are all sessile in the adult condition. The Lepadidae are the least
modified and are rarely parasitic; the Balanidae are more modified
and frequently become embedded in the skin of whales. The
Abdominalia live as parasites buried in the shells of other Cirripeds
and of molluscs. The Apoda live as parasites in the mantle of
other Cirripeds, whilst the Rhizocephala live chiefly on the abdomen
of Decapod Crustacea, sending burrowing root-like nutritive processes
into their tissues.
Insecta. — A very large number of insects are temporary or permanent parasites of animals or plants, the adult stages being chiefly ectoparasitic, the larval stages endoparasitic. The Ilemimeridae, allies of the earwigs, are ectoparasites on rats. The Mallophaga or bird-lice are degenerate wingless insects spending their whole lives as ectoparasites on birds and mammals. The larvae of Hemerobiinae are parasitic on Aphides. The saw-flies are parasitic on plants. There are over 200,000 species known of the Hymenoptera parasitic or Terebrantia. The adults deposit their eggs in the eggs, caterpillars or adults of other insects, particularly Lepidoptera. The clothes-moth, for instance, is known to be subject to the attack of over sixty species of Hymenoptera. To such; an extent has parasitism been developed in this group, that the parasites themselves are attacked by other parasites, giving rise to the phenomena known as hyper parasitism. The gall-flics (see Galls) are included amongst the Terebrantia, but in their case the early stages are passed in vegetable galls more frequently than in the bodies of other insects. The ruby-flics {Hymenoptera Tiibulifera), in the larval condition are parasitic on the larvae of wasps and bees. The Demidatae are bees that in the larval stage are parasitic on other bees, the larvae of the parasites being deposited in the food-cells prepared for their own larvae by other bees. Many of the fossorial Hymenoptera form no special nests for their young, but take advantage of the abodes and food-stores prepared by other insects. The very large number of Hymenopterous insects that collect living larvae to be shut up as provender for their developing young are in a sense parasitic. The complex relations of ants with other insects must be referred to in this connexion. The nests of many species are inhabited by foreign insects of various orders, such insects being termed myrmecophilous or ants'-nest insects. The relations between the ants and their guests are very complex, and the guests migrate with their hosts. Aphidae, Coccidae and other bugs that secrete sugary matter are cherished and tended by ants; so also the caterpillars of some Lycaenid butterflies are kept as a kind of domesticated animal for some useful purpose. There are also many Orthoptera, Hemiptera, and other insects, as well as some acarids and wood-lice found only in ants'-nests as cherished or tolerated guests. The relations between ants and plants is also interesting; the ants live as parasites on the plants or trees, but in return protect them from more harmful intruders. Such phenomena are on the border-line between symbiosis and true parasitism. Although most beetles live on decaying animal or vegetable matter, a large number are parasitic in the adult or larval condition on animals or plants. The curious beetle known as Platypsyllus castoris is known only as an ectoparasite of the beaver, whilst the Leptinidae are parasites of several species of mammals. The minute beetles of the families Mordellidae and Rhi pi phoridae are endoparasites of wasps and cockroaches, whilst the larvae of many of the Cantharidae are parasites of locusts. The Strepsiptera are endoparasites of Hymenoptera and Hemiptera. The habits of the Diptera easily pass over into parasitism, and a very large number are temporary or permanent parasites in the adult or larval stages. Most of the larvae of the Cecidomyiidae live in plants and form galls or other deformities. The bloodsucking habits of mosquitoes and gnats and sand-flies have not led to any special development in the direction of parasitism. The larvae of Bombyliidae are endoparasites of the larvae of mason bees, and some of the Cyrtidae similarly infest spiders, whilst the Tachinidae deposit their larvae in other living insects, caterpillars being especially selected. The larvae of some of the Sarcophagidae may be deposited in the nostrils of man and other animals, where they may cause death, whilst those of the South American genus Lucilia infest the nasal fos, sae and frontal sinuses of man, producing great suffering, and the larvae of the numerous kinds of bot-fly attack man and many animals. The very large group of Pupipara live by sucking the blood of mammals and birds, and many of them are reduced to wingless permanent parasites. The single member of the family Braulidae is a parasite of the bee. All the known fleas (Aphaniptera or Siphonaptera) are ectoparasites in the adult condition; the larval stages are usually to be found in organic refuse. The larvae of most Lepidoptera are temporary ectoparasites of plants, but a few attack other insects, such as coccids and aphids. All the Hemiptera (bugs) have sucking-mouth organs and the majority of them are temporary parasites of plants or other animals. Some, such as the bed-bug, have been so modified by parasitism as to be found only in human dwellings, others, such as the aphids or plant-lice, are permanent parasites of plants, many of them producing galls. The coccids. or scale insects, have been still further modified as plant ectoparasites. The Pediculidae, or lice, are the most completely parasitic of insects, and are degraded wingless insects found on almost any kind of bird or mammal,
but in most cases so highly modified as to be capable of existence only on the particular species with which they are associated.
