94 



ZOOLOGY 



o-ab.o) instead of an indefinite number as in the former case. These special sub- 

 ordinate axes are usually 3, 4, or 5, or some multiple of these numbers. The num- 

 ber however may be reduced to two in which the four poles are all alike. Many of 

 the medusae, coral polyps, and some echinoderms illustrate this type of symmetry. 

 (b) A further variation of (a) is seen in the fact that in some animals, otherwise 

 similar to those described in (a), one of these special axes perpendicular to the 

 principal axis comes to differ from the other. The two poles of each of the sub- 

 ordinate axes are essentially alike, but are unlike the poles of the other subordinate 



FIG. 50. 



ab. o. 



Pic. 50. Diagram of the sea-anemone, illustrating another type of radial symmetry. A, 

 cross section; B, longitudinal section. Lettering as in Fig. 49. c, the chamber between the mes- 

 enteries (m). 



Questions on the figures. How does the symmetry of the anemone compare 

 with that of the Medusa? Express the difference clearly in terms of the axes and 

 their poles. Are aa l and bb l strictly similar axes? Do their planes divide the 

 animal into halves? Are the four halves thus obtained equivalent? In B what 

 difference in the position of the section will account for the differences on the right 

 and left side of the figure? 



axis. This arrangement is found in the sea-anemone (Fig. 50). The differences 

 between aa l and bb l 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 symmetri- 

 cal 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 



