"When the vesicle breaks," the Doctor answered, "the end of the worm shows itself and hangs outside. It is gently pulled and coiled round a piece of linen or a small stick, like a section of a toothpick, and then fastened over the wound with sticking-plaster and a compress. Twice a day the performance is repeated, and as much as possible of the worm is coiled away. It takes all the way from a fortnight to three or four months to remove a worm in this way. The worms vary from six inches to three yards in length, and their circumference is about that of small wrapping-twine. If a worm is broken in the process of extraction it is liable to cause inflammation, fever, deformities, loss of the limbs, mortification, and death. So you see it is not to be trifled with." "What a terrible scourge!" said one of the boys. "I shall take good care not to go into the water in the region where this worm abounds." "It has been known and mentioned in ancient as well as in modern writings," the Doctor continued; "and some authorities argue that the 'fiery serpents' which attacked the Israelites in the wilderness were in reality guinea-worms." "How could that be?" Fred exclaimed. "They could not be anything like serpents; and, besides, the pictures we have of the events of the Exodus show that the Israelites were bitten by something larger than the little threads you have described." "That is quite true," was the reply; "but bear in mind that the pictures in our books were not made at the time, but many centuries afterward. The words in the original Hebrew--which are translated in our version as 'fiery serpents'--refer unmistakably to something which caused an inflammatory wound, and do not describe the serpent any farther than this. By the Greeks the _Filaria_, or guinea-worm, was reckoned among the serpents, on account of its form as well as the results of its bite; and those who have studied the subject say that the theory is supported by the natural conditions of the country through which the Israelites passed, while the mortality among them can be accounted for by their ignorance of the proper treatment. From a scientific point of view, if not from a popular one, the subject is an interesting study." [Illustration: NESTS OF WHITE ANTS.] Doctor Bronson paused, as his attention was drawn to some conical mounds on the shore near which they were passing. "They are ant-hills," said the Doctor, after a brief survey. "They are made by the white ants, which are found in various parts of Africa, and display considerable skill in the construction of their homes." The steamer halted for wood at a point close to several of the mounds, and thus gave the youths an opportunity to examine them. They found the ant-hills varying in height from six to ten feet, and composed of a yellowish earth, nearly as hard as brick, and quite capable of resisting the action of the rain.