Skin structure in the snout of the Australian lungfish, Neoceratodus forsteri (Osteichthyes: Dipnoi)

• Lymphatic vessels in the skin of Neoceratodus forsteri prevent infection in the snout.
• Lymphatic vessels appear within the dermis as soon as the hatchling is able to feed.
• As the fish grows, the vessels are enclosed in thick connective tissue and enter the epidermis as lymphatic capillaries.
• Fossil lungfish and other Sarcopterygii have the same system in mineralised dermal tissues.
• Mineralisation is lost in the course of evolution. The only living fishes with the derived structure are lungfish.

Many fossil lungfish have a system of mineralised tubules in the dermis of the snout, branching extensively and radiating towards the epidermis. The tubules anastomose in the superficial layer of the dermis, forming a plexus consisting of two layers of vessels, with branches that expand into pore canals and flask organs, flanked by cosmine nodules where these are present. Traces of this system are found in the Australian lungfish, Neoceratodus forsteri, consisting of branching tubules in the dermis, a double plexus below the epidermis and dermal papillae entering the epidermis without reaching the surface. In N. forsteri, the tubules, the plexus and the dermal papillae consist of thick, unmineralised connective tissue, enclosing fine blood vessels packed with lymphocytes. Tissues in the epidermis and the dermis of N. forsteri are not associated with deposits of calcium, which is below detectable limits in the skin of the snout at all stages of the life cycle. Canals of the sensory line system, with mechanoreceptors, are separate from the tubules, the plexus and the dermal papillae, as are the electroreceptors in the epidermis. The system of tubules, plexus, dermal papillae and lymphatic capillaries may function to protect the tissues of the snout from infection.