AbstractThe internal anatomy of freshwater crayfish is strictly bilaterally symmetric, with the conspicuous exception of the vertically oriented descending artery (sternal artery), which originates from the heart and terminates in the subneural artery. Serial sectioning of 133 juveniles of the parthenogenetic marbled crayfish revealed that the descending artery was bilaterally symmetric in 4.5% of the specimens, right asymmetric in 45.1%, and left asymmetric in 50.4%. In the bilaterally symmetric variant two branches arise from the left and right chambers of the bulbus of the heart, run laterally around the hindgut, and fuse underneath it. The asymmetric variants have only one dorsal branch, which loops around the hindgut on either the left or the right side. Other structures of the heart, such as the paired antennary and hepatopancreatic arteries and the ostia or the unpaired anterior and posterior aortae, showed no symmetry variation. Because of the genetic identity of the experimental animals and their culture under identical environmental conditions, the variation in symmetry of the descending artery observed is interpreted as the result of developmental variation. We recommend that the marbled crayfish be considered for investigation of the epigenetic mechanisms responsible for the maintenance and breaking of bilateral symmetry in metazoans. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.