84 The Ottawa Naturalist. 



thread of undifferentiated protoplasm. From the surface of a cell there 

 may project one to fifty cilia. This surface may appear like a hyaline 

 layer, but it is a false appearance, and is due to the swollen bases of 

 the cilia. Each cilium, indeed, is enlarged at its base, but narrows 

 immediately above. This narrow neck is succeeded by a swollen 

 portion which gradually becomes attenuated towards the tip. They 

 vary in length, those i -3000th of an inch long being of medium size, 

 some are shorter, others longer. Vigorous lashing movements are 

 characteristic of cilia. The movements are too rapid to be distinctly 

 seen, the vibrations being usually 700 to 800 per minute. 



If a fragment of the ciliated lining, say the mucous membrane from 

 the roof of a frog's mouth, be microscopically examined in normal salt 

 solution, the surface shows an unceasing shimmering appearance, com- 

 parable to a rapidly waving field of barley. Each cilium, it has been 

 found, is erect and straight. Then it bends rapidly on itself, and, very 

 much more slowly, resumes the straight condition. The force of the 

 vibrations is in one direction, and as successive rows of cilia do not 

 bend simultaneously, but in regular succession, the result is a progres- 

 sive rythmic undulation. When the cilia are arranged in a circle or 

 crown, as in a Rotifer, say Melicerta, the appearance produced is 

 that of a swiftly rotating wheel. Hence the Rotifers have been 

 erroneously called wheel-animalcules. 



The vibrations of the cilia continue for some time after death : but, 

 in an injured, feeble, or dying condition, they are abnormally slow, and 

 can then be best observed. Heat (up to 104" F.) increases their 

 vigour, carbonic acid gas arrests them, while under the influence ot 

 oxygen, and of induced electrical currents, the vibrations may be 

 repeatedly revived. They are independent of nervous control. They 

 are automatic and as inexplicable as the inherent contractibility of 

 muscle. Ranvier's experiments, indeed, show that the living protoplasm, 

 of which cilia are composed, is essentially the same as that of ordinary 

 protoplasmic cells. 





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