ROTIFERA. 



mind of the observer upon this point. The whole fringe of cilia 

 may be instantly set in motion, and as instantaneously stopped, or 

 their action regulated to every degree of rapidity. Sometimes one 

 or two only of the waves are seen continuing their action, whilst 

 the remainder are at rest ; or isolated cilia may be observed slowly 

 bending and unbending themselves, while the others are quiescent. 

 It is by the constant succession of these movements that the eye is 

 seduced to follow the waves which they seem to produce, and thus 

 the apparent rotation of the wheels is easily understood. 



(158.) Such being,as we conceive, the nature of the ciliary motion, 

 we will proceed to examine the uses to which it is made subservient in 

 the class of animals under consideration. A very slight examina- 

 tion of one of these creatures under the microscope will show that 

 the cilia answer a double purpose : if the Rotifer fixes itself to 

 some stationary object by means of the anal forceps, it is precisely 

 in the position of a Bryozoon ; and the ciliary action, by producing 

 currents in the water all directed towards the oral orifice, ensures 

 a copious supply of food by hurrying to the mouth whatever 

 minute aliment may be brought within the range of the vortex thus 

 caused ; or, on the other hand, if the animal disengages itself from 

 the substance to which it held by its curious anchor, the wheels 

 acting upon the principle of the paddles of a steam-boat carry it 

 rapidly along with an equable and gliding movement. 



(159.) The whole ciliary apparatus when not in use is retracted 

 within the orifice of the shell, and lodged in a kind of sheath formed 

 for it by the inversion of the tegumentary membrane. The muscular 

 fasciculi by which this is effected are very conspicuous ; they arise 

 from the lining membrane of the shell, and run in distinct fasci- 

 culi in a longitudinal direction to be inserted into the lobules 

 whereon the cilia are arranged (^g. 50, A, h). 



But, besides these retractor muscles, other fasciculi of muscular 

 fibres are seen to run transversely, (Jig. 50, z, i 9 ) crossing the for- 

 mer at right angles : these are, most probably, the agents pro- 

 vided for the extrusion of the wheel-like apparatus; for, aris- 

 ing, as they do, from the inner membrane of the hard integument, 

 they will, by their contraction, compress the fluid in which the 

 viscera float, and, forcing it outward towards the orifice of the shell, 

 it will, of course, push before it the wheels, so as to evert the te- 

 gumentary membrane connecting them with the shell, by unrolling 

 it like the finger of a glove, and thus they will cause the rotatory 

 organs to protrude at the pleasure of the animal. 



