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 (fig. 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 ProfessorEhren 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 sets its cilia in 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 (fig. 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 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.