Microanatomy of the heart and associated structures of two scorpions, Centruroides sculpturatus ewing and Uroctonus mordax thorell

Spiroid orientation of the circumferential heart wall muscles is described for Centruroides sculpturatus Ewing. This muscle arrangement accounts for differences in ostial position when the heart of this species is compared to that of Uroctonus mordax Thorell. Other differences, such as number of lateral arteries present, cannot be explained on the basis of circumferential muscle orientation. The histology of the heart and associated vessels, but not the supraneural vessel, was found to be similar in both species. The lateral, posterior, communicating and sternal arteries all possess a muscularis composed of irregularly spaced, apparently branched, striated muscle fibers. External to this is a covering of connective tissue. The lumina of these arteries, the aorta, and the supraneural vessel are lined with a homogeneous, PAS‐positive membrane. This membrane is also seen in blood vessels which penetrate the nervous system. It was not observed in vessels accompanying major nerves. Findings are compared to those of other authors. Differences in the structure of the hearts of these two species are discussed in relation to the microanatomy of other arachnid hearts.