Lower Invertebrates. — No true Chactopods are parasitic, but a few are commensal. The leeches are probably Chaetopods modified by parasitism; and Myzostomes are still more highly modified relatives of the group, very degenerate and parasitic on Crinoids. A few rotifers are ecto- and endo-parasites. No Brachiopods, Folyzoa or Echinoderms are true parasites. The flat-worms and round-worms contain the most characteristic endoparasites, and parasitism is so characteristic a feature of most of the groups that it is discussed in the separate articles dealing with the various natural assemblages of such worms. All the Cestodcs (see Tape-WOR}*), most of the Treniatodes (q.v.), and a few of the Planarians {q.v.) are parasites of animals. Most Nemertines are free-living, but Cephalothrix galatheae is endoparasitic in the ovaries of the Crustacean Calathea strigosa, whilst Eunemertes and Tetraslemma occur on Ascidians, and Malacobdella in lamellibranch Molluscs. The degraded Mesozoa (q.v.) are endoparasites of Planarians, Nemertines and Ophiurids. The Nematoda (q.v.) or typical round-worms, exhibit every degree from absolute free-life to absolute parasitism in animals and plants. The Echiuroidea (q.v.) are mostly free-living, but the male of Bonellia lives as a very degenerate parasite in the uterus and pharynx of the female. Although Coclcntera and I^orifera arc usually sessile, very few are true parasites; young stages of the Narcomediisae are parasitic in the mouth of adults of different species, whilst Mnestra parasites is a degenerate medusa living on the pelagic mollusc Pbyllirhoe. The Protozoa, from their minute size and capacity to live in fluids, naturally include an enormous number of parasitic forms, the importance of which in producing disease in their hosts is so great that a very large special literature on parasitic protozoology is being formed (see Pathology). Of the Sarcodina (q.v.) many forms of Amoeba such as Amoeba coli are associated with dysentery and kindred diseases. A very large number of the Mastigophora (q.v.), including such forms as the trypanosome of sleeping-sickness, are parasitic; in fact, observation by adequate means of the juices of almost any animal reveals the occasional presence of some kind of mobile protozoan, provided with a whip-like process. The enormous group of Sporozoa (q.v.) are entirely parasitic, and have been found in every group of animals except the Protozoa and Coelentera. Infusoria (q.v.) contain a considerable number of parasitic forms, some endoparasitic; others like the Suctoria, ectoparasitic.
B.— Plants.
Bacteria. — Every degree of adaptation to parasitism occurs amongst bacteria, a majority of which pass at least some stage of their lives in a parasitic condition.
Fungi. — As in the case of Bacteria, the absence of chlorophyll from the tissues of fungi makes it necessary that they should take up carbon compounds already assimilated by other organisms, and accordingly they are either saprophytes or parasites. The mycelium is, so to say, the parasitic organ of the fungus, ramifying in the tissues of the host. The plant may obtain access to its host by means of spores which enter usually by wounds in the case of animal and plant hosts, but occasionally by natural apertures such as the stomata of plants. The fungi Ihat develop in the organs of warmblooded animals reach the blood-stream through wounds, and thence spread to the tissues where germination takes place. Many fungi, especially those that are epiphytic, reach the tissues of their host by germ-tubes which emerge from the spore and penetrate either by a natural or artificial aperture, whilst in other cases the germ-tubes or hyphae actually penetrate uninjured tissues or membranes.
The fungi parasitic on animals are in most cases little known, and additions to the list, of which the pathological rather than the botanical features have been worked out, are constantly being made. A number of species of Eurotium and Aspergillus, usually saprophytic, may migrate to the bodies of animals, spreading in the tissues and exciting a disease known as mycosis or aspergillosis. They were first discussed in the disease of the human ear known as otomycosis, but they occur also in lungs and air-passages of mammals and birds. Recent pathological investigations conducted at the Prosectorium of the Zoological Society of London, show that mycosis is extremely frequent and fatal in birds and reptiles, and rather less frequent in mammals. Almost any organ of the body is liable to attack. The Laboulbenieae are probably Ascomycetes restricted to parasitism on insects, chiefly beetles and flies, sometimes forming a thick fur on the bodies and spreading by spores. The Entomophthoreae, possibly Mucorini. are also restricted to insects, the fungus that kills the common house-fly being the most familiar example. Cordyceps mililaris and Botrylis bassii are familiar examples of Ascomycete fungi that attack the caterpillars of insects, the latter producing the fatal disease " muscardine " of the silkworm. The group of Saprolegnieae usually vegetate as saprophytes but readily settle on aquatic animals such as goldfish, salmon, salamanders and frogs, with fatal results. It is not yet entirely certain if diseases of this kind, of which the salmon disease is the most notorious, are produced on healthy animals, by the
