FORMS OF LIFE 



characteristic antithesis of back (dorsum) and belly (venter) ; 

 hence, in botany this type (found, for instance, in most green 

 leaves) is called the dorsiventral, and in zoology the bilateral 

 in the narrower sense. One characteristic of this important and 

 wide-spread type is the relation of three different axes, vertical 

 to each other; of these three straight axes (enthyni) two are 

 uneqiiipolar and the third equipolar. Hence, the centroplanes 

 may also be called tri-axial (triaxonia). In most of the higher 

 animals (as in our own frame) the longest of the three axes is 

 the principal one (axon principalis) ; its fore pole is the oral or 

 mouth pole, and its hinder pole is the aboral or caudal (tail) 

 pole. The shortest of the three enthyni is, in our body, the 

 sagittal (arrow) or dorsiventral axis; its upper pole is at the 

 back and its lower pole at the belly. The third axis — the 

 transverse or lateral axis — is equipolar, one pole being called 

 the right and the other the left. The various parts which make 

 up the two halves of the body have relatively the same dis- 

 position in each half; but absolutely speaking (namely, in 

 relation to the middle plane) they are oppositely arranged. 



Further, the centroplane or bilateral forms are also charac- 

 terized by three vertical axes which may be drawn through 

 each of the normal axes. The first of these normal planes is 

 the median plane; it is defined by the chief axis and the sagittal 

 axis, and divides the body into cwo symmetrical halves, the 

 right and left. The second normal plane is the frontal plane; 

 this passes through the chief axis and the transverse axis (which 

 is parallel to the frontal surface in our body), and divides the 

 dorsal half from the ventral half. The third normal plane is 

 the cingular (waist) plane; this is defined by the sagittal and 

 transverse axes. It divides the head half (or the vertical part) 

 from the tail half (or the basal part). 



The name "bilateral symmetry," which is especially applied 

 to the centroplane and dorsiventral types, is ambiguous, as I 

 pointed out in 1866 in an exhaustive analysis and criticism 

 of these fundamental forms in the fourth book of the General 

 Morphology. It is used in five different senses. For our present 

 general purpose it suffices to distinguish two orders of centro- 

 plane types, the bilateral-radial and the bilateral-symmetrical; 

 in the former the radial (pyramidal) form is combined with the 

 bilateral, but not in the latter. 



The bilateral-radial type comprises those forms in which the 



radial structure is combined in a very characteristic fashion with 



the bilateral. We have striking examples in the three-rayed 



flowers of the orchids (A-f, 74), the five-rayed blooms of the 



11 177 



