PROMORPHOLOG Y. 



161 



FIG. 84. Diagraraof an actinian. (After 

 Angelp Andres, from Hatschek.) 

 Side view, perpendicular to the much- 

 lengthened main axis. 



in which the arrangement of parts is not regularly defined in 

 any direction of space, and 

 they therefore may grow irreg- 

 ularly in any direction. In it no 

 fixed central point is given, and 

 there is no possibility of run- 

 ning definite axes through the 

 body or of dividing the body 

 into symmetrical parts. (Many 

 sponges and many protozoa.} 



2. Hoinaxial or splierical 

 animals have the sphere as their 

 fundamental form ; the parts of 

 the body are arranged concen- 

 trically around a fixed central 

 point, so that any number of 



axes and planes of symmetry can be passed through it ; that 

 is to say, all lines and planes which run through the cen- 

 tral point of the sphere. (A few spherical protozoa, chiefly 

 I'ddiolarians.) 



3. Monaxial or radial symmetry is brought about, if 

 growth goes on in a definite direction, and correspondingly 

 also, if the formation of organs takes place in directions other 

 than perpendicular to this. We call the line which marks this 

 direction of growth 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 cer- 

 tain 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 number and are dis- 

 tributed regularly around the main axis in the direction of 

 the radii. Through such an animal a great number of sec- 

 tions can be made, which pass through the long axis and 

 halve the body symmetrically. If we cut the animal in the 

 direction of all the possible planes of symmetry, we obtain 

 pieces which, in essential points, are similarly constructed. 



