This paper is part of a comparative survey on the evolutionary morphology of the circulatory system in Malacostraca. Tethysbaena agentarii is a representative of Thermosbaenacea, a group which exhibits a relict distribution in cave or groundwater habitats. The study presents the first three dimensional data concerning the circulatory organs and their spatial relationships to other major organ systems. T. argentarii has a short dumbbell-shaped heart which lies in the posterior cephalothorax and the first free thoracic segment. It is equipped with one pair of incurrent ostia. The only vessel to emanate from the heart is the anterior aorta, which runs into the anterior cephalothorax and supplies the central nervous system and both pairs of antennae. Comparative and functional aspects of the circulatory system are discussed. The features of this highly reduced vascular system point to a closer phylogenetic relationship between Thermosbaenacea, and Spelaeogriphacea and Mictocarididae (Mictacea).