CILIA. 



607 



Fig. 289. 



Leucophrys 

 patula. 



sets its cilia in 



is beset with them. They may be often 

 most distinctly seen when their motion be- 

 comes languid or impeded, as is the case 

 when the water round the animal is diminished 

 by evaporation to such a degree as not to afford 

 scope for their full and rapid play. 



The cilia of the Infusoria in their arrange- 

 ment are either separate and independent, or 

 combined, forming in the latter case the rota- 

 tory or wheel-like organs of the rotiferous tribes 

 of animalcules. 



In the first or simple form, which exists in 

 the Polygastric Infusoria (Jig. 289), the cilia 

 are usually set round the 

 mouth or spread over the 

 body generally, in which 

 case they are often disposed 

 in regular rows. Their struc- 

 ture has been carefully in- 

 vestigated by Professor Ehren- 

 berg, who states that each is 

 furnished with a bulb at the 

 root, to which minute muscles 

 are attached. A slight degree 

 of rotation communicated to 

 the bulb causes a much more 

 extensive motion in the rest 

 of the organ, which in its re- 

 volution describes a cone. 

 From time to time the animal 

 motion, and then, if its body be free, the cilia, 

 acting like fins or oars, move it onwards through 

 the water, serving in this case as organs of lo- 

 comotion. If the body is fixed, the cilia com- 

 municate an impulse to the surrounding water 

 and excite a current in it. This may always 

 be made evident by mixing with the water 

 some colouring matter, the particles of which 

 are hurried along by the current. Many of 

 these particles are conveyed towards the mouth, 

 where some are swallowed and the rest thrown 

 back, the cilia in this case serving the animal 

 as a means of seizing its food. 



In their combined form the cilia constitute 

 the singular and well-known rotatory or wheel- 

 like organs of the Rotiferous Infusoria. These 

 are formed of one or more circles of cilia, 

 placed on the fore part of the animal, as in 

 Philodina (Jig. 290), in which the organ is 

 double, consisting of two cir- 

 cles of cilia set on two short 

 processes, one on each side of 

 the mouth. This apparatus 

 can be retracted or pushed 

 out at the will of the ani- 

 mal. When in motion, the 

 circles of cilia have the ap- 

 pearance of toothed wheels 

 turned round on their axes, 

 first in one direction and then 

 in the opposite. Various ex- 

 planations of this apparent 

 revolution have been given. 

 According to Ehrenberg it is 

 an optical deception, which 

 he thus explains : the individual cilia com- 

 posing the rotatory organ move in the same 

 manner as the separate cilia above men- 

 tioned, that is, they each revolve in such a 



Fig. 290. 



Philodina 

 erythropthalma. 



way as to circumscribe a conical space. When 

 viewed sideways, in performing this revolution 

 they must necessarily pass at one moment a 

 little nearer, at another a little more distant 

 from the eye, or, in other words, become alter- 

 nately more and less distinct to the view at 

 short intervals ; and this alternation occurring 

 over the whole circle gives rise to a seeming 

 change of place in every part of it, and a con- 

 sequent appearance of rotation. Perhaps it 

 would be an equally satisfactory and a more 

 simple explanation to consider the appearance 

 as occasioned by an undulatory motion of the 

 cilia, such as that produced by the wind in a 

 field of corn ; the undulations following one 

 another in every part of the circle would give 

 the appearance of rotation. Such a waving 

 motion of the cilia undoubtedly occurs in 

 other animals. The Rotifera set in motion or 

 retract their ciliary organs apparently by a 

 voluntary act; they use them for similar pur- 

 poses as other Infusoria use their simple cilia ; 

 when the body is free, the rotatory organ pro- 

 pels it through the water; at other times the 

 animal fixes itself by its tail, and setting in 

 motion its wheels, produces currents in the 

 water, by means of which it seizes its food. 

 These currents in most of the Rotifera have a 

 determinate and regular direction. 



The cilia of the Infusoria, then, serve as 

 organs of locomotion; and in the greater 

 number of species they are the only visible 

 organs for this purpose ; indeed it is not im- 

 probable that they may exist in others in 

 which from their smallness they have hitherto 

 eluded observation ; as in such cases cur- 

 rents are observed which are most probably 

 produced by invisible cilia. Secondly, the 

 cilia are employed by the animals in catching 

 their food. Thirdly, it is extremely probable 

 that, by bringing successive portions of water 

 into contact with the surface of the animal, 

 they serve also for respiration. 



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 volvox, as short 

 slender organs projecting a little from the body, 

 by means of which the animal produced a re- 

 volving motion and moved onwards. Again,f 

 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 



* Continuatio Arcanorum Naturae, 1719, p. 382, 

 Epist. 144. 



t Continuatio Epistolarum, 1715, p. 95, Epist. 

 17, Oct. 1687. 



