The axial complex (proboscis complex or heart-kidney) is well-studied for the shallow-living enteropneusts that burrow into the sediment and use the muscular proboscis as the primary locomotor organ. However, the deep-sea enteropneusts in the family Torquaratoridae are epibenthic surface-dwellers with a dispersed, poorly developed proboscis musculature. There is little data on the structure of their axial complex (AC). We have studied in detail the microscopic anatomy of the AC of three specimens of torquaratorid Quatuoralisia malakhovi Ezhova et Lukinykh, 2022. The proboscis of Q. malakhovi is widely fused with the collar along the entire dorsal side rather than being connected by a narrow stalk. The AC includes the buccal diverticulum (stomochord), pericardium, glomerulus (axial organ) and associated blood vessels, perihaemal coeloms, and proboscis skeletal elements. The latter are two separate symmetrical plates supporting the buccal diverticulum laterally. The buccal diverticulum is elongated in transverse direction and packed with large, vacuolated cells without a lumen. It is completely separated from the main course of the gut. The pericardium lies on the dorsal side of the buccal diverticulum. The heart is absent. The glomerulus consists of two symmetrical extensions separated by the confluence of the dorsal blood vessel. The latter is accompanied by the perihaemal coeloms and splits into several branches resembling the mouth of a river, which then flow into the glomerulus. Two buccal blood vessels with folded walls run from the sides of the glomerulus to the ventral blood vessel. A protocoelomic coelomopore is absent, however we found a pair of mesocoelomic coelomoducts and coelomopores. The torquaratorid AC demonstrates varying degrees of reduction. None of the previously studied species of Torquaratoridae possesses a complete set of AC components characteristic of shallow-living enteropneusts. Torquaratoridae are derived in terms of structure and composition of the AC compared to non-torquaratorid hemichordates.
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