XIV.] ORGANIC FUNCTIONS. 159 



mistaken for microscopic animals), and that "ciliary" 

 motion, which is the only motor action of sponges, and 

 appears to be universal in the animal kingdom. Motions 

 of this class are found where there is no nervous system, 

 as in plants and sponges: and even where there is a 

 nervous system they are quite independent of nervous 

 agency, as is proved by the fact that the " cilia " in man Cilia. 

 and the higher animals continue in motion long after 

 death, and even when they are detached from the body. 

 So far as has been ascertained, they are also independent 

 of any structure, but are simply due to the primary power 

 of living matter to transform energy. ISTo structure has 

 been as yet discovered in the "ciliated cells" of even the 

 highest animals.-^ 



The next kind of motor action is that which is per- Motion in 



„,. . .-1 jj. -J response to 



formed m response to a stimulus, and not accompanied ^ stimulus, 

 with sensation. This is generally confined to animals, 

 though there are instances of it among plants, as in the in plants : 

 sensitive-plant and in Venus's fly-trap. The fact that it is 

 found among vegetables at all proves that it cannot be essen- 

 tially dependent on nervous action ; and a similar proof is 

 afforded by the Hydrozoa among animals, which have no in animals 

 vestige of a nervous system, yet spontaneously close on jjgpyes : 

 their food when it touches the tentacles. And all muscular 

 fibre appears to have the power of contracting in response 

 to various kinds of stimuli, such as electrical excitement,^ 



1 Carpenter's Comparative Physiology, p. 125. See also Carpenter's 

 Human Physiology, p. 674. 



Cilia are minute hair-like projections, which are in constant and rapid 

 motion during life. Their use varies according to position ; in animalciiles 

 and in the Ciliograda they are organs of motion ; in very many animals 

 they are used to produce currents in the water, sometimes for the purpose 

 of bringing food, sometimes to keep the respiratory organs bathed witii 

 fresh supplies of water. In land animals, their only known function is to 

 produce currents of fluid towards the outlets of the body. If cilia are to 

 be called organs, they are organs the formation of which is independent of 

 any differentiation of the tissue. 



2 Thus in Helmholtz's experiment (p. 138) to determine the velocity of 

 the nervous current, electricity is seen to be capable of acting on muscle 

 directly, as well as of acting on it through the means of nerve ; causing 

 the muscle to contract in either case. 



