ORICIN OF PARASITIC LIFE. A knowledge of this subject film best be gained by a study of certain groups where all gradations (went' from free-living to parasitic forms. '1'lle first ex ample may be taken from the group Copepods. :Ilanv of these small crustacea swim free in the sea and in ponds. They feed 011 all sorts of organic diibris. One species. Argulus, trawli 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 flow ing through them. It is doubtless harmful; it is a true external parasite. Finally other Cope poda, e.g. Lem:comma, have penetrated between the scales and are found imbedded deep in the muscles of menhaden 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 Asearid:e, 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 COMMENSAL 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 spimlis. (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 those 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 (Fla ra immitis) has been found to be transmissible from parent to fcctus. as seems to occur in few species. Most parasites depend either upon the flesh of the host being eaten by a second host, or else the young are discharged. encapsuled (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 eon timiity of life from one individual to the next, the fertility of parasites has become extraordi nary. Indeed, in the group of copepods. where
the embryos are carried in external pouches, one may see how the parasitism becomes inure complete as the embryo pouches become larger.
Van Beneden states that a single nematode pro duces 60.000,00o ova. The rich food supplv ,.1 the parent makes this great fertility possible. lit extreme cases the parasite becomes little but an egg-sae. To increase the fertility still fission, in the case of cestodes :It least, Ins been added; .o that the number of fertile individuals is increased. Since a species is little• likely 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. First, temporary parasites most 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 eestodes have both slickers and hooks; in still other cases, as in the degenerate cirriped Saeuhim. roots or bold fasts are developed, which also serve as imMlltory tubes; (3) endoparasites lose their locomo tor organs, for large legs would lie a disadvan tage to burrowing parasites, such as Demodex, among mites, or unnecessary. as in the copepod Caligus; (4) endoparasites lose also their sense orgaws, because no longer useful. as is the case in Copepoda; (5) the alimentary tract becomes degenerate in extreme forms, because food is gained by osmosis through the body wall (e.g. cestodes). In less extreme cases the ali mentary tract is simplified on account of the absence of necessity for digestive apparatus. ECONOM ICAL CONSIDERATIONS. The number of animal parasites harbored by one host may be enormous. Thus, in a young horse Krause f , 300 Asearis. 190 Oxyuris, several mill' s of Strongylns, •'14 Sclerostomum, •3S7 Filarial, 69 Drilla adult and 9 immature. The destruction wrought by these parasites is sometimes very great. It is estimated that half a million pul lets the yearly in England from `gapes.' caused by a threadworm (Syngamus traelowlis). England in 1SSO 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 United States Bureau of Animal Inilu•try makes otlieiaI investigations into epidemics of parasites among the higher animals.
Bninmia.tritv. Leuckart, The Parasitis of and the Diseases Induced by Them (trans. by Hoyle. Edinburgh. 18961; Glienniann, Treatise on the Parasites and Parasitic Diseases of the Domestica led A n Is ( tr 8114. by Fleming. don. : Vail iteneulen. _tabard Parasiti s and C.58 mates (ih.. 1S711) : \!a aiez, Leg parasites de l'hommr 1SRS).