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Show 1 9 0 4 .] ANATOMY OF PELAGIC SERPENTS. 1 5 3 surface of the lung is marked by conspicuous thickenings; but these do not form a honeycomb network as in Platyurus. Ihe folds run across the lung-that is at right angles to its longitudinal axis, and are wavy in outline, which of course allows of expansion during inspiration (text-fig. 27, p. 152). The trachea (or bronchus) is continued as a gutter down the lung as far as the commencement of the liver, that is for a very short distance behind Text-fig. 28. A portion of the internal surface of the non-vascular part of the lung ot Hydrus platyurus. the heart. I The lung itself is extraordinarily long; it is indeed co-extensive with the body-cavity, reaching as far back as the vent. Its calibre too is very considerable, and its walls have the tough, almost shiny, appearance of a fish's swim-bladder. The dorsal wall is firmly fixed to the parietes. The latter part of the lung, i. e. after the end of that region which is respiratory, has undergone some modification in relation to its undoubted function as a swim-bladder. The folds already spoken of in the vascular region of the lung persist; but their object is no longer that of merely increasing the respiratory surface, and thus the efficiency of the lung as a breathing organ. They exist only as bands of muscular fibre, which, since their direction is mainly circular and parallel to each other, must act as constrictors and expel air from the swim-bladder part of the lung. On a naked-eye inspection of this portion of the lung, it looks almost as if the bands in question were the bronchial rings which had in this region taken on a new development. They are, however, serially continuous not |