Chap. II. AURELIA FLAVIDULA. 29 



not one individual; but, even in the most questionable cases, we have finally seen 

 the lower animal throw open its mouth and the upper one creep away. 



In the next stage the embryo normally has sixteen tentacles, but they do not 

 develop so nearly synchronically as in the eightrarmed period : the irregularity, 

 however, appears greater than it really is, on account of the increased number of 

 tentacles, and the difficulty of distinguishing between the members of the different 

 sets. The mode of development is the same as heretofore : the new tentacles (PI. 

 X*. Fig. 13 e) arise in the intervals of the former sets. Neither in Aurelia nor in 

 Cyanea have we actually traced the development of the tentacles beyond the number 

 fourteen (PL X^ Fig. 15); and all the figures in Plates XL and XP., whether with 

 more or less than fourteen tentacles, were drawn from specimens collected among 

 the wharves in Boston harbor. We have not been able to trace the development 

 of Cyanea beyond the fourteen-armed stage, and therefore what follows relates to 

 Aurelia excli^sively. The scyphostoma and strobila forms of these two plates are 

 so irregular in their development, Ijoth in regard to the shape of the body and 

 the development of the tentacles, that we suspect they have already cast off one 

 In-ood of Ephyraj, and that the circle of tentacles now present is not the original 

 primary one, but wms developed below the pile of Ephyra^, as in PI. XL Figs. 1, 

 4, 5, G, 11, 13, 1-i, 10, 17, 25, and 29. On this account we are not surprised 

 to find more than sixteen tentacles, but less than thirty-two, on the oldest scyphos- 

 toma. The sixteen-armed specimens (PI. XP. Fig. 3 A B, Fig. 4, with one tentacle 

 forked. Figs. 8 and 10), we might suppose, were originally four-armed ; and the 

 twenty-armed ones (PI. XL Fig. 7; PI. XL. Figs. 7 and 11) began with five tentacles. 

 This assumption seems the more probable from the fact, that we have never seen 

 a single scyphostoma or strobila which had more than twenty tentacles. We may, 

 therefore, consider the normal number of tentacles of the scyphostoma of Aurelia 

 flavidula to be sixteen; and, occasionally, twenty. 



The four buttress-like projections, which we pointed out in the eight-armed 

 stage, do not increase in number with the tentacles, but develop in breadth (PI. 

 X^ Fig. 13 V') and thickness. By the constancy of their number, and the llict that 

 they originate opposite the first four tentacles, we are enabled to determine the 

 relative age of every tentacle of a full-grown scyphostoma, no matter whether 

 there are sixteen, or, in exceptional cases, twenty of them. Thus, those which 

 are opposite the projections, as in PI. X^ Fig. 5 1?, belong to the first group 

 and are only four in number; and in an eightrarmed individual those which 

 alternate with these last appertain to the second set. In a sixteen-armed embryo 

 there will be three tentacles in each interval between those of the first grouj?, 

 and the middle one of the three belongs to the second group of four ; whilst 

 the remaining two, out of the three, altogether eight in number, belong to the 



