The arthropod circulatory system

Abstract

Arthropods have a genuine circulatory system. Their exoskeleton encloses a liquid-filled body cavity, the haemocoel. Thereby all organs and tissues are permanently exposed to a fluid medium, the haemolymph which consists of plasma with suspended haemocytes. The circulation of haemolymph is actively forced by special pumping organs referred to as hearts. Emanating from the hearts, arteries deliver the haemolymph to the various body regions and compartments. These arterial systems are developed to differing extents in arthropods and together with the heart constitute the cardiovascular system or haemolymph vascular system. As haemolymph leaves the vascular system, it empties into the lacunar system, where it follows distinct routes that are determined by the anatomy of the internal organs and by channels or diaphragms formed of connective tissue. This article reviews major features and functions of the arthropod circulatory system. It describes the great disparity in the anatomy of the circulatory organs among the major arthropod lineages, their development and evolution. A special focus refers to the evolutionary changes which this organ system has undergone along the major environmental transitions from aquatic to terrestrial habitats and the evolution of flight.