POLYGASTRICA. 55 



of no very great magnifying power, by the peculiar tremulous 

 movement which they excite in the surrounding fluid, somewhat 

 resembling the oscillations of the atmosphere in the neighbour- 

 hood of a heated surface ; but on applying higher magnifiers, espe- 

 cially if the animalcule is in a languid state, the motion is 

 seen to be produced by the action of the delicate filaments 

 of which we are speaking. It is extremely difficult accurately 

 to define the motion of the individual cilia ; it is most probable 

 that each forms by its rotation a cone, the apex of which will be 

 at the root of the organ this at least is the opinion of the best ob- 

 servers, and the combination of such movements gives rise to cur- 

 rents in the water, serving a variety of purposes in the economy of 

 these minute creatures. The vibrating organs, notwithstanding 

 their indescribable minuteness, vary considerably in size ; and it is 

 more than probable that in those monads, and other species, in 

 which their existence has not been detected, the apparent want of 

 them is owing to the imperfection of our means of investigation. 

 A few years ago, indeed, some species now distinctly proved to be 

 covered with cilia, were looked upon as being absolutely deprived 

 of locomotive apparatus, as the Volvox globator (Jig. 20) ; and 

 few greater proofs can be given of the superiority of the microscopes 

 now at our disposal, than the fact of our being able, not only to 

 detect with facility their existence on the surface of the parent 

 volvox, but even upon the young volvoces before their birth. 



(77.) The cilia, as has been already observed, are sometimes 

 dispersed over the whole body, either arranged in parallel rows or 

 scattered irregularly ; they are, however, most frequently only met 

 with in the neighbourhood of the mouth, in which position they 

 are always most evident : here they produce, by their vibration, 

 currents in the surrounding fluid which converge to the oral aper- 

 ture, and bring to the mouth smaller animalcules, or particles of 

 vegetable matter, which may be floating in the neighbourhood, and 

 thus ensure by an admirable contrivance an abundant supply of 

 food, which without such assistance it would be almost impossible 

 for these little creatures to obtain. 



(78.) We may be expected, in this place, to make a few obser- 

 vations concerning the agency by which these numberless and 

 almost invisible organs are made to perform their rapid move- 

 ments. The subject is one of no little difficulty, and in the 

 present state of our knowledge probably inexplicable. Ehrenberg 

 indeed asserts, that round the base of every cilium is an appa- 



