1899] 



ANIMAL SYMMETRY 



59 



account the siphonoglyphs and mesenterial muscles, give a good example 

 of di-plano-symmetry, whilst tri-plano-symmetry is found in certain 

 Acantharia (Haeckel), and more or less perfectly in some of the 

 Ctenophora. 



There are several points to notice with regard to these three types 

 of symmetry. These bear out our argument that they represent a 

 natural classification. 



Firstly, it has been already shown that a great number of the 

 organisms which in other respects take their position at the bottom of 



B. 



A- 



"A. A.- 



C 



Fig. 4. — Tri-plano-symmetry. 

 (2a + 2b + 2c.) 



A- 



b: 



Fig. 5. — Di-plano-symmetry. 

 (2a + b + b' + 2c.) 



,C! 



B'. 



Fig. 6. — Piano-symmetry. 

 (2a + b + b' + c + c'.) 



the animal scale belong either to no type of symmetry whatever or 

 exhibit at one stage of their career a centro-symmetry (see Fig. 9). 



Again, others of the Protozoa and almost all of the Coelentera, 

 together with other groups showing degeneration, exhibit the second or 

 axo-symmetry ; and lastly, the great majority of the Metazoa exhibit 

 the third or piano-symmetry more and more markedly as the highest 

 types are reached. 



This classification, therefore, clearly indicates the phyletic progress 

 in course of time from forms with no symmetry and ever-changing 

 shape, through symmetry in all dimensions, to that in two, and finally, 

 the successive types of symmetry. 



In this connection we may look at the increasing influence of 

 locomotion in the determination of an animal's symmetry. The 

 locomotion in centro-symmetric forms is quite indefinite, with freedom 

 to rotate about the centre and nothing more. That of axo-symmetric 

 forms is in a definite direction in a considerable number, but may be 

 in many cases entirely absent (sedentary). At most it is definite in 

 only one dimension. In piano-symmetric forms the vast majority have 

 definite locomotion, with a definite direction, in two dimensions. The 

 gradual development of symmetry from a lower to a higher type 

 is therefore in agreement with the general facts of evolution (see 

 Fig. 7). Similarly in ontogeny the same history is recapitulated. With 

 exceptions mainly traceable to secondary coenogenetic modifications, the 

 ontogeny of a piano-symmetric organism commences with the centro- 

 symmetric egg, which in the most primitive forms retains its centro- 

 symmetry up to the free swimming centro-symmetric blastula-larva. The 



