The Axial Complex of Echinoderms Represents the Kidney and Is Homologous to the Hemichordate Heart-Kidney

The question of the presence of kidneys in echinoderms is a subject of discussion. Many guides state the absence of a special excretory organ in the echinoderms. However, there is a special excretory organ (kidney) in echinoderms. It is the axial complex. The blood network of the axial complex is represented by the system of haemocoelic spaces, which lie between the folds of the coelothelium of axial coelom. This haemocoelic capillary system is an axial organ. Contractions of the heart, which is enclosed into the pericardial coelom on the aboral side of the body, provides 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 the axial organ 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 coelothelial lining, which contains the podocytes and epithelial-muscle cells. The extracellular diaphragms are stretched between the processes of the podocytes. Contractions of the heart and epithelial-muscle cells create the pressure, which provides the ultrafiltration of liquid from the haemocoel to the axial coelom. The coelomic liquid with the products of excretion is removed from the axial coelom to the environment via the pores of madreporic plate. The hemichordate heart-kidney consists of the proboscis coelom, which develops from the left axocoel of tornaria, the pericardium, which develops from the right axocoel, the heart, and the so-called glomerulus, i.e., a network of haemocoelic spaces between the folds of the proboscis coelom. The fluid is filtered from the haemocoelic spaces of the glomerulus through the basal lamina, passes between the finger-like processes of the podocytes, and reaches the proboscis coelom, from which it is excreted to the environment via the coelomoduct. The axial complex of Echinodermata is an undoubted homologue of the heart-kidney of Hemichordata. It is formed from the homologous larval coeloms, has a significant similarity in the structure, and is analogous in function. Probably, the excretory organ based on the dissymmetric preoral coeloms was formed in the common ancestor of hemichordates and echinoderms. It represents the most important synapomorphy of Ambulacraria.