9. W the ciliary motion of the embuo of 1VIollusca.—The embryo of Mollusca exhibits, while within the egg, a peculiar rotatory mo tion which belongs to the class of phenomena we are here considering, and is referable to the same cause. This motion has been observed in the Gasteropodous and Bivalve Mollusca, and may perhaps be found in others.
Gasteropoda.—Swammerdam§ states that in examining the young of the viviparous water snail, while they were yet inclosed in the mem branes of the ovum, he observed the embryo turning round in the contained fluid with con siderable rapidity, and, he adds, " in a very elegant manner." He again. mentions the fact in another place.II Baker observed the same appearance in the ova of a fresh-water snail, which appears to have been the common Lyin nwa. He says,11 " when the eggs are about a week old, the embryo snail may be discerned in its true shape, turning itself very frequently within the fine fluid in which it lies." These brief notices of this remarkable fact by Swam merdam and Baker seem to have failed to ex cite the curiosity of succeeding naturalists, for there would appear to be no account of any subsequent researches on the subject till those of Stiebel published in 1815," who seems not to have been aware that the fact had been pre viously noticed. Stiebel's observations were made on the ova of the Lyrimmus stagnalis. They were followed by those of Hugi* in 1823, and Carus in 1824,t on the same species, to which Carus afterwardst (in 1827) added cor responding observations on the Paludina vivi para. About the same time (1827) Dr. Grant extended the inquiry to salt-water Gasteropoda, both naked and testaceous, and, as far as I know,l,vas the first to point out the cilia, which are very conspicuous in salt-water species, as the agents which cause the rotation.
The eggs of the Lymnmus (or Lymnwa) are deposited in clusters, being imbedded in oblong masses of gelatinous matter that are found ad hering to stones or water-plants. Each egg consists of an oval pellucid membrane, con taining within it the yolk surrounded by a con siderable quantity of limpid fluid. The yolk is at first round, without any obvious distinc tion of parts, but in the progress of develop ment it changes its figure, and is gradually converted into the embryo, of which the shell and several principal organs can soon be dis tinguished. From the descriptions of the au thors above mentioned, .as well as from some observations made by myself, it appears that the embryo is at first motionless, but that as soon as the distinction can be perceived be tween the anterior or cephalic extremity and the rest of the animal, its rotatory motion com mences. This invariably goes on in the man
ner indicated by the larger arrows (c, c) in the annexed figure, the head or anterior extremity continually receding. After a. time the rota tion is combined with a progressive motion, by which the embryo, while turning on its axis, moves onwards at the same time along the inside of the egg, performing a circuit like a planet in its orbit. The path described by a point on the surface is indicated by the spiral line in the figure.
Stiebel, as well as the earlier observers men tioned, is silent as to the cause of this curious phenomenon. Carus§ at first denominated it a primitive or cosmic motion, without clearly explaining what he meant by the term. Having subsequently discovered that a current existed in the fluid in an opposite direction to that followed by the embryo, he ascribed the mo tion to an attraction and repulsion exerted by the substance of the embryo on the surround ing fluid,* more especially at the region of the body where the respiratory organ was afterwards to be developed, and justly conceived that the chief purpose served by it was to renew the water on the respiring surface of the embryo. The attraction and repulsion again he supposed to be produced by an oscillatory motion which he perceived on the surface of the embryo. This oscillatory motion, although he describes it as taking place in the substance of the animal, seems to be nothing else than the usual undu latory play of moving cilia, such as has been already described in other instances,—indeed be himself compares it to the undulation on the arms of polypi. I have distinctly perceived the cilia, though they are very small, in the embryo of the small species of Lymna2a com mon in this country. It is the one represented in the figure, but considerably magnified. The current takes place along the whole of the sur face indicated by the small arroN.vs, which also mark it,s direction, being opposite to that in which the embryo moves. The cilia, though they probably exist over all this surface, were distinctly seen only on the part inclosed be tween the dotted lines at a ; it required a dou blet of one-thirty-fifth of an inch focus to make them visible.