Appearances similar to those described were discovered hy Dr. Grant in the ova of Marine Gasteropoda. In examining the embryos of the Buccinum undatum and Purpura lapillus, which are inclosed in groups within transparent sacs, he was struck with a rapid and incessant motion of the fluid in the sac towards the fore: part of the embryo, and he observed that this motion was produced by cilia placed around two funnel-shaped projections on the fore part of the young animal, which form the borders of a cavity in which he, perceived a constant revolution of floating. particles. He also ob served these circles of cilia in the young of other testaceous Alollusca, as the Trochus, Nerita, &c. in which the embryo was seen re volving round its axis. He met with the same appearance in the naked Gasteropoda, as the Doris, Eolis, &c. The. embryo of these re volves round its centre, and swims rapidly forward by means of its cilia, when' it escapes from the ovum. My own observations on the ova of the Buccinum agree generally with those of Dr. Grant. The larger cilia are placed round the prominent border of a cavity on the fore part of the body, but the surface of the foot and other neighbouring parts is also ciliated, though the cilia are there much smaller. Dr. Grant assigns various uses to these motions ; it seems not to have occurred to him that they were connected with respiration, although there can be little doubt that they are principally subservient to that function.
Acephala.—The rotation of the embryo of bivalves was discovered by Leeuwenhoek, and described by him in one of his epistles, dated October, 1695.$ On examining the ova of a species of Fresh-water Mussel with the micro scope, he observed the embryo turning slowly round within the egg, like a sphere revolving on its axis. This was at a time when the shell could be distinctly perceived on the young mussel ; he had failed in discovering the phe nomenon in some ova of the same species which he had examined at an earlier period of advancement.t He adds, that he was so much delighted with the spectacle of the young Mus sels turning round within the egg, that he spent two hours along with his daughter and his draughtsman in contemplating it. Baster,t who wrote in 1762, seems to have observed an appearance of the same kind in the ova of the Oyster, if we may judge from a reference by Cavolini, for I have not been able to consult the original. More recently (1827) Sir E. IIome and M. Bauer§ perceived the motion in the embryo of the Fresh-water Mussel, as de scribed by Leeuwenhoek, but erroneously attri buted it to a small worm which pierces the egg and preys on the young mussel, and which, according to their vieiv, by dragging on it pulls it round in the manner described. Lastly, Carus subjected the phenomenon to a more careful investigation, in the course of his re searches on the development of the River Mus sell' According to his observations the em bryo, at the time the motion becomes percepti ble, has acquired a flattened triangular shape (fig. 308), the two halves of the shell cover its
two surfaces, and are united together by the hinge at the base of the triangle. When the ovum is placed under the microscope, the embryo is seen moving round in a ho rizontal direction, as indica ted by the larger arrows, ap pearing as if it turned on the centre of the lowermost shell.
When the embryo is extract ed from the egg, a current is perceived in the water opposite that part where the current en ters and issues in the adult animal, (as shown by the small arrow,) and Carus therefore attri butes its rotatory motion to an attraction and repulsion exerted on the water by that part of the embryo, which is afterwards to form the respiratory organ. The attraction and re pulsion of the water he supposes to be pro duced by an oscillatory motion observable in the substance of the animal at its surface, as in the embryo of the snail, which motion, as we have already seen, is in reality an undulatory movement of minute cilia. As in the snail also, he conceives the phenomenon to be con nected with respiration. For an account of his observations on the velocity and direction of the motion, and its supposed influence in de termining the figure of the animal, I must refer to the paper itself.
The analogy of these motions of the embryo of the Mollusca with the phenomena exhibited by the ova of Infusoria, Polypi, Sponges, and Actinite, already described, scarcely requires to be pointed out. We shall afterwards see that it extends to the ova of Batrachian Reptiles."4 11. Phenomena (1 the ciliary motion in the ciliary motion exists very ex tensively in vertebrated animals. Until lately it had been found only in the larvce of Batra chian Reptiles, but Purkinje and Valentirrf have recently made the important discovery that it exists also in adult Reptiles, Birds, and Mammiferous animals; and it seems to prevail generally throughout the three classes, having been found by these naturalists in all the nume rous examples of each class examined by them in the course of their investigations. It has not been found in Fishes, though many species have been submitted to examination.t The parts of the body which exhibit the ciliary motion in the Vertebrata are, the lining membrane of the respiratory organs, and that of the generative organs in the female. Besides this general situation, it is found on the external gills and surface of the body in the larvw of Batrachia, and on the surface of the embryo of these reptiles while contained within the ovum.