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North America, which presents us with all the anomalous forms that we have been considering, with the exception of the Proteus, produces two or three species which are still more eel-like in their appearance than the Sirens. The body is greatly lengthened, flexible, and formed for swimming, terminating in a thin, and vertically compressed tail. The skull is solid. There are four limbs, but in one of the genera, these are so widely removed, so short and slender, and so rudimentary, the toes being almost evanescent, as to convey the idea of tentacula rather than feet. But what is most remarkable in these animals, is the peculiarity on which the name of the Order is founded, the absence of branchiæ, or gills. These organs have not been observed at any period of life, but there is an orifice on each side of the neck. Respiration is performed exclusively by lungs, the structure of which is reticulated, and puckered into longitudinal folds: hence these animals are air-breathers, though habitual residents in water. It is believed that this is the permanent condition of their existence, and that they undergo no metamorphosis.