On the sandy bottom of the seas, white-cream or slightly pinkish translucent animals called lancelets lead a benthic lifestyle. Their sizes are from 5 to 8 cm. The body is laterally flattened, its front end is cut obliquely, and there is a mouth framed with tentacles on it. The back of the body looks like a surgical knife - a lancet. Comparative anatomy and zoology study such outwardly unremarkable animals quite seriously for one reason: the lancelet is considered a link between the two most important groups of animals - invertebrates and chordates.
In this article we will compare the structure of the lancelet with bony fish, and also give an answer to the following question: what is the circulatory system of the lancelet? Russian biologist A. O. Kovalevsky in 1860 proved that this animal has similarities with vertebrates, retaining the features of invertebrate organisms.
Circulation of blood
Consider the structure of the circulatory systemlancelet. A red fluid without pigments moves along the abdominal aorta, which constantly pulsates due to contractions of the myoepithelial layer of the coelom cavity. Then the blood with an excess of carbon dioxide enters the head of the lancelet. Gas exchange takes place in the gill vessels. The arteries flow into the posterior pharynx, where the right and left parts of the dorsal aorta are located. The anterior part of the body of the lancelet is provided with blood from the carotid arteries emerging from the aorta. Through smaller arterioles, oxygen-rich blood flows to all organs of the animal. The venous part of this system begins with a network of intestinal venules containing carbon dioxide. From them, blood enters the axillary vein.
The portal system of the liver is formed here. Anatomically, it is located under the intestinal tube of the lancelet, breaking up into a network of venules that braid the walls of the digestive system. Its function is to carry detoxified blood with a high carbon dioxide content further into the venous sinus. From both parts of the body of the lancelet, it goes to the cardinal (otherwise called jugular) veins, then to the Cuvier ducts.
Cuvier ducts
These veins of vertebrates are first isolated in the lancelet and are formed by the confluence of the cardinal vessels. In them, a red liquid comes from the anterior and posterior ends of the animal's body. Cuvier's ducts directly flow into the venous sinus, which is considered the beginning of the abdominal aorta. These vessels are clearly expressed in vertebrate embryos, and inin the postembryonic period are inherent in cyclostomes (lampreys and hagfish), as well as fish and amphibians. The circulatory system of the lancelet and cyclostomes has the greatest similarities, although the latter have a real heart, consisting of an atrium and a ventricle.
Venous sinus
It is the initial part of the abdominal aorta, and such a lancelet system is a vicious circle. Thus, the structure of the lancelet's circulatory system proves that its circulation is closed. In mammals, birds and other vertebrates, this part of the organs belongs to the right atrium. From it, venous fluid enters the ventricle and then into the pulmonary arteries. This is how the pulmonary circulation begins in organisms with a four-chambered heart. In the lancelet, like other representatives of the cephalochords, the heart is absent and the venous sinus is represented by an unpaired vessel into which venous fluid enters from the hepatic vein. It then passes into the abdominal aorta. If you remember the structure of the circulatory system of the lancelet and bony fish, you will find that the changes affected primarily the abdominal aorta, which in fish is modified into a two-chambered heart. In addition, the respiratory surface of the gills of bony fish also increased due to branching of the capillary network of their gill arteries.
Portal system of hepatic outgrowth
The circulatory system of the lancelet, like other vertebrates,anatomically related to the digestive organs. The digestive organs of all vertebrates are morphologically connected, and the products of dissimilation: glucose, amino acids - enter its capillaries. Continuing to study the structure of the lancelet's circulatory system, we will clarify that all the liquid from the animal's digestive organs enters the hepatic outgrowth. Similar to the liver of fish, amphibians and other vertebrates, this organ of the lancelet performs a detoxifying function, cleansing the blood coming from the intestines from decay products - metabolites. Then it enters the venous sinus. We add that blood enters the hepatic outgrowth from the subintestinal vein.
Abdominal and dorsal aorta
It is the main arterial vessel. If you remember the structure of the lancelet's circulatory system, then on a micropreparation you will see that under the animal's pharynx there is an abdominal aorta, from which paired arteries depart symmetrically. They branch in the septa of the gill cavities. The dorsal aorta is formed at the posterior end of the pharynx by the fusion of the supragillary arteries. Anatomically, it is located under the chord and stretches to the posterior end of the body of the lancelet, branching into arteries that feed the internal organs of the animal. In the lancelet, metabolic products in the blood are filtered using special tubes called protenephridia. From the abdominal aorta to the body cavity - the whole - an arterial vessel approaches. It branches into capillaries. Plasma is filtered through their walls, and toxins in dissolved form enter the pronephridia, then into the mesonephric duct and then intocesspool.
Circulatory system of lancelet and bony fish
Let's consider the similarities and differences in the structure of the cardiovascular system of the superclass Bony fish and the type of head-chordidae, to which the lancelet belongs. Both groups of animals have one circle of blood circulation. But the lancelet does not have a heart, its function is taken over by a part of the abdominal aorta, which contracts together with the afferent branchial arteries and creates a blood flow. Fish have a heart, it, like cyclostomes, has two chambers (atrium and ventricle).
The formation of this organ is associated with a more active metabolism. The heart of fish is located next to the intergill arches under the lower jaw. As we have seen from the above facts, the structure of the lancelet circulatory system, which provides the transport of oxygen and nutrients, differs from that of bony fish.
Features of the blood supply of the gill apparatus
If you remember the structure of the lancelet circulatory system, compare it with bony fish, you will find differences in the blood supply to the gill apparatus. On the underside of the pharynx is the abdominal aorta. From it, arteries that carry venous blood approach each pair of gill arches. The decrease in the number of septa in the gills (the lancelet has 150 pairs, and the fish has 4 pairs) is explained by an increase in metabolism, as well as an increase in the total area of the capillary network in representatives of bony fish. The lancelet is capable of saturating its blood with oxygen not only through the system of branchial arteries, butand direct diffusion of the gas through the skin into superficial blood vessels.
Carotid arteries
If you compare the circulatory systems of the lancelet and bony fish, you will find differences regarding vessels called carotid arteries. They carry the arterial red fluid to the anterior end of the animal's body. In bony fish, 4 pairs of branchial arteries flow into the dorsal aorta, the roots of which separate the carotid arteries. In the lancelet, the number of gill vessels is much larger. They supply oxygen to the brain, which is an extension of the neural tube and is not differentiated into sections. It controls the reflex activity of the animal. The provision of brain neurons with oxygen and nutrients occurs due to the branching of the carotid arteries to the capillary system. It also receives products - metabolites, sent through the veins to the venous sinus.
In this article, the circulatory system of the lancelet and the features of blood circulation in cephalochords were studied.
