CILIA. 



[ 143 ] 



CILIA. 



thaeae (Polypodiaceous Ferns). With a bi- 

 valve indusium. Exotic. 



Fig. 131. 



^^(  



Cibotium macrocarpum. 

 A pinnule vnth sori. Magnified 10 diameters. 



CILIA (plural of ciliiim) of Animals. — 

 These are microscopic filaments attached by 

 one end to the surfaces of various parts of 

 animals, and exhibiting a vibratory or rota- 

 tory motion. They are usually rounded and 

 broadest at the base, tapering towards the 

 free end; sometimes they are flattened. 

 Their length is very variable, having been 

 estimated at 1-50,000 to 1-500"; probably 

 1-15,000 to 1-500'^ would include most of 

 them. The latter large size is attained by the 

 cilia existing on the point or angle of the gills 

 or branchial laminae of the whelk {Buccinum 

 undatum). 



Numerous examples of animals furnished 

 with cilia, showing their appearance when at 

 rest, are figured in PI. 23, 24, 25, 34, & 35. 

 During life, and for some time after death, 

 they are usually in constant motion, giving 

 the parts of the field of the microscope in 

 which they are situated a tremulous appear- 

 ance when their motion is very rapid and the 

 cilia are very minute. When they are large, 

 as on the gills of the common sea-mussel 

 (Mytilus), especially w^hen then' motion is 

 slackening, they are seen waving to and fro, 

 or lashing the water, and producing in it 

 strong currents, rendered visible by the mo- 

 tion of minute particles accidentally con- 

 tained in the water. The motion is mostly 

 uniform, or in one direction; occasionally, 

 however, it has been observed to cease for a 

 moment, and then to assume an opposite 

 direction to that previously exhibited. Du- 

 ring the motion, the whole filament is usually 

 more or less curved ; but in some instances 

 among the Infusoria, the basal portion of the 

 cilia remains rigid, whilst the terminal por- 

 tion vibrates ; under these circumstances, 

 the cilia are distinguished as flagelliform 

 filaments. Sometimes the cilia move around 

 an imaginary perpendicular axis, in a rotating 

 direction. 



Cilia are found in all the Vertebrata and 



the Invertebrata, excluding the Crustacea, 

 Arachnida, and Insecta. In Man, they 

 spring from epithelial cells ; the localities 

 in which they are found are stated under 

 Epithelium. 



The uses of the cilia are of two kinds : 

 when the body to which they are attached is 

 of no great bulk or specific gravity, compared 

 with that of the medium in which they re- 

 side, the cilia become organs of locomotion, 

 as in the Rotatoria, Infusoria, the young 

 Acalephse, the ovum, &c. But if the inertia 

 of the body be too great to be overcome by 

 the feeble power of the cilia, they produce 

 motion in the surrounding medium, as on 

 gills of fishes, of young reptiles, and of the 

 Mollusca, the gill-tufts of the Annulata, and 

 the various mucous surfaces of the Vertebrata 

 uponwhich they exist, in which they favour re- 

 spiration and excretion. By the same agency 

 they also bring particles of food suspended 

 in the medium towards the mouth. It need 

 scarcely be remarked, that the motion of 

 cilia must be stronger in one dhection than 

 the other, otherwise there could be no current. 

 The cause of the motion of ciUa has long 

 formed a subject for discussion; it is un- 

 known. In some instances, as in the Infu- 

 soria, it appears to be voluntary. In some 

 cases it might be attributed to the action of 

 a contractile amorphous tissue, such as that 

 composing the AmcEhcB. It would naturally 

 be attributed to muscular agency. But no 

 muscular tissue can be detected; in fact, 

 cilia are quite structureless. Moreover, they 

 are often of less breadth than the ultimate 

 fibrillse of muscle. Neither the most power- 

 ful poisons, as strychnine, prussic acid, opium 

 and belladonna, nor electricity, produce any 

 efi'ect upon ciliary motion, provided the 

 structure upon which the cilia are situated 

 be not injured. It also lasts a long time 

 after death, having been observed in the 

 low^er animals nineteen days after this occur- 

 rence, and when putrefaction was far ad- 

 vanced. The question has however lost its 

 interest in regard to its necessary dependence 

 upon muscular action, because cilia are com- 

 mon among the lower plants, where this is 

 out of the question. 



The cilia and their motion may readily be 

 observed in the common Rotatoria and Infu- 

 soria, in a thin piece cut fi'om the margin of 

 the gills of the oyster, or still better, the 

 sea-mussel ; in the latter, they form a most 

 beautifid and interesting object. Fresh- 

 water almost immediately arrests the motion 

 of the cilia in marine animals. In some cases. 



