186 THE EVOLUTION OF THE METAZOA 



with the first four septa. This can rightly be considered as a 

 recapitulation of a state reached in their ancestors. 



The supposition that the primary tetraradial symmetry repre- 

 sents the first stage reached in the transition from a creeping 

 way of life with an explicit bilateral symmetry, is justified in 

 view of the fact that during such a transition it is first the 

 difference between the ventral and dorsal sides which dis- 

 appears, and, in connection with this, the difference between 

 the tw^o lateral sides and the median sides. In this way a natu- 

 ral basis, or precondition, has been developed, wx may say, for 

 the formation of the tetraradial (as the first possible) symmetry. 

 Soon the polymerization of the so-called antimeres took place 

 in anthopolyps which have grown larger and larger, and which 

 have become more and more sessile and solitary animals. This 

 led to a polymerous radial symmetry with clear traces of a 

 primary bilateral symmetry. This was followed by the for- 

 mation of cormi and, in connection with this, by a secondary 

 diminution of the polypoid individuals; oligomerization finally 

 occured as a consequence of this development, so that ulti- 

 mately the hexa- and octo-merous states have been developed 

 in Anthozoa. Simultaneous with the disappearance of the 

 tendency to the old bilateral symmetry and with the decrease 

 of the size of polyps wxnt the reduction until the number of 

 antimeres was reduced to four and a complete radial sym- 

 metry has been developed. It can be easily understood that 

 this reduction of the polymerous parts was least effective in' the 

 tentacles; it is even possible that the number of tentacles has 

 been considerably increased as can be seen in the case of the 

 Naiisithoe polyp among Scyphozoa, and in numerous other 

 instances among hydropolyps that had inherited their tendency 

 to tetraradial symmetry from their scyphozoic ancestors. 



When we pass over to hydropolyps we must first state that 

 in these the last trace of a formation of sarcosepta has dis- 

 appeared while simultaneously their size has been further 

 decreased. Not infrequently we can find in the oral part of the 



