Page:Popular Science Monthly Volume 8.djvu/702

682 their particular species, which in many cases are the cause of peculiar skin-affections. Since the presence of these animals constitutes the disorder, it may be easily caught; man may communicate it to the domestic animals, and they may also give it to him; it is only the genus Sarcoptes however, that may be thus transferred from animals to man.

The true parasites just described, and many others. like them, are nourished by the blood of their neighbors, but they never establish themselves in the organs of their host, being free throughout their lives. There is another class that live in freedom while young, but when arrived at mature age, and the cares of a family are soon to be assumed, they change in appearance, choose a host, and settle down for life. The chigoe, a parasite of man in South America, is one of these. It is only the female, however, that demands both lodging and provisions, the male (Fig. 18) being contented with pillaging his victim as he passes by. It is a small species, which pierces the shoes and clothes with its pointed beak (Fig. 19), and penetrates into the substance of the skin, generally selecting that of the toes. The male, as

just remarked, takes his food and resumes his wanderings, but the female seeks a hiding-place for permanent abode, and then grows to such a monstrous size that the entire insect appears to be nothing more than a mere appendage to the abdomen, as may be seen in Fig. 20, Besides man, this parasite infests the dog, the cat, the pig, the goat, the horse, and the mule.

Another form coming within this category, and the terror of travelers on the coast of Guinea, is the Guinea-worm, Filarla medinensis (Fig. 21), also found in other parts of Africa, and said by Mitchell to have been observed in South Carolina. It was long supposed that this filaria could introduce itself into the cellular tissue of the body directly through the skin, in the form of a microscopic embryo, but several recent observers concur in the belief that it is transmitted by means of the cyclops, a little fresh-water crustacean. This is swallowed in drinking-water, and at the end of six weeks the presence of the filaria is revealed by tumors, which later develop into open sores, caused not by the worm itself, but by the dissemination of its