a single loop is a signal

A circuit in which all parts are connected in a single loop

A servo, or servomechanism, is an automatic device that uses error-sensing negative feedback to correct performance. http://en.wikipedia.org/wiki/Servomechanism

no they have double loop circulatory system but the difference is that they have single ventricle.

well you see a single loop circulation only goes through the heart and body one at a time meanwhile a double loop circulation does it all at the same time ,,, suss