1902.] NATURAL SCIENCES OF PHILADELPHIA. 788 



It would be singular indeed if there were no exceptions at all to 

 this general rule. Many variations from the regular cyclic sym- 

 metry do occur, but only as many as would be expected from the 

 marked tendency to variability in all parts of the medusa. These 

 variations no more obscure the normal definiteness of plan than the 

 occurrence of six or seven-rayed star-fish obscure the normal penta- 

 merous form in echinoderms. Text-figure 21 shows an irregular 

 condition, the fourth tentacle in each quadrant having arisen 

 aberrantly, following instead of preceding the first sense-organ (1). 



Fig. 21. 



In PI. XXXIII, fig. 19, one of the latest arisen quartette had 

 not put in its appearance (see arrow in quadrant A). In the 

 older specimens, the number of irregularities increases. It seems to 

 me that the bell-margin increases in extent subsequent to and as a 

 consequence of the increase in the number of tentacles, rather than 

 that the tentacles arise, haphazard, wnerever there is space enough 

 on the margin to accommodate them (Hargitt, 1901). Certain it 

 is that the most crowded part of the bell-margin at any particular 

 moment is that from which new tentacles are in the process of 

 arising. 



