As to the nature of the damage done by parasites, it may be enough to mention the most outstanding effects :—robbing their host of much half-digested food ; absorbing much blood ; causing serious pressure on adjacent parts (the sturdie-worm on the sheep's brain) ; perforating the intestinal wall (as large thread worms sometimes do) ; blocking passages, as bee-mites (Acarapis woodi) in the tracheae of bees afflicted with "Isle-of-Wight" dis ease. More unusual is castration, e.g., that of crabs infected by parasitic epicarids; or the formation of open sores by emerging guinea-worms ; or the production of cancerous growth in fishes and rats by the irritation of nematode worms (Gongylonema).
It has been proved that some animal parasites are toxic. (a) Thus the malaria organisms produce toxic substances in the red blood corpuscles, and these are liberated when the corpuscles break up, causing fever. Sarcosporidia of sheep contain toxic sub stances which are fatal in very small quantities when injected into rabbits. (b) When the big bladders of Taenia echinococcus burst and the fluid escapes into the body cavities, there is violent poison ing; and the fluids of other bladderworms have been shown to be toxic. (c) Adult tapeworms, such as Dibothriocephalus latus, are also toxic, producing a haemolytic lipoid, which is liberated when segments disintegrate, and perhaps also as a secretion. Thus the anaemia of human patients suffering from worms becomes more intelligible. (d) Ascaris produces in the routine of its metabolism volatile aldehydes and fatty acids like valerianic and butyric. If the worms die and disintegrate in the intestine, the liberation of these substances may produce toxic effects. The poisoning observed in trichinosis is probably due to products of the parasite's metabolism and of the muscle-disintegration.
like roots right through the body of the parasitized crab. The head of the pigmy parasitic male of the mid-water angler is concrescent with the tissue of the female who carries him. The strange gastro pod parasite Entoconcha has its head thrust into a blood-vessel of its synaptid host. When the available food from the host is very abundant it may be utilized by the parasite in a somewhat uneconomical fashion; thus some nematodes ferment glycogen into valerianic acid, carbon dioxide and hydrogen, which is far from making the most of the material. In tapeworms the whole surface of the body absorbs liquid food. Parasites deeply im bedded in tissues must be nourished by the lymph just as if they were parts of the host. (3). It is characteristic of many thorough going endoparasites that they can survive in conditions where free oxygen is apparently very scarce. In most cases they seem to ob tain a sufficient supply from the blood or tissues of their host, just as if they were parts of the body. There is no modern corrobora tion of the older view that some parasites can live anaerobically. (4). Many parasites, such as nematodes, crustaceans, insects, mites have a chitinous cuticle, which is very resistant, e.g., to bacteria and to digestive juices. (5). There are some noteworthy adapta tions in connection with reproduction. Thus resistant egg-shells are characteristic of platyhelminths, and the tapeworm's libera tion of an entire joint, capable of some independent movement, must often be advantageous. The difficulty of securing fertiliza tion, when the parasites do not occur in large numbers together, is met in various ways, e.g., (a) by a very prolonged association of the sexes as in Bilharzia, where the male carries the female, or in Chondracanthus where the female carries the male; or (b) by self fertilization or autogamy, as in the liver-fluke; or (c) by an emer gence of the sexually mature forms into freedom, as in horsehair worms (Gordiacea). There are some very remarkable cases, notably the trematode, Diplozoon paradoxurn, where two mature hermaphrodite individuals are united in a permanent coition. In W edlia two individuals are found together inside a cyst, the smaller one—the male—imbedded into a protrusion of the vesicu lar posterior body of the larger one—the female. But each individual shows traces of the gonads of the opposite sex, so that this looks like a secondary abandonment of hermaphroditism, when arrangements for securing fertilization had been in the course of time established.