1 14 THE MECHANICAL FUNCTIONS. 



ture. This is the case with all the lower Zoophytes, 

 such as the Itijusoria, Polypi, Medusce, and the 

 simjiler kinds of Entozoa. 



Among Polypi and Infusoria we meet with a 

 singular mode of acting upon the surrounding fluid 

 by means of very minute and generally microscopic 

 organs, resembling short hairs, termed cilia, which 

 the animal, by some unknown power, causes to 

 vibrate with great rapidity. These organs are 

 also met with in many parts of animals belonging 

 to the liigher classes. Wherever they exist, they 

 perform, as will hereafter be shown, very important 

 functions; either serving as instruments of pro- 

 gressive motion, or determining currents of fluids 

 which contribute to respiration, to the supply of 

 food, or to internal circulation. 



Cilia are long slender filaments of a conical, and 

 sometimes slightly flattened form, broad at the 

 base, and tapering to an invisible point. Their 

 length varies, in diflerent animals, from the 13,000th 

 to the 500th part of an inch. They are arranged 

 in regular order ; often in rows, and sometimes in 

 circular, or spiral lines ; and are generally set close 

 together in the same row. Being transparent, and 

 for the most part colourless, they are not easily 

 seen, especially as during the life of the animal 

 their vibrations are too rapid and incessant to be 

 followed by the quickest eye, even when assisted 

 by powerful microscopes : so that they can, in 

 general, be detected only at those times when, in 

 consequence of the failing vigour of the animal, 

 their motions have become comparatively languid. 



* 



* Milne Edwards states, however, that he has repeatedly seen tlie 

 ciha on various polypi, when they were at rest. Ann. Sc. Nat. 2"^* 

 ser. vi. 22. 



