426 M ERISTIC VARIATION. [PART I. 



tn there are ////, m//// as a specific character four diadems 

 1 >et ween each pair of radial tubes, making in all sixteen instead of 

 ////(/, which is the usual number in the genus. ROMANES, G. J., 

 /-. I. luii. Sue. Zool., xn. p. 525. 



i ;:{(>. Aurelia aurita. This form exhibits an exceptional frequency 

 f Mi-ri>tic Variation. In the normal there are 16 radial canals, 

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

 partures from this normal form have been described in detail 

 by KHUK.\]:I i:<.' and by ROMANES*. 



.Men-tir Variation in Aurelia may occur in two distinct ways, 

 tir.-t in tin- decree to which there is complete separation between 

 tin ^.-ii'-rat ive sacs, and second in actual numerical change. 



Imperfect division of generative sacs. 



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

 met from its nri^lilxuns, but iu some specimens the generative epithelium is 

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

 cliunilii-r. though opening by 4 openings as usual. (Such absence of complete sepa- 

 I'ati'in lii-tween 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 

 ;i triii- circle, but is sacculated to form 4 (as normally) 3, 6, or some other number 

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

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

 pouch, luit cadi of these pouches opened by 3 openings. There was thus a bilateral 

 -\ -1111111 -try, i -ai-h h a lt' containing three lobes of ovarian epithelium incompletely 

 -i ]iarat'<l t'mni each other. Complete union of all the generative organs was 

 very ran . 



Tin- specimens differ greatly with regard to the degree to which the generative 



pithelium i- 1'ol.lcd off, and in the shapes of the generative organs. Commonly 

 tin (.'enerative epithelium is of a horse-shoe form, the two limbs of the horse-shoe 



i"t Hinting eac-h ntlier; but in some specimens the two limbs maybe to various 

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

 roughly circular. (Cases figured by EURENBERG, 1. c., PI. u.) [Here note the 

 Simultaneous Variation of the single quadrants.] 



Numerical Variations. 



< >f 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 



|u:idrants (Fig. ll'S, 1). 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. 

 l->. V, and II. These figures are from ROMANES. Symmetrical forms 

 ha\ing live segments and eight segments are described and figured by 

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

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



(iiadrant are repeated in each segment of the disc, the number of parts 

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

 mental numliiT of the specimen. 



iii-aas, C\ (I., M,h. I;. .I/.-. HV.-x.. /.v///,,, ls;r>, pp. 1<,)9 202, Plates. 

 ' J HOMANKS, (i. J., Jour. Linn. Soc., /<</., xn. p. ,V_'s, and xin. p. I'.lO, Pis. xv. 

 and xvi. 



