Animal Symmetry. 

 By A. T. Masteeman, B.A, D.Sc, F.R.S.E. 



The least attention to the subject of the symmetry of organisms will 

 serve to convince one that the present state of our knowledge, or, to 

 speak more accurately, our present method of expressing the facts, is 

 far from satisfactory. In most modern text-books professing to deal 

 with animal morphology the subject is either left severely alone or is 

 dismissed with a few remarks upon so-called " radial " and " bilateral " 

 symmetry, illustrated by one or two well-known examples. Thus 

 Hatschek 1 divides the Metazoa into Protaxonia and Heteraxonia, relyiug 

 mainly upon the position of the blastopore ; and if we go further back 

 we find that the heterogeneous group Eadiata owed its name to the 

 emphasis of a particular prevalent form of symmetry. 2 Standard works 

 upon botany devote more attention to this branch of morphology, as 

 must needs be the case when the structure of flowering -plants is 

 described. 



As to a correct definition of the term " symmetry," when applied 

 to organisms, there appears to be a considerable divergence of opinion. 

 Thus, many botanists regard the symmetry of a flower as meaning the 

 similarity of its constituent parts about one or more axes or planes, the 

 term polysymmetrical being, from this point of view, regarded as 

 equivalent to " actinomorphic," or, in other words, symmetry about a 

 median axis, whereas " zygomorphic " implies a symmetry about a plane. 

 Sachs draws distinctions between monosymmetry and polysymmetry 

 on the one hand, and between bilateral and multilateral symmetry on 

 the other, pointing out that the former are special cases of the latter. 

 His term " monosymmetrical " alone corresponds to bilateral symmetry 

 as usually accepted amongst zoologists. Geddes, in his suggestive 

 article on " Morphology," 3 remarks in this connection that " botanists 

 since Schleiden " have contented " themselves with thro wins: organisms 

 into three groups- — first, absolute or regular; second, regular and 

 radiate; third, symmetrical bilaterally or zygomorphic. . . . Burmeister, 

 and more fully Bronn, introduced the fundamental improvement of 



1 "Lehrbuch der Zoologie." 2 Cuvier, " Regne animal." 



3 Geddes, P., "Encyclopaedia Britannica," 1885, p. 843. 



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