426 MERISTIC VARIATION. [part i. 



diademata there are normally as a specific character four diadems 

 between each pair of radial tubes, making in all sixteen instead of 

 eight, which is the usual number in the genus. Romanes, G. J., 

 Jour. Linn. Soc. Zool., XII. p. 525. 



*636. Aurelia aurita. This form exhibits an exceptional frequency 

 of Meristic Variation. In the normal there are 16 radial canals, 

 4 oral lobes, 4 generative organs and 8 lithocysts. The de- 

 partures from this normal form have been described in detail 

 by Ehrenberg 1 and by Romanes 2 . 



Meristic Variation in Aurelia may occur in two distinct ways, 

 first in the degree to which there is complete separation between 

 the generative sacs, and second in actual numerical change. 



Imperfect division of generative sacs. 



In the commonest form of Aurelia there are four generative organs each 

 distinct from its neighbours, but in some specimens the generative epithelium is 

 continuous all round the mouth, and there is then one continuous generative 

 chamber, though opening by 4 openings as usual. (Such absence of complete sepa- 

 ration between some of the generative organs is not rarely seen in cases of numerical 

 Variation, v. infra.) Though the epithelium is then continuous it does not form 

 a true circle, but is sacculated to form 4 (as normally) 3, 6, or some other number 

 of incompletely separated parts. Ehrenberg (I. c, p. 22) saw a case in which there 

 were 6 such sacculations, three on each side being united and having one generative 

 pouch, but each of these pouches opened by 3 openings. There was thus a bilateral 

 symmetry, each half containing three lobes of ovarian epithelium incompletely 

 separated from each other. Complete union of all the generative organs was 

 very rare. 



The specimens differ greatly with regard to the degree to which the generative 

 epithelium is folded off, and in the shapes of the generative organs. Commonly 

 the generative epithelium is of a horse-shoe form, the two limbs of the horse-shoe 

 not meeting each other; but in some specimens the two limbs may be to various 

 degrees approximated, so that each generative organ is kidney-shaped or even 

 roughly circular. (Cases figured by Ehrenberg, I.e., PI. n.) [Here note the 

 Simultaneous Variation of the single quadrants.] 



Numerical Variations. 



Of these the most striking and also the most frequent are variations 

 consisting in a perfect and symmetrical change in the fundamental 

 number of segments composing the disc. Normally there are four 

 quadrants (Fig. 128, I). Varieties are found having only half the usual 

 number of organs, the disc being made up of two halves, each contain- 

 ing one generative organ (Fig. 128, IV). Other symmetrical varieties 

 having three, and six, as their fundamental numbers are shewn in Fig. 

 128, V, and II. These figures are from Romanes. Symmetrical forms 

 having five segments and eight segments are described and figured by 

 Ehrenberg. As to the comparative frequency of these forms facts are 

 given below. In each of them all the parts normally proper to one 

 quadrant are repeated in each segment of the disc, the number of parts 

 being greater or less than the normal in correspondence with the funda- 

 mental number of the specimen. 



1 Ehrenberg, C. G., Abh. k. Ah. Wiss., Berlin, 1835, pp. 199—202, Plates. 



2 Romanes, G. J., Jour. Linn. Soc., Zool., xn. p. 528, and xm. p. 190, Pis. xv. 

 and xvi. 



