The blood vascular system of Branchiostoma is of closed type. The blood is colourless and devoid of corpuscles and respiratory pigment. Blood is found not only in blood vessels but also in lymph spaces, around fin-rays, and in metapleural folds. Its main function is to transport the food and excretory wastes.
Heart is not found in Branchiostoma. Most blood vessels are similar, but due to their homologies with blood vessels of vertebrates they are called arteries or veins. The principal arteries have muscular walls and only dorsal aorta has an endothelial lining.
Sinus Venosus:
It is a thin-walled sac below the hind end of pharynx. It is formed by the union of numerous vessels (veins) which collect blood from the entire body. It anteriorly leads into the ventral aorta.
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Ventral Aorta:
Lying in the subendostylar coelom, below the pharynx, is a contractile ventral aorta which pumps blood forwards by peristaltic contractions. At the anterior end the ventral aorta passes into two external carotid arteries. From the ventral aorta paired afferent branchial arteries arise and pass upwards in the pharyngeal wall on both sides.
The afferent branchial arteries at their commencement have contractile dilations called bulbilli which also pump blood, then they pass through the primary gill bars as three branches and through the synapticula they give two branches to each secondary gill-bar.
They form vascular plexuses which supply nephridia. Blood is collected from the gill-bars and nephridia by aortic arches or paired efferent branchial vessels. In the afferent and efferent vessels the blood is exposed to a respiratory water current, but there is no proof of its oxygenation since there is no respiratory pigment in the blood.
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The efferent branchial vessels of each side open into a lateral dorsal aorta lying on the side of the epipharyngeal groove. The right lateral dorsal aorta is more dilated than the left one; each is continued in front into the rostrum as an internal carotid artery to supply the blood to oral hood region.
Dorsal Aorta:
The two lateral dorsal aortae unite behind the pharynx to form a median dorsal aorta lying between the notochord and intestine. The dorsal aorta gives out many small, paired parietal arteries to the body wall and intestinal arteries to the intestine where they form plexuses in lymph spaces, then it is continued backwards as a caudal artery into the tail. Blood flows backwards in the dorsal