44 



has apparently crowded in between the sense-orn;an and the 

 perradial tentacle which comes before it, as the hands of a 

 watch go. Text-figure 3 shows this stage, and is made from 

 a camera di-awing of the eight-tentacled medusa. And the 

 relation which is here exhibited in the youngest stage of 

 the free-swimming gonosome is the same throughout the 

 growth of the creature: "/her ever a_ rudimentary or n ewl y 

 arisen tentacle lies on the bell-margin, it will always , 

 normally . be found to lie ,1 u s t in frorit qf_ a_ newly arisen 

 sense-organ and just after a_ large tentacle . i,e, , one of a 

 much earlier cycle, 



r'uch has been written to show that coelenterates , 

 and especially the Hydrozoa, sliow bilateral symmetry, eith- 

 er in the normal condition or when they depart from the 

 normal form and may be supposed to revert to a more prim- 

 itive type (Mayei', 1901, e.g, ). Gonionema shows a very 

 different plan fx-om that of bilateral symnnetry. It is rath- 

 er a certain sort of radial symmetry v/hich has nothing bi- 

 lateral about it - one in which the radial parts cox-i-espond 

 exactly to each other and are superimposable , but none of 

 which is the reflected image of any other. I shall call 

 this relation one of cyclic symmetry, With reference to 



