There is a long tubular heart that extends through much of the body, but usually few, if any, blood vessels.

Later, this will form the myocardial primordium, which will contribute to the tubular heart.

The tubular heart or primitive heart tube is the earliest stage of heart development.

The haemocoel of anostracans is pumped by a long, tubular heart, which runs through most of the animal's length.

The circulatory system consists mainly of a dorsal tubular heart with anterior and posterior aortae.

In the grasshopper, the closed portion of the system consists of tubular hearts and an aorta running along the dorsal side of the insect.

Xiphosurans have well-developed circulatory systems, with numerous arteries that send blood from the long tubular heart to the body tissues, and then to two longitudinal sinuses next to the gills.

Once activated Pitx2 will be locally expressed in the left lateral mesoderm, tubular heart, and early gut which leads to the asymmetrical development of organs and looping of the gut.

The main body cavity is a hemocoel that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward.

Blood is conveyed to the wall of the sac by the primitive aortæ, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo.

The developing tubular heart then folds ventrally and bulges in five regions along its length: the first one and closest to the arterial end is the truncus arteriosus, then follow the bulbus cordis, the primitive ventricle, the primitive atrium and the sinus venosus.