608 



CILIA. 



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 such 

 motion takes place in a living body. Lastly, 

 in describing a small animal which he 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 :f " 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 matters that float 

 in the water, for their nourishment and growth 

 and for covering their bodies." 



Baker,! next to Leeuwenhoek, takes notice 

 of the cilia of animalcules. He observed them 

 in many species, and named them fins, or feet, 

 and sometimes fibrillae. 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.JJ 

 He there describes its rotatory apparatus as 

 " a couple of semicircular instruments round 

 the edges of which many little fibrillae move 

 themselves very briskly, sometimes with a kind 

 of rotation, and sometimes in a trembling or 

 vibrating manner,"^ " 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.ff 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.jj It appears that he was 

 aware of the true structure of the so-called 

 wheels, and though he often speaks of their 



* Continuatio Arcanorum Naturae, 1719. p. 386, 

 Epist. 144. 



t Epistolae Physiologicae, 1719, p. 66. Epist. 7. 



^ I cite his work entitled " Of Microscopes, 

 and the Discoveries made thereby," London, 1785, 

 although his observations were previously related 

 in separate memoirs of a much earlier date. 



S Of Microscopes, vol. i. p. 71, p. 80. 



\\ Reprinted in op. cit. ii. p. 267. 



IF P. 271. 



* P. 273. 



tt P. 284. 



n P. 292. 



being turned round, he was still doubtful of 

 the reality of the apparent rotation. 



Spallanzani, in his curious and interesting 

 researches on the production 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 for its aliment 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.;}: In the account of his singular experi- 

 ments on the apparent resuscitation of the 

 Rotifer, he describes its wheel organs as two 

 circles of filaments, exactly like the vibrating 

 filaments of other Infusoria, which by their 

 continued motion give 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,|| about the same time as Spallan- 

 zani, correctly observed the cilia, and recog- 

 nized their uses. Saussurelj 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 Itoti- 

 fera, and notices their cilia, but far less clearly 

 than his predecessors.. Wrisbergft observed the 

 currents and eddies produced by the vorticellae; 

 at least he saw smaller Infusoria and particles of 

 floating matter hurried on towards their mouths, 

 but he seems not to have perceived the cilia. 



Otto Frederick Miiller,^ 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 

 Infusoria excite. But while he assigns 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 

 always again thrown out. In this, however, 

 he was undoubtedly mistaken. 



* Opuscules de Physique, torn. i. p. 180. 

 t P.183. 



* P. 199. 



$ Tom. ii. p. 227. 



|| Spallanzani, Nouvelles Recherches sur les 

 Decouvertes Microscopiques, &c. 1769, p. 161. 



^ See Letter by Bonnet, in Spallanzani Opus- 

 cules, torn. i. p. 176. 



** Elenchus Zoophytorum, 1766. 



ft Observationum de Animalculis Infusoriis sa- 

 tur'a, 1765, p. 52, p. 63. 



JJ Vermium Terrestrium et Fluviatilium His- 

 toria, 1773, and Animalcula Infusoria, 1788. 



