The largest number of parasitic forms and the greatest variety of parasitic adaptations, are found among animal organisms. Al most every order or phylum has parasitic representatives and cer tainly every animal can act as host to a variety of parasites be longing to several phyla. Protozoa, helminths and insects are the most widely distributed parasites, occurring in the greatest variety of hosts.
Parasitic helminths occur in representatives of most of the orders of the animal kingdom; and parasitic insects are found to attack a great variety of terrestrial animals. A great number of parasitic forms are known among Crustacea, but the range of their hosts is more limited. It is interesting to note that while para sitic insects have a special predilection to invade other insects, so the parasitic Crustacea have a tendency to live upon other Crustacea. Parasitism occurs also in Coelenterata, Rotif era, An nelida, Mollusca and even in fishes (Cyclostomata, q.v.).
Parasitism is so common in nature that it is hardly possible to find representatives of any species of animal which do not har bour parasites of some sort ; and often the same individual is invaded simultaneously by parasites of various orders.
Hyperparasitism.—There is no protection in nature against parasitic invasion. The parasite will reach its host however se cluded may be its position. An insect burrowing itself deep into
the tissues of a plant is not protected from invasion by another parasitic insect, which will find a way to insert its eggs on the skin or inside the body of such a burrowing host. Even the parasite living right inside the body of its animal host is not secure against attack by another parasite. The latter, which is called a hyper parasite, will reach, and will live upon the primary parasite which may ultimately be destroyed, and all this takes place inside the body of the primary host. Hyperparasitism is known among protozoa and helminths, but is more frequent among parasitic Crustacea and is very common among parasitic insects.