Single-celled organisms can sense and react. Much of what they do counts as behavior only in a very broad sense, but they can control how they move and what chemicals they make, in response to what they detect going on around them. In order for any organism to do this, one part of it must be receptive, able to see or smell or hear, and another part must be active, able to make something useful happen. The organism must also establish a connection of some sort, an arc, between these two parts.
