ИСТИНА

The question of the presence of the kidneys in echinoderms, comparable to kidneys of other invertebrates, is the subject of discussion. The kidney must to contain some morphological components: firstly, the net of haemal vessels, secondly, the coelomic cavities, where the liquid from the haemal vessels gets during the ultrafiltration, and thirdly, the excretory canal, which connects the excretory region of the coelom with the environment. In Echinodermata, the kidney's functions are performed by the axial complex of organs (ACO). The blood net of ACO is represented by the system of haemocoelic spaces, which lie between the folds of the coelothelium of axial coelom. Contractions of the heart, which is enclosed into the pericardial coelom on the aboral side of the body, provide the directional movement of the blood. The heart accepts the blood from two aboral haemal rings: the gastric ring and the genital ring. Haemocoelic spaces of ACO are separated from the axial coelom by the basal lamina. The surface of this basal lamina from the side of coelom is covered by the coelomic lining, which contains the podocytes and epithelial-muscle cells. Contractions of the epithelial-muscle cells create the pressure, which provides the ultrafiltration of liquid from the haemocoel to the axial coelom. The liquid passes through the filter, which is formed by the basal lamina, and is modified during the passing through the narrow spaces between the processes of the podocytes. The coelomic liquid with the products of excretion is removed from the axial coelom to the environment via the pores of madreporic plate. The echinoderm kidney is homologous to the heart-kidney complex of another deuterostome phylum, Hemichordata. The hemichordate's heart-kidney consists of the proboscis coelom, which is developed from the left axocoel of tornaria larva, the pericardial coelom, which is formed from the right axocoel, the heart, and so-called glomerulus, i.e. the net of haemocoelic spaces between the folds of the proboscis coelom. The liquid is filtered from the haemocoelic spaces of the glomerulus through the basal lamina, passes between the finger-like processes of the podocytes, and gets to the proboscis coelom. From there the liquid is excreted to the environment via the coelomoduct. The heart-kidney of Hemichordata is undoubted homologue of the ACO of Echinodermata. It is formed from the homologous larval coeloms, has a significant similarity in the structure, and in addition is analogous in function. Probably, the predecessor of the heart-kidney has appeared in common ancestors of echinoderms and hemichordates. We can assume that the heart-kidney = ACO represents one of the most important synapomorphies of Ambulacraria.