CNIDARIA AS THE ONLY COELENTERATA 57 



of the difference between the dorsal and ventral sides of its 

 body. As a result of this, the form of the transverse section of 

 the animal becomes circular. In some animal groups, whose 

 body is otherwise completely bilaterally symmetrical, the in- 

 clination to accept the radial symmetry can be developed in 

 one part of its body only, e.g. in its anterior part. This depends 

 on how much these organs are used by the animal to stick to 

 the ground or to adhere to it by means of suction. Clear 

 examples of such a development can be seen in Cestoda (scolex) 

 and in Nematoda (the coroniform distribution of various pa- 

 pillae and of sensory organs of the skin) (see Fig. 9.). A complete 

 radial symmetry can only be reached in the aproctous types of 

 animals, e.g. in Cnidaria, or in such euproctous animals where 

 the absence of the anal orifice is a secondary phenomenon in 

 their development (some Echinodermata, especially among the 

 Ophiuroidea). In Infusoria, a complete radial symmetry can 

 rarely, if ever, be observed, even if in these the anal orifice has 

 frequently not developed. This is due to their strong inclina- 

 tion to adopt an asymmetrical structure; the latter has also 

 been inherited in their digestive apparatus by animals that live 

 a sessile way of life (epitrichial Infusoria). 



The combination of a primary bilateral symmetry and of 

 a secondary radial symmetry can lead to a bisymmetry. This 

 can be observed in the interior of numerous anthopolyps and 

 of other animals, and it is best and most purely developed 

 in Ctenophora. In my opinion, the origin of this bisymmetry 

 can be explained rationally in one way only if we consider the 

 bilaterally symmetric form as primary, and we presuppose 

 that there had been a development caused by the loss of con- 

 tact with the firm ground which made the ventral and dorsal 

 sides similar. The two lateral sides preserve their original 

 similarity with each other, yet they are more rounded off and 

 therefore better developed. Moreover, they still continue to 

 be different from the ventral and dorsal sides, though the latter 

 have become similar with each other. In Ctenophora this change 

 of symmetrical conditions occurs in connection with their 



