On the sandy bottom of the seas, a benthic lifestyle of white-cream or slightly pinkish translucent animals, called lancelets, leads. Their sizes are from 5 to 8 cm. The body is flattened from the sides, its front end is obliquely cut, and there is a mouth framed by tentacles on it. The back of the body looks like a surgical knife - a lancet. Comparative anatomy and zoology are studying these seemingly unremarkable animals quite seriously for one reason: the lancelet is considered the connecting link between the two most important groups of animals - invertebrates and chordates.
In this article, we compare the structure of the lancelet with bone 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 traits of similarity with vertebrates, while retaining signs of invertebrate organisms.
Blood circulation
Consider the structure of the circulatory system of the lancelet. The red liquid, which does not have pigments, moves along the abdominal aorta, which constantly pulsates due to contractions of the myoepithelial layer of the coelomic cavity. Then, blood with an excess of carbon dioxide enters the head of the lancelet. In gill vessels, gas exchange occurs. Arteries flow into the posterior pharynx, where the right and left portions of the dorsal aorta are located. The front of the lancelet is provided with blood from the carotid arteries exiting the aorta. In smaller arterioles, oxygen-rich blood enters all organs of the animal. The venous part of this system begins with a network of intestinal venules containing carbon dioxide. Of these, blood enters the papillon vein.
Here the portal system of the liver is formed. Anatomically, it is located under the intestinal tube of the lancelet, breaking up into a network of venules, which braid the walls of the digestive system. Its functions are to transfer purified blood of toxins with a high content of carbon dioxide further into the venous sinus. From both parts of the body of the lancelet, it is sent to the cardinal (otherwise called jugular) veins, then to the Cuvier ducts.
Cuvier ducts
These veins of vertebrates are isolated for the first time at the lancelet and are formed by the fusion of cardinal vessels. In them, red fluid comes from the front and rear ends of the animal's body. The Cuvier 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 in the postembryonic period they are characteristic of cyclostomes (lampreys and myxines), 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 the atrium and 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 circulatory system of the lancelet proves that its blood 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. So begins a small circle of blood circulation in organisms with a four-chambered heart. In the lancelet, like other representatives of the cephalic chordates, the heart is absent and the venous sinus is represented by an unpaired vessel into which the venous fluid comes from the hepatic vein. Then he passes into the abdominal aorta. If you recall the structure of the circulatory system of the lancelet and bone fish, you will find that the changes affected primarily the abdominal aorta, which in fish is modified into a two-chamber heart. In addition, the respiratory surface of the gills of bone fish also increased due to branching of the capillary network of their gill arteries.
The collar system of the hepatic outgrowth
The circulatory system of the lancelet, like other vertebrates, is anatomically connected with 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 circulatory system of the lancelet, we clarify that all the fluid from the digestive organs of the animal enters the hepatic outgrowth. Similarly to the liver of fish, amphibians and other vertebrates, this lancelet organ performs a detoxifying function, cleansing the blood coming from the intestine from the decay products - metabolites. Then it enters the venous sinus. We add that blood enters the hepatic outgrowth from the papillon vein.
Abdominal and dorsal aorta
It represents the main arterial vessel. If you recall the structure of the circulatory system of the lancelet, then on a micropreparation you will see that under the pharynx of the animal there is an abdominal aorta, from which the paired arteries depart. They branch in the partitions of the gill cavities. The spinal aorta is formed at the posterior end of the pharynx as a result of the fusion of suprajugular arteries. Anatomically, it is located under the chord and stretches to the posterior end of the lancelet body, branching into arteries that feed the internal organs of the animal. In a lancelet, the 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 is suitable. It branches into capillary glomeruli. Plasma filtration occurs through their walls, and toxins in dissolved form enter protenephridia, then into the mesonephral duct and then into the cloaca.
The circulatory system of the lancelet and bone fish
Consider the similarities and differences in the structure of the cardiovascular system of the superclass Bony fish and type Golovokhordovye, to which the lancelet belongs. Both groups of animals have one circle of blood circulation. But the lancelet has no heart, part of the abdominal aorta takes over its function, which contracts along with the gill arteries and creates a flow of blood. Fish have a heart; it, like cyclostomes, has a two-chamber (atrium and ventricle).
The formation of this organ is associated with a more active metabolism. The heart of the fish is located next to the inter-branchial arches under the lower jaw. As we have seen from the above facts, the structure of the circulatory system of the lancelet, which provides the transfer of oxygen and nutrients, differs from that of bone fish.
Features of blood supply to the gill apparatus
If you remember the structure of the circulatory system of the lancelet, compare it with bone fish, you will find differences that affect the blood supply to the gill apparatus. The abdominal aorta is located on the underside of the pharynx. From it, arteries that carry venous blood are suitable for each pair of gill arches . The decrease in the number of septa in the gills (150 pairs in the lancelet and 4 pairs in the fish) is explained by increased metabolism, as well as an increase in the total area of the capillary network in representatives of bone fish. The lancelet is capable of saturating its blood with oxygen not only through the system of gill arteries, but also by direct diffusion of gas through the skin into the superficial blood vessels.
Carotid arteries
If you compare the circulatory systems of the lancelet and bone fish, you will find differences regarding vessels called the carotid arteries. They carry arterial red fluid to the anterior end of the animal's body. In bone fish, 4 pairs of gill arteries flow into the spinal aorta, the roots of which separate the carotid arteries. In lancelet, the number of gill vessels is much larger. They supply oxygen to the brain, representing the expansion of the neural tube and not differentiated into departments. It controls the reflex activity of the animal. The supply of brain neurons with oxygen and nutrients occurs due to the branching of the carotid arteries into the capillary system. It also receives products - metabolites that travel through the veins to the venous sinus.
In this article, the circulatory system of the lancelet and the features of blood circulation in the cephalic chords were studied.