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Arachnida . Vertebbata.. Species Many parasites among Acarina (mites) Demodex lolliculorum ; Pentas- tomum. Myxinidte (hag-fishes). Host Mammals, birds, and insects. Itch-mite of man: of young hare, cat, ox : of adult dog, wolf, horse. Bore into the body cavity of fishes. HemarlcB A greatl.T modified arthropod. Ttie onl.r parasitic vertebrates.
about on the surface of the body of fish and feeds on their slime. Another genus, Caligus, has taken a further step; it has migrated under the gill-cover of a fish and has attached itself to the gills, receiving food from the blood fiow- ing through them. It is doubtless harmful; it is a true external parasite. Finally other Cope- poda, e.g. Lern.'eonema, have penetrated between the scales and are found imbedded deep in the muscles of menliadcn and other marine fishes living entirely on the juices elaborated by them, and typifying a complete internal parasite. We see in this case how living or waste organic matter is by easy gradations to live on and in living organisms.
The nematode worms are chiefly parasitic. Some of the Anguillulidfe. or vinegar eels, how- ever, live in organic fluids. The Ascaridse, or stomach worms, live for the most part in the organic fluid contents of the intestine, which they still have to digest somewhat, so that they are on the line between messmates (see Commexsal- ism) and true parasites. They are, however, in great danger of being indubitable parasites, and this is the fate that befalls some of their kin- dred; e.g. Trichina spiralis. (See Trichina.) Anguillula, Ascaris, and Trachina show how para- sitism has arisen in a group that originally had come to live on organic fluids, and especially how tho.se animals that live on the surface of other animals, devouring their excretions (or waste fluids), will easily come to penetrate into the flesh, devouring the vital secretions (or func- tional fluids). They will pass from scavengers to parasites.
The question why animals which are in a posi- tion to become parasites often do become para- sites is not difficult to answer. Parasitic life brings great advantages to the parasite. First. it affords an abundant food supply ; second, it diminishes the chance of direct attack from other organisms. The great disadvantage of abject parasitism is this, that the parasite is restricted in its environment, and since the body that it inhabits is mortal, it must make special provision for the continuation of the species. In mammals the capacity of infecting the embryo while at- tached to the mother, would be of great service in Insuring this continuity, and a certain thread- worm of the dog (Flrirr'i immifis) has been found to be transmissible from parent to ftetus, as seems to occur in few species. ]Wost parasites depend either upon the flesh of the host being eaten by a second host, or else the young are discharged, cncapsuled (to protect them from desiccation or other untoward conditions), and take their chances of being picked up by a suit- able host. To increase the chances of the con- tinuity of life from one individual to the next, the fertility of parasites has become extraordi- nary. Indeed, in the group of copepods, where theembryos are carried in external pouches, one may see how the parasitism becomes more complete as the embryo pouches become larger. Van Beneden states that a single nematode pro- duces 60,000,000 ova. The rich food su])ply of the parent makes this great fertility possible. In extreme cases the parasite becomes little but an egg-sac. To increase the fertility still more, fission, in the ease of cestodes at least, has been added; so that the number of fertile individuals is increased. Since a species is little likel}- to devour the flesh of its own kind, many parasites may pass successively into two hosts, e.g. the pig and the rat. This has given rise in some cases to an obligatory alternation of hosts. The adaptations in structures of parasites are striking. Fir.st, temporary parasites must move over the body of their hosts or go from one host to another. Hence ( 1 ) sense organs are developed to direct them in their migrations, and they are provided with locomotor apparatus, e.g. the springing legs of the flea : (2) stationary parasites gain apparatus for holding on, as suck- ers in cestodes and nematodes, and hooks in mites and copepods. Certain cestodes have both suckers and hooks; in still other cases, as in the degenerate cirriped Saculina, roots or hold- fasts are developed, which also serve as imbibi- tory tubes; (3) endoparasites lose their locomo- tor' organs, for large legs would be a disadvan- tage to burrowing parasites, such as Demodex, among mites, or unnecessary, as in the copepod Caligus: (4) endoparasites lose also their sense organs, because no longer useful, as is the case in parasitic Copepoda ; ( .5 ) the alimentary tract becomes degenerate in extreme forms, because food is gained by osmosis through the body wall (e.g. ce.stodes). In less extreme cases the ali- mentary tract is simplified on account of the absence" of necessity for digestive apparatus. Economical Consideration.s. The number of animal parasites harbored by one host may be enormous. Thus, in a yoimg horse Krause found 500 Ascaris, 190 Oxyuris, several millions of Strongj'lus, 214 Sclerostomum, 287 Filaria, 69 Ticnia^ adult and 9 immature. The destruction wrought by these parasites is sometimes very gi-cat. It is estimated that half a million pul- lets die yearly in England from. 'gapes,' caused by a threadworm " (.'?.i/rt<7a"n(S trachealis) . England in 1S80 was estimated to be losing about 1.000,- 000 sheep annually from liver-flukes. Fortunate- ly, the United States has been visited by no such scourges, but scores of cattle regularly die of the liver-fluke in this country. The T'nitcd States Bureau of Animal lndu>try makes official investigations into epidemics of parasites among the higher animals.
BiBLiOGRAniY. Leuekart, The I'arasitcs of Man and the Diseases Induced 6.7 Them (trnns. by Hoyle, Edinburgh, 1896) : Ghenmnnn, Treatise on the Parasites and Parasitic Diseases of the Domesticated Animals (trans, by Fleming, Lon- don, 1802) ; Van Beneden, Animal Parasites and Messmates (ib., 1876) ; JIaniez, Les parasites do horn me (Paris, 1888).
