Aurelia. (Aurelia flavidula Pér. et Les.)
Fig. 49. Aurelia seen in profile, reduced. (Agassiz.)
Another large Discophore, though by no means to be compared to the Cyanea in size, is our common Aurelia ([Figs. 49],[ 50]) Its bluish-white disk measures from twelve to fifteen inches in diameter, but its dimensions are not increased by the tentacles, which have no great power of contraction and expansion, and form a short fringe around its margin, widening and narrowing slightly as the tentacles are stretched or drawn in. It is quite transparent, as may be seen in [Fig 49], where all the fine ramifications of the chymiferous tubes as well as the ovaries, are seen through the vault of the disk. [Fig. 50] represents the upper surface, with the ovaries around the mouth, occupying the same position as those of the Cyanea, though they differ from the latter in their greater rigidity, and do not hang down in the form of pouches. The males and females in this kind of Jelly-fish may be distinguished by the difference of color in the reproductive organs, which are rose-colored in the males, and of a dull yellow in the females. The process of development is exactly the same in the Aurelia as in the Cyanea, though there is a very slight difference in their respective Hydroids. They are, however, so much alike, that one is here made to serve for both, the above figures being taken from the Hydroid of the Aurelia. It is curious, that while, as in the case of the Aurelia and Cyanea, very dissimilar Jelly-fishes may arise from almost identical Hydroids, we have the reverse of the proposition, in the fact that Hydroids of an entirely distinct character may produce similar Jelly-fishes.
Fig. 50. Aurelia flavidula, seen from above; o mouth, e e e e eyes, m m m m lobes of the mouth, o o o o ovaries, t t t t tentacles, w w ramified tubes.(Agassiz.)
The embryos or little planulæ, hatched from the Cyanea and Aurelia in the fall, seem to be gregarious in their mode of life, swimming about together in great numbers till they find a suitable point of attachment, and assume their fixed Hydroid existence. The Cyaneæ, however, when adult, are usually found singly, while the Aureliæ, on the contrary, seek each other, and commonly herd together.