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The slender dorsal vessel, provided with ostia and valvules, pulsates along the entire length of the trunk; an aorta passes into the head.

The internal genital organs of both sexes are paired, and extend along the greater part of the trunk; in either sex they may be compared to two long, slender, straight cords extending from the fourth to the tenth pair of legs. The two oviducts do not unite before reaching the sexual opening (Fig. 15, ovd).

The male sexual organs are more complicated than the feminine. The paired testicular tubes lie in trunk-segments 6 to 12, on each side, and partly under the intestinal canal, communicating with each other by a cross-anastomosis situated under the intestine, and which, like the testes, is filled with sperm. Of the paired seminal ducts (vas deferens) in trunk-segment 4, each unites again into a thick tube, sending a blind tube forward into the third segment. Under the place of union of the two vasa deferentia arise the paired ductus ejaculatorii, which open beneath in the uterus masculinus. The anterior blind ends of the vasa deferentia form a sort of small paired vesiculæ seminales in which a great quantity of ripe sperm is stored. The uterus masculinus is in its structure homologous with the evaginable penis of Pauropus, Polyxenus, and some diplopods, and the sexual opening has without doubt become secondarily unpaired. The sexual opening is rather long and is closed by two longitudinal folds. “In several respects the male sexual organs of Scolopendrella are like those of Pauropus; in the last-named form we have indeed an unpaired testis, but also in Scolopendrella we see the beginning of such a singleness; namely, the presence of an anastomosis uniting the two tubes, their communication by means of a transverse connecting canal and a glandular structure in the epithelium forming them. The male sexual organs of Pauropus differ only through a still greater complication.” (Schmidt.)