Parasitism itself occurs in various ways and degrees. According to R. Leuckart, we should distinguish between OCCASIONAL (temporary) and PERMANENT (stationary) PARASITISM. Occasional parasites, such as the flea (_Pulex irritans_), the bed-bug (_Cimex lectularius_), the leech (_Hirudo medicinalis_), and others, only seek their “host” to obtain nourishment and find shelter while thus occupied. Without being bound to the host, they usually abandon the latter soon after the attainment of their object (_Cimex, Hirudo_), or they may remain on the body of their host throughout their entire development from the hatching of the egg (_Pediculus_). It follows from this mode of living that the occasional parasites become sometimes distinguishable from their free-living relatives, though only to a slight extent. It is, therefore, seldom difficult to determine the systematic position of temporary parasites from their structure. In consequence of their mode of life, all these temporary parasites live on the external surface of the body of their host, though more rarely they take up their abode in cavities easily accessible from the exterior, such as the mouth, nose and gills. They are therefore frequently called EPIZOA or ECTOPARASITES; but these designations do not cover only the temporary parasites, because numerous epizoa (as for instance the louse) are parasitic during their entire life. In contradistinction to these temporary parasites, the permanent parasites obtain shelter as well as food from their host for a long period, sometimes during the entire course of their life. They do not seek their host only when requiring nourishment, but always remain with it, thus acquiring substantial protection. The permanent parasites, as a rule, live within the internal organs, preferably in those which are easily accessible from the exterior, such as the intestine, with its appendages. Nevertheless, permanent parasites are also found in separate organs and systems, such as the muscular and vascular systems, hollow bones and brain, while some live on the outer skin. Here again, the terms ENTOZOA and ENDOPARASITES do not include all stationary parasites; to the latter, for instance, the lice belong, which pass all their life on the surface of the body of their host, where they find shelter and food and go through their entire development. The ectoparasitic trematodes, numerous insects, crustacea, and other animals live in the same manner.