PROMORPIIOLOG T. 



135 



body of an animal, and up to a certain degree may characterize it 

 according to the nature of these axes; further, we may cliaracterize 

 it according to tlie planes by which it can be symmetrically halved, 

 the planes of symmetry. Thus we find the following fundamental 

 forms : 



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

 mental form (fig. <S-i). 



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

 mental form (fig. 85). 



3. Monaxial, radially symmetrical (fig. 80). 



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



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



1. Anaxial or asymmetrical animals, so called, are tlidse in wliioli the 

 arrangement of parts is not regularly de- 

 fined in any direction or space, and the\' 

 therefore may grow irregularly in any direc- 

 tion. Tiiere is no fixed central point, and 

 tiiere is no possibility of running definite 

 axes through the body or of dividing it intu 

 symmetrical parts. (Many sponges and 

 many Protozoa.) 



3. 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 

 can be passed tlirough it ; tliat is to say, 

 all lines and planes whicli run through the 

 central point of the sphere. (A few spheri- 

 cal Protozoa, chiefly radioUxrians.) 



■3. Monaxial or radial sijmmetry is 

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

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

 perpendicular to this. The line wliich marl<s this direction of growtli is 

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

 all similar to each other. The main axis can be determined, becanso 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 otlier planes, fn radially 

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

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

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

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

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

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



FlG. ST.— Diagram el' an actiniaii 

 (after Aiidrts, from Hatschek). 

 Side view, perpendicular to the 

 much-lengthened main axis. 



