PROMORPHOLOG Y. 



I5C, 



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 

 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 Jlut'iatilis. (After Huxley.) a, superficial layer with dermal pores ; lie 

 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, 

 8 5 ). 



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

 i. Anaxial Q* asymmetrical animals, so called, are those 



