138 Proceedings of the Royal Physical Society. 



It is very easy, even in immature adults, to detect cases of twinning, 

 for there is an enlarged notch on the margin of the umbrella and an 

 enlarged hood protects the tentaculocysts, each of which is removed 

 slightly to the side at the end of a branched canal. It is seen that 

 in nine of the ten specimens the twin is situated perradially, and that 

 in one example there is a double case of twinning, giving this particular 

 individual nine branched canals and eleven tentaculocysts. Four of the five 

 twinned examples which Browne noticed had the twin tentaculocysts 

 perradially placed, and these had respectively 9, 9, 9, 10, 11 tentaculocysts. 

 It is not easy to agree with Browne when he says that " it is possible for 

 two tentaculocysts to become widely separated by the growth of the 

 umbrella, and each to have its own canal system " (I.e., p. 90). It seems 

 rather to be a case of once a twin always a twin. 



With this case of a double sense-organ in Awrelia aurita may be 

 compared that in another ccelenterate, Olavatella prolifera, which has 

 normally a single eye at the base of each arm. " Clapadere figures a case 

 of duplicity of an eye, and says that specimens occur in which each eye 

 is doubled, so that there are two eyes at the base of each arm instead of 

 one " (Bateson, I.e., p. 425). 



6. The Question of Symmetry. 



In Nature, regularity of form generally shows itself in symmetry, which 

 is all but a universal feature of organisms. While radial symmetry is the 

 characteristic type of symmetry found in the Ccelenterata, bilateral symmetry 

 is the type usually exhibited by more highly organised forms. 



From what has already been said as to the number and arrangement 

 of the more important organs in Aurelia aurita, it is clear that a normal 

 living example shows radial symmetry. Some cases where this symmetry 

 is departed from are sufficiently obvious from the tables given. There 

 are all degrees of departure from radial symmetry to forms that are 

 purely asymmetrical. Examples are found where the separate organs 

 considered by themselves show radial symmetry (Bateson's Minor 

 Symmetry), but the correlation of the various organs has been so altered 

 that there is not complete radial symmetry among the different kinds of 

 organs (Bateson's Major Symmetry). It has been noted as a curious 

 fact that if one kind of symmetry is lost, there is a tendency in the 

 animal world to replace it by another kind of symmetry. Several examples, 

 among the thousand here considered, show that radial symmetry has been 

 disturbed in such a way that bilateral symmetry takes its place. 



