An arrangement of internal transport in which blood bathes the organs directly and there is no distinction between blood and interstitial fluid. operant conditioning A type of associative learning that directly affects behavior in a natural context; also called trial-and-error learning. operator A segment of DNA that interacts with a repressor protein to regulate the transcription of the structural genes of an operon. operon( op-ur-on) [L. opus, operis, work] A unit of genetic function common in bacteria and phages, consisting of coordinately regulated clusters of genes with related functions.