Page:The Zoologist, 4th series, vol 5 (1901).djvu/329

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process, common to all reptiles, of periodically casting off a "slough" is a very interesting one to watch, and can best be observed in serpents. It is not often the good fortune of the observer to be able to see it in natural conditions, though I have encountered several Adders engaged in the act; but in serpents in captivity the whole process can be studied with ease and accuracy. In books which touch on the subject three statements are to be found, which, in my experience, are inaccurate, or at any rate only partially true. It is said, in the first place, that the slough is cast once every year; secondly, that the reptile often makes a meal of the cast-off slough; and, thirdly, that the slough is cast entire (i.e. in one piece) when the reptile is in good health, but in several pieces if the animal is out of condition. It is quite possible that various species exhibit individual peculiarities in this matter, but in the case of British reptiles none of these statements are quite accurate. In the first place, it is usual for both Adders and Ring-Snakes to cast their sloughs three, four, or even five times in a season. The first sloughing takes place immediately after the winter hibernation is over, and the process is repeated at intervals of six weeks or two months. The same remark applies to the Slowworm (Anguis fragilis). Secondly, I doubt whether any one of the three species mentioned ever swallows the cast-off slough—at any rate, those I have kept have never done so. It is no doubt true that some amphibians do this, and probably this accounts for the idea that serpents act in a similar manner. The third point is perhaps the most interesting, and it is that to which I wish specially to draw attention in this paper. What determines whether the slough comes off whole or in pieces? To answer this question it is necessary to understand the nature of the process. There are two distinct stages in the act of sloughing.