The scaphopod vascular system is rudimentary lacking heart auricles as well as corresponding ctenidia (gills) and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. The heart, a characteristic feature of all other groups of mollusca, has been considered totally lost or reduced to a thin fold of the pericardium; however, according to more recent studies, the muscular, regularly beating perianal blood sinus is homologous to the ventricle and is therefore considered the scaphopod heart. Metabolic waste is excreted through a pair of nephridia close to the anus. The tusk shells appear to be the only extant molluscs which completely lack the otherwise standard molluscan reno-pericardial apertures. Furthermore, they also appear to be the only molluscs with openings that directly connect the hemocoel with the surrounding water (through two "water pores" located near the nephridial openings). These openings may serve to allow the animal to relieve internal pressure by ejecting body fluid (blood) during moments of extreme muscular contraction of the foot.