Chap. V. THE RHIZOSTOME^ IN GENERAL. 133 



a manner as to close up the mouth, yet the fact, that hi a more advanced stage of 

 growth, specimens found together in the same shoal, and in no way differing from 

 one another in other respects, have the margins of the arms and of the edges of the 

 mouth so united, at intervals, that they cannot be spread out or easily opened without 

 tearing, as well as the additional fact, that in still older specimens, not, however, 

 exceeding one or two inches in diameter, the extent of the union of the edge 

 of the mouth is so great, as to leave only comparatively few passages for a free 

 communication of the surrounding medium, with the main cavity of the body, shows 

 most unquestionably that the seeming absence of the mouth in Rhizostomeaj is 

 only the result of a gradual closing up of the margins of the actinostome, which 

 takes place, sooner or later, and to a greater or less extent, in different genera. 

 In the adult Aurelia the margins of the anns are approximated together closely, 

 and all but closed up in the latest period of their growth, though, when young, 

 they form simj^ly a wide funnel. In RhizostomidiB, the edges of the actinostome, 

 starting also fi-om a wide funnel, are very early closed up, leaving only passages 

 between their edges, in their pei'ipheric prolongation; so that, through life, nutrition 

 goes on through the narrow channels between the comparatively few open spaces in 

 the peripheric portion of the arms, which are very early closed in its central portion. 

 With such a tendency to the obliteration of the passage between the marginal 

 prolongation of the actinostome, in the centre of the lower floor, it is not surprising 

 that among the Rhizostomidte the central part of that system should acquire the 

 singular complications which we observe among the Cassiopea3 and in Polyclonia ; 

 but all these complications in no way conflict with the explanation I have here 

 given of the polystomy of these Acalephs. 



In order fully to appreciate the differences upon which genera, and perhaps 

 families also, may be distinguished among the RhizostomeiB, it is important to ana- 

 lyze the elements of structure of the lower surface of their umbrella, and especially 

 that of its central part. The great cavity which hangs, like a sac, imder the centre 

 of the umbrella, has walls of very unequal thickness. Very thin where the ovaries 

 are situated, this sac seems there to be perforated by holes, when, in reality, the 

 wall is only extremely thin, movable, and capable of great exj)ansion and con- 

 traction ; but the bunches of ovaries and spermaries, which project from these holes, 

 like a hernia, into the main cavity, are mostly so large as to increase the impres- 

 sion that there are real holes in those places. The spaces of the walls alternating 

 with the ovaries are much thicker, and form, as it were, j^iHars, converging toward 

 the central disk in the shape of a branching stem. Now, in this region, we must 

 distinguish three parts: first, the pillars or arches arising between the ovaries and 

 converging toward the centre. These arches may be compared to roots of the 

 stem, which hangs down in the form of arms. They are longer or shorter in 



