Animal Parasites are invertebrate animals which have taken up their abode in or upon the living bodies of other animals.
From a broad biological outlook parasitism is a negative reaction to the struggle for existence, and always implies a mode of life that is nearer the line of least resistance…
Occasional parasites are to be distinguished from permanent parasites. Among the former are the bed-bug and the leech, which usually abandon their host when they have obtained their object.
In the embryo stage they are migratory, moving from host to host, or to a free life before becoming mature…
Among the latter are the so-called fish-lice, which, with piercing mouth organs and elaborate clinging apparatus, remain in the same host always, and are among the most degenerate parasites known.
Parasites affect their hosts by feeding upon their living tissues or cells, and the intensity of the effect upon the hosts ranges from the slightest local injury to complete destruction.
The Encyclopedia Britannica.