56 THE EVOLUTION OF THE METAZOA 



bottom by means of beating of cilia, and among Gastropoda 

 where the origin of their asymmetry is a special problem which 

 cannot be discussed here at any length. In certain groups of 

 fishes the asymmetry is due to their lying constantly on one 

 side of their body, similarly as in the case of Branchiostoma 

 (Amphioxus) . 



Most frequently, however, we see that a secondary radial 

 symmetry develops out of the primary bilateral symmetry. 

 Always, and without an exception, this change stands in causal 

 relationship to the transition to a secondary, sessile, way of 

 life, i.e. to the fact that the active locomotion had been given 

 up. These changes of symmetrical conditions occur in varying 

 degrees of intensity in all the large groups of the animal world, 

 beginning with Flagellata among Protozoa and continuing up 

 to Chordoma. Radial symmetry can occur even without 

 a trace of the former bilateral symmetry or of the asymmetry; 

 the degree of evolution of this radial symmetry depends on 

 the final structural conditions, the way of life, the type of feed- 

 ing, as well as on the phyletic age of the transition to a semi- 

 sessile or finally to a fully sedentary way of life. This radial 

 symmetry can be so completely without a trace of the former 

 bilateral symmetry that it was believed, especially by older 

 zoologists (and unfortunately it still is accepted by some modern 

 scholars,) that this radial symmetry is an original property of 

 Metazoa. We cannot blame the idealistically minded pre-Dar- 

 winian morphologists that they put together Coelenterata (ac- 

 tually Cnidaria) and Echinodermata, i.e. two animal groups 

 with the most highly developed radial symmetry, and created 

 for them the taxonomic unit of Radiata. Even in the post- 

 Darwinian days there were zoologists— and there are some 

 even now— who, following Cuvier, try to preserve this arti- 

 ficial unit. As a matter of fact, however, Cnidaria are in no 

 way whatever phyletically related to Echinodermata. 



As a preliminary stage in the development towards the radial 

 symmetry we can observe first a change in the general carriage 

 of the animal and in connection with this, the disappearance 



