PROMORPHOLOGT. 135 



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 planes of symmetry. Thus we find the following fundamental 

 forms : 



1. Anaxial, asymmetrical, irregular, or amorphous funda- 

 mental form (fig. 84). 



2. Homaxial, symmetrical in all directions, spherical funda- 

 mental form (fig. 85). 



3. Monaxial, radially symmetrical (fig. 86). 



4. Simple heteraxial, biradially symmetrical (figs. 87, 88). 



5. Double heteraxial, bilaterally symmetrical (fig. 89). 



1. Anaxial or asymmetrical animals, so called, are those in which the 

 arrangement of parts is not regularly de- 

 fined in any direction or space, and they 



therefore may grow irregularly in any direc- 

 tion. There is no fixed central point, and 

 there is no possibility of running definite 

 axes through the body or of dividing it into 

 symmetrical parts. (Many sponges and 

 many Protozoa.) 



2. Homaxial or spherical animals have 

 the sphere as their fundamental form ; the 

 parts of the body are arranged concentri- 

 cally around a fixed central point, so that 

 any number of axes and planes of symmetry 

 <?an be passed through it ; that is to say, 

 all lines and planes which run through the 



central point of the sphere. (A few spheri- FIG. 87.-Diagramof anactinian 

 cal Protozoa, chieny radiolarians.) ( |t^^5S^So?ltoti^ 



3. Monaxial or radial symmetry is much-lengthened main axis, 

 brought about, if growth go on in a definite direction, and correspond- 

 ingly also if the formation of organs take place in directions other than 

 perpendicular to this. The line which marks this direction of growth is 

 the main axis, in distinction from the accessory axes or radii, which are 

 all similar to each other. The main axis can be determined, because it is 

 longer or shorter than the accessory axes ; but it may also be of the same 

 length and still be entirely distinct, since it contains certain organs (e.g., 

 the mouth-opening) which are lacking in the other planes. In radially 

 symmetrical animals the same organs are always present in greater num- 

 ber and are distributed regularly around the main axis in the direction of 

 the radii. Through such an animal a great number of sections can be 

 made, which pass through the long axis and halve the body symmetri- 

 cally. If we cut the animal in the direction of all the possible planes of 

 symmetry, we obtain pieces which, in essential points, are similarly con- 



