49 



symmetry, or th-^t such a condition may ajjpTar in occasional 

 instances, as is perhaps the case in Aurelia, In Gonionema 

 the rule holds with reriarkable constancy. 



From a study ol' the successive stages of growing me- 

 dusae the following table is compiled, to show the I'elation 

 in time of appearance of tentacles and sense-organs. The 

 numbers in brackets, in the coliunn of sense-organs, indi- 

 cate half-quai'tettes , the corresponding pair in each case 

 having been delayed in appearing. Since the sense-organs 

 are only half as numerous as tentacles, they appear with 

 half the rapidity, and are therefore more frequently found 

 in this incomplete condition. That is, if a jelly-fish 



were killed when the tenta- 



Tentacles Sense-organs 



rval I 2 



olyp)| 4 



Adult 

 (Medusa) 



'2) 



f S (6) 



12 8 



1^3 (10) 



^0 12 



24 (14) 



28 16 



36 20 



40 (22) 



44 24 



48 



cles and sense-organs were 

 in the precise stage indi- 

 cated by the line A.B, the 

 fifth quartette of sense- 

 organs would be found only 

 half formed - five sense- 

 organs appearing in two op- 

 posite ciuadrants, and only 

 four in the other two. This 



