84 ZOOLOGY. 



116. Types of Symmetry. It is desirable to distinguish the follow- 

 ing types : 



I. In a spherical organism in which no differentiation is apparent (as 

 in a simple spherical cell, or blastula, Figs, n A, 3; 44) any plane passing 

 through the centre divides it into symmetrical parts and all the axes are 

 essentially equal. In such a case there may be said to be an infinite num- 

 ber of similar axes, and the poles of each axis are similar. 



FIG. 44. 



Fid. 44. Spherical cell (resting stage of Amoeba) illustrating general or universal 

 symmetry. Any plane passing through the centre will divide it into two essentially 

 equal portions. 



Question on the figure. What prevents this animal being a perfect 

 illustration of universal symmetry? 



FIG. 45. Amoeba in active condition. Entirely unsymmetrical. 



2. An organism may be wholly asymmetrical, without any definite 

 form, the axes being without regular arrangement. {Amoeba in its active 

 stages, Fig. 45 ; some Sponges.) In other instances the form may be 

 definite and axes developed, but the structure is such that no plane will 

 separate the animal into symmetrical parts (Paramccium and many other 

 active Protozoa; see Figs. 66-69). 



3. As a variation of the universally symmetrical condition seen in I, 

 a limited number of axes may become distinguishable from the others by 

 some specialization of structure (Fig. 46). These special axes are sim- 

 ilar and their two poles are alike. 



4. Starting again from the undifferentiated spherical form, one of its 

 numerous similar axes may come to differ from all those perpendicular 

 to it by increased or diminished length, or by a difference in construction. 

 This special axis is to be known as the principal axis. The poles of the 

 principal axis do not usually remain alike. Perpendicular to this princi- 

 pal axis one may select an indefinite number of subordinate axes which 

 are essentially similar to one another. The poles of each subordinate 

 axis are alike. Such a condition is realized in the simplest gastrulae (Fig. 



