1902.] NATURAL SCIENCES OF PHILADELPHIA. 775 



members of the hydrozoa, show bilateral symmetry, either in the 

 normal condition or when they depart from the normal form and 

 may be supposed to revert to a more primitive type (Mayer, 1901, 

 e.g.). Gonionema shows a very different plan from that of 

 bilateral symmetry. It is rather a certain sort of radial symmetry 

 which has nothing bilateral about it — one Iq Avhich the radial parts 

 correspond exactly to each other, and are superimposable, but none 

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

 tion one of cyclic symmetry. With reference to the order of ap- 

 pearance of the marginal organs I shall speak of cyclic sequence. 



Fi^. 11. 



These terms were suggested by Prof. Morley, of the mathematical 

 department of Johns Hopkins University. 



New tentacles make their appearance four at a time, or, so to 

 speak, in quartets; they are 90° apart, so that they occupy iden- 

 tical positions in the four quadrants of the marginal ring. But 

 while the tentacles, and the sense-organs as well, appear to rise in 

 fours, the condition in the larva, and in frequent instances among 

 the adults, indicates that a paired origin is more primitive and 

 fundamental. It is the rule in the early larvae that two tentacles 

 appear opposite to one another (PI. XXXII, fig. 12), and later 



