138 THE MICROSCOPE. 



summit. Many, also, can extend and withdraw their bodies at plea- 

 sure, in a similar manner to the snail or leech. 



One of the most interesting and important organs possessed by the 

 infusorial animalcule is scientifically known by the term cilium, which 

 is the Latin word for eyelash, the plural being cilia. Its appearance 

 is that of a minute delicate hair. 



The cilium is not only useful in the act of progression, but also as 

 an assistance in procuring food ; the two duties being performed at the 

 same time, the motion of the organs that propels it forward causing a 

 current to set towards the mouth, which carries with it the prey on 

 which the animal feeds. From the cilia being found in the gills or 

 beard of the tadpole, the oyster, and mussel, it would appear that they 

 are serviceable as organs of respiration, by imbibing oxygen, and emit- 

 ting the carbonic acid generated in the blood during its circulation 

 through the body ; they are also believed to be the medium of taste 

 and touch. It is not only at the mouth, but over the whole body, that 

 cilia are discovered ; and it is now satisfactorily shown that cilia exist 

 also in the internal organs of man and other vertebrated animals ; 

 and are agents by which many of the most important functions of the 

 animal economy are performed. They vary in size from the 1000th to 

 the 10,000th of an inch in length. These minute organs would often 

 be invisible, were it not from the water being coloured when placed 

 under a microscope ; then the little currents made by the action of the 

 cilia are easily perceived ; and when the water is evaporated, the deli- 

 cate tracing of their formation may be observed on the glass. They 

 are differently placed, and vary in quantity in the numerous species of 

 Infusoria. In some they are in rows the whole length of the body, in 

 others on the base ; many have them over the whole of the body ; some- 

 times they fringe the mouth, form bands around projections on the 

 body ; and many have but two projecting from the mouth, as long as 

 the body of the creature. Ehrenberg says they are fixed at their base 

 by the bulb moving in a socket, in a similar manner to a man's out- 

 stretched arm and by their moving round in a circle, they form a 

 cone, of which the apex is the bulb. Poison, galvanism applied to the 

 .animal, and death, do not immediately stop the motion of the cilia, as 

 they will continue in action some hours afterwards ; even longer than 

 nervous or muscular action can be sustained, until the fluids dry up 

 and they stiffen. 



Very little is known of their muscular development, from their 

 extreme minuteness ; but there can be no doubt of the existence of this 

 structure in all. Now in the wheel-animalcules the cilia are in circular 



