The Segmental Vessels of Insects. The morphology and histology of the dorsal vessel and the segmental vessels in the American cockroach Periplaneta americana were investigated. P. americana exhibits 6 pairs of excurrent ostia which are associated with segmental vessels. The valves of the excurrent ostia consist of an unusual type of muscle cells containing only a few bundles of myofibrils, which extend at various angles within each cell. The segmental vessel wall is continuous with the connective tissue sheath of the dorsal vessel and contains no muscle cells. Among insects only Blattodea and Mantodea possess segmental vessels whereas excurrent ostia are quite common in the Paurometabola.