Parasite animals are divisible, although not strictly so, into ecto-para-sites (epizoa), and into ento-parasites (entozoa). The former infest the surface of the body, the latter its different cavities and parenchymata.

Some of them are parasitic during their entire existence; others only at certain periods of it. For this purpose the latter migrate, and enter into various metamorphoses.

Some of them inhabit exclusively certain definite parts of the body, both cavities and parenchymata, others on the contrary occur in various regions of the body, and in great numbers all at once. All this is contingent upon their habitudes, and perhaps still more upon the mode in which they obtain access to their place of abode.

With regard to the difficult question of their origin and propagation, modern researches in the least promising domain, namely, of the helmin-thes, have pretty well succeeded in subverting the older doctrine concerning the generation of parasites, and their relation to the animals which they infest.

They get introduced into the organism as ova, as larvae, or even as developed creatures; and wherever they meet with a nidus congenial to their nature, live and thrive upon it. For this habitation to last, however, a peculiar disposition on the part of the subject is no doubt indispensable. In the different parasites this sort of predisposition differs materially. Much is assuredly not required to incur a visitation of ascarides. On the other hand, notwithstanding the extensive dissemination of the ova of the helminthes, the disproportionately small number of persons affected with worms; the circumstance that, under certain conditions (for example, disease), worms for the most part, if not altogether, abandon the individual they had infested; and lastly, the fact that different kinds of worms are proper to different animals; testify to the necessity of the peculiar disposition adverted to, existing in persons affected with worms.

1. Infusoria

The most frequent are the vibriones, in purulent and other protein-fluids in the progress of decomposition. Donne has detected a vibrio in the pus of chancre, and rated it beyond its worth.

In pus, the vorticella, and also the colpoda cucullulus (Vogel) occur.

The trichomonas vaginalis, detecting by Donne in the vaginal mucus of syphilitic females, is probably not an infusorium, but a misshapen ciliary cell from the uterus or the tubes.

Lastly, we have to cite the haematozoa occurring in the blood; if they be not rather the embryones of worms, which is probably the case with many of them.

2. Insects

Besides the various flies which infest putrid ulcers with their ova and maggots, and the exotic [still problematic] oestrus hominis, we have the flea, the lice, and the bug.

(a.) Pulex irritans, the common flea.

(b.) Pulex penetrans, the sandflea, common in the "West Indies and in South America. The impregnated female burrows into the skin, especially beneath the toe nails, where the brood gives rise to malignant sores.

Of lice there are -

(a.) Pediculus capitis, the head-louse.

(b.) Pediculus pubis, the crab-louse, infesting, the scalp excepted, every hairy part, and penetrating the skin with its head.

(c.) Pediculus vestimenti, the clothes-louse, infesting parts of the body devoid of hair, and uncleanly vestments.

(d.) Pediculus tabescentium, the louse of wasting disease, in which it occurs in great multitudes. The notion, however, that there is a disease in which lice are generated beneath the skin, is without doubt fallacious.

Of bugs, we have only to mention the ordinary bed-bug, cimex lectu-larius.