CLASSIFICATION Or ANIMAL PKEPAKATIONS. 441 



regularity ; but in detached scales, it is often so violent, that 

 the scale itself is whirled about in a similar manner to an 

 animalcule provided with a locomotive apparatus of the same 

 description, and has frequently been mistaken for such. The 

 animals more commonly employed for the examination of the 

 cilia are the oyster and the mussel, but the latter is generally 

 preferred. To exhibit the movement to the best advantage, 

 the following method must be adopted : — Open carefully the 

 shells of one of these moUusks, spilling as little as possible of 

 the contained fluid ; then, with a pair of fine scissors, remove 

 a small portion of one of the gUls (branchise), lay this on a 

 slide or the tablet of an animalcule cage, and add to it a drop 

 or two of the fluid from the shell, and, by means of the 

 needle-points, separate the filaments one from the other, cover 

 it lightly with a thin piece of glass, and it is ready for exami- 

 nation. The cilia may then be seen in several rows beating 

 and lashing the water, and producing an infinity of currents 

 in it. If fresh water, instead of that from the shell, be added, 

 the movement will speedily stop, hence the necessity of the 

 caution of preserving the liquid contained in the shell. To 

 observe the action of any one of the cilia, and its form and struc- 

 ture, some hours should be allowed to elapse after the prepara- 

 tion of the filaments above given, the movement then will have 

 become sluggish ; if a power of four hundred linear be used, 

 and that part of the cilia attached to the epithelium scale 

 carefully watched, each one wiU be found to revolve a quarter 

 of a circle, whereby a "feathering movement" is effected,* 

 and a current in one direction constantly produced. In the 

 higher animals, the action of the cilia can only be observed a 

 short time after death. In a nasal polypus, when situated at 

 the upper and back part of the Schneiderian membrane, the 

 cilia jnay be beautififlly seen in rapid action some few hours 

 after its removal; but in the respiratory and other tracts 

 where ciliated epithelium is found, it would be almost 

 impossible ever to see it in action, unless the body were 

 opened immediately after death. In some animals, it may be 

 * See a paper by the author, in. vol. ii. of Transactions of the 

 Microscopical Society. 



