444 MOVEMENTS NOT DEPENDENT ON MENTAL DIRECTION. 



Contractile Tissues : Muscular Contractility. 



577. When we examine into the nature of the movements 

 of the lower tribes of Animals, however, we find that they 

 bear a much closer analogy to those of Plants, than they do 

 to those which are the expressions of the self-determining 

 power of higher Animals. Among the simplest Protozoa, it 

 seems as if the change of form of the single cell of which each 

 individual is composed, were the sole means of movement 

 which it possesses ( 129) ; and this change of form often 

 appears rather to be due to the nutrient actions taking place 

 within the cell, than to occur in respondence to any im- 

 pression made upon its exterior. In such movements it is 

 impossible to suppose with any probability that consciousness 

 can participate. So, again, among Infusory Animalcules, all 

 the movements of the body are effected by the action of cilia 

 ( 133), which we know in our own experience to be entirely 

 removed from any mental direction, and which we see to be 

 exhibited under a no less remarkable aspect by the " zoo- 

 spores " or motile buds of the Algae (BOTANY, 767). 



578. Although the movements of the Hydra ( 121) and 

 other Zoophytes may appear to indicate the existence of a self- 

 determining power, yet it is very doubtful whether such an 

 endowment is really possessed by these animals. For their 

 contractile tissue is of the simplest possible character, resem- 

 bling that which is found in the early state of newly-forming 

 parts of higher Animals ; and when the movements executed 

 by it are carefully compared with our own, it becomes obvious 

 that they are analogous, not to those of the Human body and 

 limbs generally, but to those of the muscular coat of the 

 alimentary canal and of the muscles concerned in deglutition, 

 which not only take place without any voluntary determina- 

 tion on our parts, but may even be performed without our 

 consciousness. In like manner, the rhythmical movements of 

 the umbrella-like discs of the Medusce ( 120), by which many 

 species of them are propelled through the water, bear a much 

 closer analogy to the rhythmical movements of the heart of 

 higher Animals, than they do to any other of their actions ; 

 and are probably performed, like these, without any exercise 

 of will, and even without the guidance of consciousness. 



579. In proportion, however, as we ascend the scale, we 



