By means of the remarkable provision here de scribed, the ova of these fixed zoophytes are dis seminated, and conveyed to situations suitable to become the abode of the future individuals. The same provision undoubtedly serves also to move the water along their suiface for the pur pose of respiration. It exists, as will be after wards shown, in the ova of many other ani mals.
4. Acalephe.—Many species of Medusw are furnished with cilia, or at least with moving organs bearing a close resemblance to the cilia of other animals, though in the Medusze they present several peculiarities. The cilia. are found in all the AIedusae belonging to the order Ciliograda of Blainville, or Ctenophora. of Eschscholz, of which the genus Beroe is a good example. Eschschole describes them as small pectinated or comb-like organs, ranged in longitudinal rows or stripes on the external surface of the body, with their flat surfaces in apposition. Each comb-like organ consists of many small, flattened, pointed filaments, united together by a common base, the points being directed towards the posterior extremity of the body. They are moved like fins, being slowly raised and suddenly struck back, by which means the body is carried through the water. In the Beroe and others of similar form, the cilia point towards the closed extremity of the body, so that the opposite or open end is carried forward. The animal seems to have the power of ,moving more or fewer of' these organs as it may incline, by which means other motions besides direct progression are per formed. The cilia, w.hen separated Jrom the body with a piece of skin, continue to move briskly for some time. A longitudinal vessel runs under each row of cilia, com municating with the rest of the vascular system, and containing a fluid, in which yel lowish particles are suspended. Eschscholz regards these vessels as arteries, and considers the cilia as respiratory as well as locomotive organs. Dr. Grant, in describing the cilia of the Bera pileus,t represents the parallel fila ments of which the comb-like organs consist, as united together by a membrane as far as their points, like the rays in the fin of a fish.
Schweigger compares the vessels which run underneath the rows of cilia, to the canals com municating with the tubular feet of the Sea urchin and Asterias ; and Dr. Grant seems
also inclined to ascribe the motion of the cilia, whose filaments he conceives to be tubular, to their being alternately filled and emptied of fluid derived from the longitudinal vessel, like the tubular feet of the Echinodermata. This view of their mode of action, however, is scarcely reconcilable with the observed phe nomena, as will be afterwards shown in con sidering the structure of the cilia in general. Audouin believed that in the Idya, a genus nearly .allied to the Beroe, the fluid of the longitudinal vessel, which he supposes to be water, is sent into the cilia; he therefore regarded them as respiratory organs. If the vessel under the cilia in this case, as in the Ben* communicate with the rest of the vas cular system, and its contained fluid be re aarded as blood, then the cilia ,of the Idya, which, according to Audouin, are permeated by the fluid, would bear a certain analogy to the gills of fishes.
Cilia appear also to exist in other tribes of Medusw besides the Ciliograda but they differ in form and situation from tflose described, and have not been investigated with equal accuracy.
In Rhizostoma there are certain membranous appendages attached to the arms or tentacula, and bearing on their free edge a fringe of short filaments which are constantly in motion, and continue so for some time after the arm or portion of membrane supporting them is de tached from the body. These filaments are described and figured by Eysenhardt,* who regards them as organs of generation ; they are probably of the nature of cilia. Similar fila mentary organs seem also to exist within the body in some Medusw. (See ACALEPHX7 p. 48.) 5. Actiniee.—In a paper published on the present subject in 1830,1 I mentioned that I had found the ciliary motion in the Actinia or Sea-anemony, but gave no description of it. I have since re-examined various species of Actiniw with this view, and shall now describe the appearances ; but to make the description intelligible, it may not be improper to remind the reader of some points in the anatomy of these animals which require to be kept in wiew.