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may be observed to unroll themselves and, in a very short time, to swim about, more suo, with great activity. Manifestly these larvæ come from a guinea-worm lying in the tissues and communicating with the surface through the little hole in the derma. If the douching be repeated after an hour or longer a further supply of larvæ can be obtained; and this can be continued from time to time until the worm has emptied herself. Apparently the cold applied to the skin of the host stimulates the worm to contract and thereby force out her uterus, inch by inch, until it is completely expelled. When the protruded portion of uterus ruptures, under the pressure of the vis a tergo, it shrivels up, dries, and thus effectually seals up the end of that part of the organ which is still inside the worm, thereby preventing what might be inopportune and too prodigal discharge of larvæ.

Fig. 151.—D. medinensis.
a, Side view; b (After Looss.) front view.

The larva (Fig. 151).—The larvæ are not cylindrical; they are distinctly flattened. In swimming they move by a sort of side-to-side lashing of the tail, and tadpole-like motion of the body. The movements are intermittent; sudden, short swims alternating with brief pauses. When progressing, the greater transverse axis of the body is perpendicular to the plane travelled over; but when, from time to time, the little worms pause, they gradually roll over on to their flat surfaces. As soon as they come to rest on the flat they