The Vascular System of Octochaetus thomasi

Abstract

ABSTRACT Posteriorly to the gizzard the dorsal vessel of O. thomasi is double, consisting of two tubes joined by a short connecting vessel anterior to the septum in each segment. There are six pairs of strongly contractile hearts in segments 8-13. The latero-intestinal hearts in segments 10-13 receive blood from the dorsal vessel and from a common trunk from the supra-intestinal and latero-oesophageal vessels, and supply blood to the ventral vessel. The lateral hearts receive blood from the dorsal vessel and supply ventro-tegumentary and ventral vessels. For complete circulation all blood must pass through the hearts. The ventral vessel, which is not contractile, receives blood from the hearts; posteriorly to the hearts the flow is backward in the ventral vessel and outward via ventro-tegumentaries and ventro-intestinals to the body and gut; anteriorly to the hearts the flow is forward, the blood leaving the ventral vessel by the commissures from which the ventro-tegumentaries of the anterior region arise. The ventral vessel is the main arterial trunk. Posteriorly to the hearts the dorsal vessel collects blood from the body and gut by dorso-tegumentaries and dorso-intestinals. The flow in the dorsal vessel, which is contractile, is forward. Blood leaves the dorsal vessel through small vessels to the latero-intestinal hearts, and larger vessels to the lateral hearts. Anteriorly to the hearts, the commissures and all vessels connected with the dorsal vessel receive blood from it. Thus, posteriorly to the hearts the dorsal vessel is venous, and anteriorly to the hearts arterial, in character. Blood from the anterior region is returned by paired latθro-oesophageal vessels to the supra-intestinal vessel and to the latero-intestinal hearts. The latero-oesophageal vessels and the supra-intestinal vessels are the main venous trunks in the anterior region.s There is no sub-neural vessel.