428 MEMSTIC VARIATION. [part i. 



are not quite accurately distributed). Similarly a particular quadrant 

 may be represented by two sets of parts or by three sets (Fig. 128, VIII), 

 the other three quadrants being normal or nearly so. I have seen a 

 case also in which the chief symmetry was arranged as for three seg- 

 ments (having 3 oral lobes), but one of the three segments was imper- 

 fectly divided into two. 



In a case of 6 segments, 3 on one side may be large and the other 3 

 small, somewhat as in Fig. 128, VIII, but the whole disc was not circular, 

 the radius on the side of the large segments being the greater. 



In the figures (after Romanes) all the discs are represented as 

 circles, but my own experience was that when there was not a truly 

 symmetrical distribution of the generative organs the half quadrant or 

 other segment in which the number of parts was greatest bulged out- 

 wards, thus exemplifying the general rule that when an organ divides 

 the two resulting parts are together larger than the undivided organ. 



Besides those specified, there are also irregular cases, e.g., specimens 

 with 3 generative organs but 4 oral lobes and other parts in multiples of 

 4, but as Ehrenberg says in such cases it is generally possible to detect 

 that one of the generative organs is larger than the others or even 

 partially double. He also saw cases otherwise arranged in a symmetry 

 of 6, but having 22 chief radial canals instead of 24, &c. Also 14 radial 

 canals (instead of 12) were found in some cases of 3 generative organs. 



As everyone will admit, it is impossible in regular threes, sixes, &c. 

 to say that any particular segment is missing or is added rather than 

 another. 



Comparative frequency of the several forms. 



•637. Among thousands of individuals seen by Ehrenberg only two were 

 8-rayed, 15 — 20 were 6 rayed, some 20 — 30 were 5- and 3-rayed, the 

 remainder being 4-rayed. In percentages, 90 are 4-rayed, 3 are 3-rayed 

 3 are 5-rayed, 2 are 6-rayed and 2 have other numbers. 



The result of an attempt to ascertain these percentages in a great 

 shoal of Aurelia washed ashore on the Northumberland coast on 4 Sept. 

 1892 is given below. The radial canals were not counted, and the 

 numbers apply strictly to the generative sacs only. It will be seen 

 that the pi-oportion of abnormals is lower than that given by Ehrenberg. 



2 generative sacs 



3 ,, symml. : 3 oral lobes in 4 unbroken cases ...10 (0 - 57°/ o ) 



3 „ 2 large, 1 small : 3 or. lobes 1 



4 ,, normal 1735 



4 „ 3 large, 1 small : 5 or. lobes 1 



4 „ 2 large, 2 small : 3 lobes 1 



5 ,, symmetrical 5 lobes in one 2 



5 „ not quite symmetrical 1 



6 ,, sym. : 6 lobes in 2 unbroken cases 7 (0*397.) 



6 ,, not symmetrical 1 



6 „ 4 large, 2 small 1 



6 ,, 4 large, 2 united: 6 lobes in 1 unbroken ... 2 

 G „ 3 large, 3 small : 6 lobes 1 



1763 



