Circulatory System Of Branchiostoma
Blood Vascular System or Circulatory system of Branchiostoma is of the closed type, well developed and peculiar in many ways. Blood is colourless due to lack of any respiratory pigment and corpuscles. Besides blood vessels it also occurs in some lymph spaces such as in metapleural folds and the dorsal and ventral fins. Its main function seems to be the transportation of food and excretory products rather than oxygen and Carbon dioxide for gaseous exchange. A heart is lacking so that all blood vessels are muscular and contractile in nature.
Except dorsal aorta, they lack an endothelial lining. There is no structural difference between arteries and veins, but these names are being given due to their homologies with blood vessels of higher chordates. The principal vessels and their branches are as follows :
Sinus venosus :-
It is a small, thin-walled sac present just below the posterior end of pharynx or midgut diverticulum. Blood from different parts of the body is collected by sinus venosus through a number of veins, and pumped forward into the ventral aorta.
Ventral aorta
It is a large median longitudinal artery extending forward from sinus venosus. It runs mid-ventrally in the wall of pharynx below the endostyle in the subendostylar coelom, hence also named as the subendostylar aorta. Blood flows in it anteriorly due to rhythmic contractions of its muscular walls. The ventral aorta gives off a series of paired lateral afferent branchial arteries, one running through each primary gill bar of pharynx.
At the base of gill bar each branchial artery forms a pulsatile tiny bulb called bulbillus or bulbule (pleural bulbilli or bulbuli), which helps in circulating blood. Through transverse vessels running in synapticulae, the different branchial vessels of primary gill bars are connected to those of secondary gill bars. The afferent vessels leave the pharynx dorsally as such in the form of efferent branchial vessels which open into a lateral dorsal aorta of that side. Before doing so, each efferent vessel splits into a small capillary network, the nephric glomerular sinus or glomus, in relation with a nephridium.
Dorsal aorta
Two longitudinal vessels, right and left lateral dorsal aortae, lie one on either dorso-lateral side of pharynx. They continue anteriorly as internal carotid arteries to supply the oral hood region. The right artery is somewhat dilated. The two lateral aortae unite just behind the pharynx into a single median dorsal aorta which ins posteriorly between the notochord and intestine and enters the tail region as the caudal artery Blood flows backwards in the dorsal aortae. From lateral and median dorsal aortae branches lead into myocoel, the lymph space between myotomes and body wall. Besides, the median dorsal aorta supplies blood through several intestinal arteries forming a capillary network or plexus in the intestinal wall.
Sub-intestinal vein
The blood from tail region is collected by a mid-ventral caudal vein and from intestine by lateral intestinal veins. These join a small median longitudinal subintestinal vein lying below the intestine in the form of a plexus. Blood flows in it anteriorly.
Hepatic portal system
The subintestinal vein continues anteriorly as the hepatic portal vein running ventrally along the midgut diverticulum into which it breaks up in a capillary network. From midgut diverticulum, blood is collected by a hepatic vein running along its dorsal border and bending downwards to join the sinus venosus. Hepatic portal system occurring for the first time in Branchiostoma is a precursor of that of higher chordates.
Cardinal veins
The blood from ventro-lateral region of body is collected on either side by an anterior and a posterior cardinal vein. The two unite just behind the pharynx to form a common cardinal vein or ductus Cuvieri. The two ductus Cuvieri then run down through the atrium to join the sinus venosus.
Parietal veins
Blood from dorsal body wall is collected by a pair of parietal veins one on either side. They run above the intestine for some distance and then turn ventrally to join the sinus venosus.
Course of circulation
Blood flows anteriorly inside parietal, subintestinal and posterior cardinal veins and ventral aorta. It flows posteriorly inside lateral and median dorsal aortae and anterior cardinal vein. The course of Circulatory System Of Branchiostoma can be graphically represented as shown.
Lymphatic system
Lymph as such is lacking. The so-called lymphatic spaces or sinuses present inside fins and metapleural folds, are filled with colourless blood lacking in leucocytes. It is regarded that most of the oxygen required by the animal is obtained by gaseous exchange between outside water and blood inside these superficial sinuses. Thank for reading our article on Circulatory System Of Branchiostoma.
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