From the elongated form of the pulmonary bag, and the large volume of air which it contains, we can understand not only how a temporary suspension of respiration can be supported, but we comprehend how it is that these reptiles can remain under water for long periods, as they often do,—not because they breathe _in_ the water, but because they can for a while do without breathing. Snakes have been seen to remain perfectly quiet at the bottom of a clear stream for half an hour or more. Sometimes in this totally quiescent state one has been supposed to be dead, until, on a stone being thrown, it has darted away like a fish. None of the aquatic birds or the cetaceous mammalia can remain so long under water without coming to the surface to breathe as serpents can. At the Zoological Gardens they remain for hours at a time in their tanks. Often you will see a head peeping out—which, unfortunately, is all we _can_ see—while the bath is being enjoyed, but as often the head is also immersed, though, of course, for a shorter interval, the snake lifting it to breathe occasionally. We can imagine also the great assistance in swimming which this long air-receptacle must be, these reptiles deriving from it the same advantage, says Professor Owen, ‘as an eel from its swim-bladder.’ In chap. XII. is described the almost swimming motion of the more active snakes when gliding through long grass, or effecting progress over a very smooth surface. In the water the action is similar—that is, the progression is by lateral undulations, the tail being the chief propelling power. Whether through the resisting medium of water, or beating the air, so to speak, when skimming over smooth or unresisting surfaces, this swimming motion is ever easy and graceful. In the chapter on Tails, we shall see what an important agent in progression is this limb, whether by pressure, as in the burrowing snakes, or by its oar-like or paddle-like use in rapid motion. To recapitulate the above in a few words—first, respiration warms the blood; snakes are cold-blooded because only a portion of the blood passes through the lungs to become oxygenated, and in proportion to the diminution of the quantity of blood transmitted to the lungs, so does respiration become weaker; therefore reptiles are less dependent on breathing. Regarding the ‘voice’ of serpents, so surprising are the qualities attributed to it, that one would imagine the existence of varieties of snakes of widely differing organizations, if we were to believe all we read of the sounds they produce. ‘Hissing loudly,’ or ‘whistling,’ is the rule. No ordinary writer or traveller who says a word about a snake ever heard it hiss anything but ‘loudly,’ a statement traceable to the same sentiment which causes persons to talk of the ‘horrid forked tongue.’ A benevolently-disposed snake who would warn you away with that terrible tongue would also strengthen his argument by a prolonged hiss, and the louder the better.