Chap. H. AURELIA FLAVIDULA. 47 



the contrary, is not so conspicuously folded as in earlier stages. The sexual organs 

 {Fi<js. 10 and 17 g) have made considerable advance ; the rows of digitate appendages 

 {Fig. 10 g g"^) have increased to six or seven in number, and render this organ 

 very conspicuous even in their natural size {Fig. 18). The exterior pouch {Fig. 

 10 a (i ;'), which opens outwardly directly underneath the sexual organs, is fully 

 as long as the semicircle of digitate appendages, and its distal side («) corresponds 

 to the margin of the semicircle, although the two are in different walls. The 

 breadth of this pouch Ls about half that of its length, its depth is about in the 

 same proportion; and it has only one wall, being a simple invagination of the 

 outer w^all of the lower floor of the disk. The eye peduncles {Fig. 17 h) have 

 changed appreciably, only in becoming hollow to the base of the eye-facets. The 

 oculiferous lobes (/ /) are less than one quarter as long as the diameter of the 

 disk, and have lost in consequence the conspicuous prominence which they held 

 in earlier stages, and in which they were the chief characteristics of the ephj'ra. 

 The tentacles have increased to fourteen in number {Fig. 17 i^) in each marginal 

 segment, and the marginal or tentacular lobes (Z^) are correspondingly numerous. 

 The oldest tentacles are at the middle of each marginal segment; and from this 

 point they decrease in age and size each way toward the oculiferous lobes. At 

 this age the relation of the tentacles to the margin of the disk appears to be 

 quite complicated ; but when fully understood it is quite simple. In a view either 

 from above (PL XP. Fig. 3) or from below {Fig. 4), the tentacidar lobes (r) which 

 project from the margin of the disk between the tentacles, in the form of vertical 

 ridges with a rounded contour, are quite as conspicuous as the tentacles themselves. 

 These lobes are simply outwardly folded diverticuli of the exterior wall alone (k) ; 

 the inner wall projects but a short distance, and stands across the base of the 

 lobes, like a bridge ()•/) ; and in this Avay the lobe becomes a completely closed 

 cavity {x). In a view from above, the inner wall folds upon itself two or three 

 times, and therefore presents as many outlines ()j & C) at different depths. Between 

 these lobes there is a deep socket, from the bottom of which a tentacle arises, 

 and the outer {a) and inner {(i) walls of the tentacle are directly continuous at 

 the base {y d) with the outer (e) and inner {■)]) walls of the labes. At the margin 

 {Figs. 3 and 4 |) of the sockets, the outer (e) and inner {tj) walls touch each 

 other, and continue so directly to the bottom, where they are continuous with 

 those of the tentacles, as we have already pointed out. The tentacles are hollow 

 {X fi) about half way to the tip, and have very thick walls {a /5). 



In the next stage (PI. XP. Fig. 29; PI. XF. Figs. 5, 6, 19, and 20; PI. XP. 

 Figs. 5, 6, and 13), the development is purely a matter of degree: the disk is 

 a little more than an inch in diameter; the marginal segments extend over more 

 than two thirds of the whole circumference; the tentacles are thirty-two iir number 



