The scaleless integument of Monopterus possesses numerous, exceptionally large, merocrine, unicellular mucous glands, specifically adapted to prevent desiccation and to furnish lubrication during burrowing. Vertical collagenous fibers ensure firm anchorage of the superficial with deeper skin layers. Large mucous cells and vertical strands of collagenous fibers have evolved independently in Monopterus, Conger, Misgurnus, and the Dipnoi. Integumentary intra-epithelial capillaries in Monopterus are lacking from the skin which contributes to 14% of the total respiration in water and 24% in air at 23? C. The esophagus of Monopterus has intra-epithelial capillaries and performs accessory respiratory functions. The postesophageal alimentary tract is non-respiratory. The number of goblet cells in the hindgut is a food dependent feature. Starvation results in a pronounced decrease in rectal goblet cells. The prestarvation number can be reestablished within twenty days by feeding. Monopterus is principally an air breather. The ratio between aerial and aquatic respiration is 3:1. When aerial respiration is prevented, Monopterus can depend solely on aquatic respiration in well oxygenated water, despite the lack of gills and counter-current flow of water across the gills. The maintenance of the normal metabolism in starved Monopterus may be correlated with its inability to estivate.
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Karel F. Liem (1967) studied this question.