Page:Popular Science Monthly Volume 8.djvu/703

Rh eggs. The filaria at last is so entirely atrophied that Prof. Jacobson, after seeing it alive on one of his patients at Copenhagen, wrote to Blainville: "This medina worm is not really a worm; it is a sheath full of eggs." In fact, all the internal organs disappear, and nothing is found in their place except the eggs and their embryos.

The ichneumons and many other insects that lay their eggs in the living larvae of other species, belong to a class of parasites that begin life as dependents, but that become free and self-supporting on arrival at adult age. The Œstrus or gadfly of the horse (Fig. 22), is

thus dependent in its early life. But, instead of making their attacks on those of their own class, the gadflies prefer to install themselves on mammals, and sometimes even on man. The eggs are received into some cavity of the body, nostrils, stomach, or a hole in the skin, where they hatch and where the larvæ feed until the adult state is reached, when they escape and afterward live in freedom.

There is a large class of parasites generally known as worms, characterized by the circumstance that during their lives they undergo certain strange transformations that can only take place by the