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. Por their 
contractile tissue is of the simplest possible character, resem¬ 
bling that wdiich 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 Medusae (§ 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 
