132 DISCOPHOR^. Part III. 



aperture, from the corners of which project four long arms, furrowed along the 

 middle, and leading into the main cavity. This structure, however, varies greatly 

 with age in Aurelia. In the young the central aperture is only a broad funnel 

 with four sides, more or less flattened, the angles of which become prominent, lobed, 

 and fringed, until regular arms, with a deep furrow in the centre, have been formed, 

 communicating with the wide central opening; and the edges of the arms them- 

 selves are so folded as to present numerous minor furrows, leading from the sides 

 towards the main central channel. In fact, the arms, with their middle and lateral 

 channels, are only the prolonged margins of the mouth, the whole surface of which 

 leads to the mouth. 



As Aurelia grows older, the arms become thicker along their centre, and the 

 thin margins are folded against one another, their edge alone remaining pliable 

 upon the sides of the stifFer axis; but as these edges are themselves wider, longer, 

 and more sjjreading than the axis, they fold, bend, and twist in every direction, 

 from both sides, until, at last, these winding folds become also harder and stiffer, 

 and can neither be fully opened nor stretched, so that, though the margin of the 

 arms is free and open, from tip to base, and can be laid out like a flat leaf, with 

 comparatively little effort, each arm of an adult Aurelia forms, in reality, a sj^stem 

 of flat channels, gaping along the margin, and uniting into fewer and fewer rami- 

 fications toward the middle line of the arm, along which runs the larger channel 

 which terminates in the mouth. The central aperture, or the mouth itself, under- 

 goes identical changes. Its walls become thicker and stiffer, and less movable, and 

 are finally thrown into such folds as fit one against the other so closely, that, in 

 the end, the oral aperture is transformed into a system of capillary surfaces, between 

 the folds of the actinostome, leading into the main cavity. 



Now such is exactly the structure of a Rhizostome, with this exception only, 

 that the margins of these capillary surfaces interlocked with one another, are sol- 

 dered uj), and present, only at intervals and in particular places along the edge, 

 which vary in different genera, apertures which through life remain open and keep 

 up a communication between the surrounding medium and the main cavity, and 

 through which the food necessary for their sustenance is absorbed. I know, from 

 direct observation of the young of Polyclonia frondosa, one of the earliest Rhizosto- 

 midie known to naturalists, that in this species at least, the young has a simjjle 

 funnel-shaped mouth, as widely open, as freely gaping, and as directly communi- 

 cating with the central cavity of the body, as in the young Aurelia and the young 

 Pelagia (Pis. X. and XII.). I know, further, that in more advanced young the 

 angles of the mouth begin to project, in the shape of arms with oj^en and free 

 mai'gins, as in Aurelia, Cyanea, and Pelagia. And though I have not actually seen 

 the margins of the mouth of any specimen of this species grow together, in such 



