ONE of the most striking effects of external disturbance on certain types of living substance is a visible change of form. . Thus, a piece of muscle when pinched contracts. The external disturbance which, produced this change is called the stimulus. ...
The body which is thus capable of responding is said to be irritable or excitable.; A stimulus thus produces a state of excitability which may sometimes be expressed by.change of form. Mechanical response to different kinds of stimuli.- This reaction under stimulus is seen even in the lowest organisms; in some of the amoeboid rhizopods, for instance.