The observations of Ehrman led Trevimnus to investigate the subject;* and he distin guished two different motions, the one a mus cular contraction, the other the peculiar motion alluded to by Ehrman. The latter motion had the appearance of a trembling or flickering of innumerable points, and seemed at some places as if produced by a moving fluid, and at others by the agitation of oblong vibrating organs. It was peculiarly distinct alongside each of the bars of the gills and appendages. He fluffier perceived that the agitation on the surface of these parts caused an eddying mo tion in the water in which they lay, and also set in motion globules of blood which had escaped from the vessels. On breaking down the parts into small fragments, he found that each retained its power of motion, by which they moved in most manifold directions, the larger masses at the same time contracting and dilating themselves. From these observations Treviranus concludes that the bivalve Mollusca afford an example of a structure in which the integrant parts possess an independent vitality. Their independent vitality shews itself in the persistence of their automatic motion after solution of organic connexion with each other, and this motion is intermediate in its nature between the spontaneous movements of organic molecules in infusions, the male semen, &c. and the motion of muscular parts, which re quires the integrity of the texture and the application of a stimulus. These reflections on the relation of the phenomenon to the general laws of organization are the sole infe rences which he draws from his observations. He notices the motion of the water only as a concomitant and subordinate circumstance, not having been aware of its determinate direction, its relation to the respiratory process, or, in short, of its being the chief end and effect of the motion of the cilia.
The next researches on the subject are those of Huschke, narrated in a paper in the Isis for 1826.f Not having seen the original, we must content ourselves with a brief notice of them to be found in Burdach's Physiologiel It is there stated that on detaching a portion of the gill of the Fresh-water Mussel (Unio pictorum), Huschke found that the water " moved up wards on one side, and then in an eddying manner back again." Raspail, in a memoir on a species of fresh water polype, published in 1828,* pointed out the analogy between the phenomena exhibited by the gills of Mollusca and those observed in infusory animalcules and polypi.
Ciliary currents were now described by vari ous other writers of eminence, but their causes were very commonly mistaken : among the number may be quoted Poli,t Delle Chiaje,t Carus,§ De Blainville,11 and Ungerlf Having observed currents produced in other instances by an impelling power inherent in the surfaces over which the fluid passed, I was myself led to suspect that the respiratory cur rent in bivalve Mollusca was of the same kind, or that it was caused by an impulsion commu nicated to the water by the surface of the gills and other parts over which it was conveyed in its pa.ssage, without being aware of any similar
view having been entertained by others. I then observed the determinate direction of the impulsion along the surface, together with the arrangement and action of the cilia. These^ observations were published at the time (1830) in a paper already mentioned," in which also the respiratory currents of the bivalve Mollusca are considered as a particular example of a more generally prevailing phenomenon.
In a paper on the circulation of the blood, in Magendie's Journal for 1831,1-1- there are some remarks pertaining to the present subject, from which it appears that the author, M. Guillot, had observed the ciliary motion of the gills of the Sea-mussel and Oyster. He has, however, like Baker, mistaken the regular un dulations of the cilia for the circulation of a fluid within vessels. Ile takes no notice of any motion or current excited in the water.
Carus,IT in a memoir on the development of the River-mussel, states that he observed an undulatory or oscillatory motion of the gills, and that by this motion, which he conceives to be in the substance of the gill, the water is propelled, and the general respiratory current through the branchial cavity produced. It is obvious that what he calls an oscillation of the substance of the gill, and which he erroneously supposes has previously escaped attention, is merely the undulatory motion of the cilia.
The last researches on this subject which we have to notice are those of Purkinje and lentin.§§ As above stated, they discovered the ciliary motion in the alimentary canal of the Mollusca, having found it in the Lymmea, Pa ludina, and the Fresh-water mussel.
Such is an outline of the observations hitherto made relative to the ciliary motion in the bivalve Mollusca. We may now shortly con sider those which refer to the other classes of these animals.
Dr. Fleming,* in describing the cilia in some species of Polypi, states that " analo gous hairs" exist on the branchiw of the Tri tonia, which may probably be considered as forming part of the aerating organs. He also mentions, in another place,t that these branchiw " readily fall off, and, as if indepen dent, are capable of swimming about for a short time in the water, by means of minute hairs with which their surface is covered, and which move rapidly, pushing forwards the distal extremity. Gruithuisen, as formerly mentioned, observed the ciliary motion, and recognised its true nature in the Valvata bran chiata, a species of fresh-water snail. Also Itaspail,t havincr seen the phenomena pro duced by the gills of the Fresh-water Mussel, was led by analogy to discover the same in the Lymnwa and Paludina. Without being aware of these previous researches, I observed the ciliary motion in several different tribes of marine Mollusca, and shewed that it prevailed extensively among Mollusca generally. Mr. Lister, as has been already stated, has subse quently discovered that it exists in the Ascidia; and since then .I have also found it in that animal, though in a different species.