4. SYMMETRY. 



spherical and has no distinction between base and apex. For 

 example, the body of Volvox can be divided into symmetrical 

 halves in any plane passing through the centre (Fig. 1). 



The more limited form of multilateral symmetry, which, may be 

 conveniently distinguished as radial, is that which obtains in 

 cylindrical bodies or members. It is multilateral symmetry about 

 the longitudinal axis. In this case the body or member can be 

 divided in various planes along the longitudinal axis into a number 

 of similar halves. 



A mushroom with a central stalk, an apple, a cylindrical tree- 

 trunk, are radially symmetrical as regards their external form. 



As regards the posi- 

 tion of the lateral 

 members, the trunks 

 of Pines and Spruces, 

 with branches arising 

 on all sides, are ra- 

 dially symmetrical ; 

 and, as regards the 

 form of the lateral 



FIG. l.-Folro* Globafor (after Cohn ; x about 100), 

 illustrating multilateral symmetry. 



members, the flowers 

 of the Rose and of the 

 Tulip are radial. 



A radial body or 

 member can be divided 

 by radial longitudinal 

 sections in two or 

 more planes, into 

 symmetrical halves, 

 which are to each 

 other as an object and its image reflected in a mirror (in Fig. 2, A, 

 the halves obtained by the sections 1-1, 2-2, 3-3, 4-4, 5-5). The 

 possible number of such similar halves is not always the same, but 

 it is in any case at least four. In a mushroom or a Fir-stem, there 

 are many possible planes of symmetrical section ; but in a Tulip, 

 the sections being taken through the longitudinal axes of the 

 floral leaves, only three are possible ; and in an apple if they pass 

 through the loculi of the core, only five (Fig. 2 A). 



The two halves are not always as exactly alike as an object and its 

 reflected image ; this is not the case, for instance, in a Fir-trunk, because 

 the lateral branches are not borne at the same level. The two halves are, 



