468 ANIMALCULES. CILIARY MOVEMENT. 



notice of them. They are always found in connection with cells, of 

 whose substance, as we have seen among Protophytes (§§ 189, 194), 

 they may be considered as extensions. The form of the filaments 

 is usually a little flattened, and tapering gradually from the base 

 to the point. Their size is extremely variable ; the largest that 

 have been observed being about 1 -500th of an inch in length, and 

 the smallest about 1-13, 000th. When in motion, each filament 

 appears to bend from its root to its point, returning again to its 

 original state, like the stalks of corn when depressed by the wind ; 

 and when a number are affected in succession with this motion, the 

 appearance of progressive waves following one another is produced, 

 as when a corn-field is agitated by successive gusts. When the 

 ciliary action is in full activity, however, little can be distinguished 

 save the whirl of particles in the surrounding fluid ; but the 

 back-stroke may often be perceived, when the forivard-stvoke is 

 made too quickly to be seen ; and the real direction of the move- 

 ment is then opposite to the apparent. In this back-stroke, when 

 made slowly enough, a sort of ' feathering ' action may be observed ; 

 the thin edge being made to cleave the liquid, which has been 

 struck by the broad surface in the opposite direction. It is only 

 when the rate of movement has considerably slackened, that the 

 shape and size of the cilia, and the manner in which their stroke 

 is made, can be clearly seen. It has been maintained by some 

 that the action of the Cilia is Muscular ; but they are often too 

 small to contain even the minutest fibrillae of true muscular tissue, 

 and no such elements can be discerned around their base ; their 

 presence in Plants, moreover, seems distinctly to negative such an 

 idea. Hence we must consider them as oi'gans sui generis, wherein 

 the contractility of the cell to which they belong is (as it were) 

 concentrated. We have seen that in the Rhizopods, the entire 

 mass of whose sarcode is highly contractile, no cilia are present ; 

 whilst in the Infusoria, whose bodies have comparatively little 

 contractility, the movements are delegated to the cilia. — Cilia 

 are not confined, however, to Animalcules and Zoophytes, but 

 exist on some of the free internal surfaces, especially the walls of 

 the Respiratory passages, of all the higher Animals, not excepting 

 Man himself. Our own experience assures us that their action 

 takes place, not only without any exercise of will on our own parts, 

 but even without affecting our consciousness; and it has been 

 found to continue for many hours, or even days, after the death 

 of the body at large. How far it is subject to any conscious con- 

 trol on the part of these Animalcules, in which the cilia serve as 

 instruments for locomotion, as well as for bringing to them food or 

 oxygen, it is impossible for any one to say with confidence. In 

 this important respect, however, the ciliary movement of Animal- 

 cules differs from that which is observable in the higher animals, — 

 that whilst in the latter it is constant, giving the idea of purely 

 automatic agency, in the former it is so interrupted and renewed 



