NOTES. 329 



scope, a transverse cut of the polypidom in its natural state, we distinguish in it an 

 analogous conformation, the external wall of the cells being not composed of 

 layers, but rather of cylinders or irregular prisms placed perpendicularly to its 

 surface. 



" As to the operculum, which serves to shut the entrance of the tegumental 

 cell of the Eschara, when the animal is wholly concealed in it, it is but a labial 

 fold of that which we may call the skin of the polype, and of which the pro- 

 jecting margin has acquired a horny consistence, whilst that portion continuous 

 with the general envelope, preserves sufficient softness to remain flexible, and 

 to obey the action of the muscles whose tendons are inserted in its thickness. 



" The changes which we have indicated above in the external formation of the 

 cells of the Eschares are not the only ones effected by the progress of age in the 

 stony integuments of these zoophytes. The form of their opening is modi- 

 dified considerably, as may be seen by the figures which accompany this memoir -. 

 the sinus or emargination situated under the operculum disappears by degrees, 

 and their interior ca\Tty becomes filled up so as not to occupy more than about 

 the quarter of their original diameter. This thickening changes even a little 

 the genend appearance of the polypidom ; for as it is more considerable in the 

 cells situated farthest from the extremities of the branches, it results that 

 these, at first almost flat, become more and more cylindrical. Lastly, it is not 

 without surprise that we have seen these same cells when they are arrived at ex- 

 treme old age, lose altogether the opening from which the polype extended 

 its tentacula. In fact, the margins of this opening swelling more and more out- 

 wardly come at last to touch and to close, so that no trace of its existence is 

 left : but the cell, now a shut cavity, still exists towards the axis of the polypi- 

 dom. 



" Thus, then, the last external mark of the individual existence of these col- 

 lected polj-pes, disappears before that life is extinct in the interior, and the most 

 remarkable character of the polypidom is lost without hope of recovery. 



" Reflecting on the fact we have just noticed, we are naturally led to ask how 

 the nourishment necessary for the support of the secretions on which the pro- 

 gress of consolidation depends, can continue when the cell containing the di- 

 gestive apparatus of the animal is shut up in this manner. Is it from its neigh- 

 bours that it receives its nutritive matters, or can it continue to absorb them direct- 

 ly from without through these stony integuments? The nature of this solid shell 

 seems at first sight to oppose great obstacles to this imbibition, particularly to 

 that which would take place by the free surface of the polypidom, but an ex- 

 periment which is, so to speak, the counterpart of that which has been already 

 detailed, shov/s that it is otherwise. 



" On boilinga fragment of the solid polypidom ofan Eschara in a solution of caus- 

 tic potash, I have extracted the major part of the substances which compose the 

 organized part of its tissue, and I have then seen that the appearance of the 

 polypidom is considerably changed. The external parietes of the cells had become 

 of an almost spongy texture, and its surface, instead of being simply granular, 

 presented a great number of very distinct pores, which were before concealed by 

 the soft parts with which they were filled. 



" We may understand, then, that the organized tissue of the old polypes finding 

 itself without covering in different points of the external surface of the cells. 



