Abstract
The ultrastructure of the circulatory organs of Leiobunum limbatum, Mitopus morio, and Rilaena triangularis (Arachnida: Opiliones) has been investigated. We studied the organization of the heart, the myocardial contractile elements, its tubular system, organelles, and cell-junctions. The epicardium exhibits a large number of longitudinally arranged microtubules and is attached to the myocardium by special membrane complexes. Because of these structures, the epicardium is interpreted as the elastic antagonist to the heart muscle. At both ends of the heart the epicardium continues into a nonbranching aorta. Consequently the ultrastructure of the aortal wall, containing numerous microtubules, is similar to the ultrastructure of the epicardium. At the posterior end of the heart is a muscular slit-valve, while at the anterior end is a lobe-valve that lacks contractile elements. The heart is innervated by a dorsal ganglion. It covers the entire length of the heart and contains neural and glial cells. Two different types of neurosecretory granules and both electron-dense and electron-lucent synaptic vesicles are described.
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