The _round-worm_ occasions the fewest symptoms, and is rather an object of disgust than of grave importance, at least in this country, where it seldom happens that more than two or three are present. In other countries, as some parts of Italy, for instance, where the drinking water is bad and stagnant, they are sometimes found in great numbers, as thirty or forty, and it is then not easy to determine whether the symptoms which accompany them are produced by the worms, or by the unwholesome character of the water in other respects. They appear to live on the contents of the intestines, and do not adhere to them, as the tape-worm does, and hence their comparative harmlessness, and they have no power, as has sometimes been mistakenly imagined, of perforating the bowels, and of thus producing grave mischief. The _thread-worm_ is the commonest variety of these creatures, and has the peculiarity of inhabiting the lowest twelve inches of the bowel, where it produces much irritation and causes very distressing itching. It is often present in great numbers, and is so rapidly reproduced, that in a week or two after it has been apparently got rid of, it may again be found as numerous as before. Certain articles of food seem to favour its development, such as pastry, sugar, sweets, beer, fruit, and anything which is apt to undergo fermentation, and thereby to impart to the evacuations a specially acid character. These worms are often accompanied with more or less marked symptoms of indigestion, but otherwise the local irritation is usually the only indication of their presence. They produce, indeed, such disturbance of the nervous system as may attend indigestion in any of its forms, but I have never but once known convulsions occur apparently due to their presence in great numbers, and ceasing on their expulsion; and this was in a child between eighteen months and two years old.