PROMORPHOLOG Y. 



159 



to ignore certain greater or less variations. If, for exam- 

 ple, we call man bilaterally symmetrical, we must ignore 

 not only the slight asymmetry of a nose awry, etc., but 



J cD ^ tf 



also what is more important that the liver has been 

 pushed to the right, the heart to the left, side; and that 

 the digestive tract throughout its entire course runs asym- 

 metrically. 



Symmetry. Now, according to the three dimensions 

 of space, we can pass three axes, perpendicular to each 

 other, through the body of an animal, and up to a certain 

 degree may characterize it according to the nature of these 

 axes ; further, we may characterize it according to the planes 

 by which it can be symmetrically halved, the plane of sym- 

 metry. Thus we find the following fundamental forms: 



i. Anaxial, asymmetrical, irregular, or amorphous fun- 

 damental form (Fig. Si). 



FIG. 81. Spongilla fluviatilis (After Huxley.) a, superficial layer with dermal pores ; be 

 region of the ciliated chambers ; d, osculum. 



2. Homaxial, symmetrical in all directions, spherical 

 fundamental form (Fig. 82). 



3. Monaxial, radially symmetrical (Fig. 83). 



4. Simple heteraxial, biradially symmetrical (Figs. 84, 



85). 



5. Double heteraxial, bilaterally symmetrical (Fig. 86). 

 i. Anaxial or asymiHi'frical animals, so called, are those 



