Soon after the invention of the microscope, the animalcula of infusions became a favourite subject for its employment, and the cilia and the motions which they produced did not escape the notice of the earlier microscopic observers. Leeuwenhoek observed them dis tinctly and recognised their use, and probably he was the first that did so. He repeatedly makes mention of them in his writings. At one place* he describes them in an animalcule, which seems to have been the vo/vox, as short slender organs projecting a little from the body, by means of which the animal produced a re volving motion and moved onwards. Againif in speaking of the animalcules which he ob tained from an infusion of pepper, he states that these animals produced a great commotion in the water by means of divers organs placed on the fore part of the head, which organs also the animals used in swimming. " In this way," says he, " they occasioned such a cir cular eddy in the water that not only several small bodies floating in the water were moved in a circular manner, but even many very minute animalcules, though able to swim vigorously, when they approached the larger animalcules, were whirled about for some time in a circular manner." In announcing his discovery of the wheel animal,* he describes its rotatory apparatus as two projecting discs set round with very slender elongated' organs. " Imagine," says he, " two wheels set round with points of needles, and moved very swiftly round from west by the south to the east." He adds that he cannot comprehend how sueh motion takes place in a living body. Lastly', in describing a small animal which lie found adhering to the water-lentil, (probably a species of vorticella,) and speaking of the currents which it excites, and by which it attracts its food, he adds the following reflection :I- " More over it is necessary that these animals, and in general all such as are fixed and cannot change their place, should be provided with an appa ratus for stirring up motion in the water, by which motion they obtain any matteks that.float in the water, for their nourishmentand growth and for covering their bodies." Baker,t next to Leeuwenhoek, takes notice of the cilia of animalcules. He observed them in many species, and named them fins, or feet, and sometimes fibrillm. He distinctly recog nised the currents produced by them, and in ferred the existence of cilia as the cause of visible currents in cases where the cilia them selves could not be seen.§ In particular, he bestowed much pains in investigating the eco nomy of the wheel animal previously disco vered by Leeuwenhoek, and addressed a letter to the Royal Society on the subject, in 1744.11 He there describes its rotatory apparatus as tc a couple of semicircular instruments round the edges of which many little fibrillw move themselves very briskly, sometimes with a kind of rotation, and sometimes in a trembling or vibrating manner,"Ilf—" by this means a cur rent of water is brOught from a great distance to the very mouth of the creature, which thereby is supplied with many little animalcules and va rious particles of matter."'" He also states that the wheels are instruments of locomotion by which the creature swims.tt Baker drew a distinc tion between the rotatory and vibratory motions of the cilia, these organs being moved in some animals in the one way, in some in the other, while in others they seemed capable of being used in both ways.n It appears that he was aware of the true structure of the So-called wheels, and though he often speaks of their being turned round, he was still doubtful of the reality of the apparent rotation.
Spallarizani, in his curious and interesting researches on the productidit and economy Of the Infusoria, made observations similar to those of Baker on the cilia and their 'motions. He describes them as small filaments or points agitated with a vibratory or oscillating motion. He conceived them to be organs of locomotion which the animals used in swimming,* *and that they also served to -excite a vortex or cur rent by means of which food was brought to the' mouth. " The oscillating filaments cause the vortex; the vortex draws the floating par ticles into the aperture or mouth of the animal cule, and the latter chooses fait§ alimerit:the most delicate, or at least thoSe which suit:it best."-f He afterwards describes the ciliary apparatus of the vorticella in a similar man ner:I In the account of his' singular exfieri ments on the apparent resuscitation of the Ratijer,' he describes its wheel organs as two circles of filaments, exactly like the vibrating filaments of other Infusoria, which by their continued motion givP rise to the appearance of two moving wheels; but he distinctly states that the rotation is only apparent, not real.
These organs, he adds, serve the' same purposes as the` simple cilia.§ • Needham,11 about the same time as Spallan zani, correctly observed the cilia, and recog nized their uses. Saussurell observed the cur rents, but did not perceive the cilia. Pallas,'" in his systematic work on Zoophytes, describes the eddies or currents'produced by certain Roti fera, and notices their cilia,' but far leSs clearly than his predecessors. Wrisbergif observed the currents and eddies produced* by the vorticellw; at least he saw smaller Irifusoria arid particles of floating, matter hurried on' towards- their mouths, but he seems licit to have perdeived the cilia.
OttO Frederick Miiller,tt in his Systematic work on the Infusoria, described the appear ance and arrangement of the cilia- in each species; and represented them in figures. He named them cilia and pili, and ascribed to their action the currents and vortices which the Inftisoria excite. I3ut while he assicins to them the office of locomotive organs, he °denies that they are employed in seizing food ; for, what is singular, in his long-continued and elaborate inquiries into the economy of these animals, he could never perceive that foreign matters drawn into the mouth were retained there as nourishment, but believed that they were alvvays again thrown out. In this, however, he was undoubtedly mistaken.
Gleichen,* in 1778, described the currents produced by the vorticeM. In an earlier work-he ascribed an agitation of small bodies, which he had observed in the neighbour hood of one of the Infusoria, to an electric or Magnetic force, not having perceived the cilia.t Fontanat described the rotatory apparatus of the Rotifer and its use ; he conceived that its apparent rotation was produced by the succes sive elevation and depression of the cilia which encircle it.
Of the more recent writers who have inves tigated or described these phenomena in the Infusoria, I may mention Dutrochet,§ Gruit huisen,II Agardh,lf Raspail," and Ehren berg.tt Raspail denies the existence of cilia, attributing their appearance to an optical de ception, an opinion which is undoubtedly erroneous. Ehrenberg, who, of all recent ob servers, has contributed most to the knowledge of the economy and natural history of the Infusoria, has particularly investigated the structure and mode of action of their cilia. The substance of his observations has been already given.
The ciliary motion has been recently ob served in the embryoof Infusoria while enclosed in the ovum.II 2. Polypi and Sponges.—a. Fresh-water polypi. The phenomena in question have not been discovered in the Hydra, which is the largest and best known of the Fresh-water Polypi; but they have been seen and described by ' many observers in another sort, viz. that known by the names of the Polype a panache, or Plumed Polype of Trembley, the Bell flower animal of Baker, and Plumatella, Cris tatella, Alcyonella, &c. of other naturalists. The Polypes of this kind are connected in groups on a common stock or stem, (a, a, jig. 291, which represents the animal magnified,) and each is furnished with a tube (b, b') into which it can wholly withdraw itself. From time to time they advance a little way out of the tubes and display a double row of arms or tentacula (c) ranged round the mouth in. the figure of a horse-shoe. When the arms are spread out in this manner, cur rents appear in the surrounding water, which are made evident by the motion of any small particles that may accidentally or intentionally be suspended in it. The currents pass along the tentacula, the water being drawn towards them from every side, and the main stream at last issues from the midst of them, appearing as if it came out of the mouth, from which, however, it really is not derived. The arms are fringed on their two borders with a mul titude of cilia, (see A, a single arm mag nified,) set close together, which vibrate in regular succession, their motion appearing like progressive undulations along the ten tacula. 'When one of the arms is cut off, it aficts the water in the same way as when con nected with the animal, its cilia impelling the fluid in a current, or carrying the separated arm through it, according as it is fixed or free.