A circulatory system in which blood is not always in vessels. 708

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.

a type of circulatory system where the blood is not contained within a system of vessels and the heart. Blood empties from vessels into sinuses and then returns through other vessels to a "heart.".