Harvestman Anatomy - Circulation

Circulation

Unlike arachnids with book lungs (scorpions, most spiders and several others), harvestmen and most other purely tracheate arachnids lack extensive arterial branching and well-defined venous sinuses. The circulatory system consists mainly of a dorsal tubular heart with anterior and posterior aortae. The heart is innervated by a cardiac ganglion. Myofibrils are mostly arranged circularly and constrict the heart during systole. There are two valves present, an anterior valve, extending into the anterior aorta seems to prevent backflow of hemolymph, and a posterior valve, which is possibly actively controlled.

Five types of blood cells have been found, none of which are unique to harvestmen. Prohemocytes are about five µm in diameter and rather scarce. They probably act as stem cells to the amoeboid plasmatocytes, which are about 11 µm and are known to ingest bacteria and dead cells. Granulocytes apparently develop from plasmatocytes and are also amoeboid, although no phagocytosis has been observed. They may act as storage cells. Spherulocytes are up to 15 µm long seem to be non-motile. They contain many spherules, whose composition changes over time, but the function is not known. Coagulocytes, which probably derived from granulocytes, release granules and disintegrate when they come in contact with uncleaned surfaces in vitro, probably due to present bacterial endotoxins. Despite their name, these cells seem not to play a role in hemolymph coagulation.