab. o, the oral and aboral poles of the principal axis; a, a 1 , 

 and b, b 1 , the similar poles of the two chief subordinate axes. 



Questions on the figure. Are the poles of the oral-aboral axis alike 

 or unlike? How many clearly differentiated secondary axes are there? 

 What would be the appearance of a section midway between aa 1 and bb 1 ? 

 Would the resulting halves be symmetrical? Compare this condition with 

 the definition of radial symmetry in the text. Find other illustrations of 

 radial symmetry in the figures of this book. 



The two poles of each of the subordinate axes are essentially alike, but 

 are unlike the poles of the other subordinate axis. This arrangement is 

 found in the sea-anemone (Fig. 48). The differences between aa 1 and 

 bb 1 are not usually so great that we cease to speak of the form as radially 

 symmetrical. It is of importance to know that in the radially symmetrical 

 animals the principal axis, whether longer or shorter than the subordinate 

 axes, is normally perpendicular to the substratum on which the animal 

 rests, or to which it is attached. 



5. If such an animal as was last described were to have its principal 

 axis horizontally placed, with one of its two subordinate axes vertical 

 and the other horizontal, and were maintained in this position, it would 

 likely happen that the formerly similar poles of the new vertical axis 

 would become unlike, because subjected to different influences. These 

 poles are known as the dorsal and ventral poles. The poles (right and 

 left) of the other transverse or subordinate axis would remain similar, 

 as they are subjected in the long run to similar conditions. This gives 

 us the condition, found in all the higher, free-moving animals, known as 

 bilateral symmetry. It consists, to recapitulate, of (i) a main axis 

 (antero-posterior axis} usually horizontal and with dissimilar poles; (2) 

 a transverse axis, usually vertical {dor so -ventral axis) with dissimilar 

 poles; and (3) a transverse axis perpendicular to the other two, hori- 



