I50 LIFE : OUTLINES OF GENERAL BIOLOGY 



persistently eliminated in the course of evolution, as would naturally 

 happen if they destroyed their host before becoming themselves 

 reproductive. But the large fact to be emphasised is that in many 

 cases a give-and-take relation is established between the parasite 

 and the host, such that the parasite does not get the upper hand and 

 the host is not too seriously prejudiced. In the intestinal caeca of 

 the grouse there are often thousands of almost invisibly transparent 

 Nematodes {TricJiostrongylus pergracilis), whose early stages are 

 found on the heather. If the grouse is otherwise in good condition, 

 its Nematodes seem to be unimportant, but if the grouse be con- 

 stitutionally below par, the parasites may multiply excessively 

 (10,000 in one bird) and fatally. When parasites or quasi-parasites 

 prove quickly destructive, it is usually when they find their way 

 into a new host that offers great advantage, as by having no natural 

 counteractives to their influence and increase. This is familiarly 

 illustrated by microbes when they find themselves in a new host, 

 which thus may not have powers or time to develop the wonted 

 natural checks, such as "anti-bodies". 



As to the nature of the damage done by parasites, it may be 

 enough to mention the most outstanding: (a) robbing their host of 

 much half-digested food (ninety specimens of a big and long tape- 

 worm (Bothriocephalus) in one patient!); (6) absorbing much 

 blood (as in the case of hookworms in man, and of gapes-worms in 

 chickens); (c) causing serious pressure on adjacent parts, e.g. the 

 sturdie-worm {Ccenurus cerebralis) on the sheep's brain; {d) per- 

 forating the intestinal wall (as large threadworms sometimes do); 

 {e) blocking passages, as bee-mites [Acarapis woodi) in the thoracic 

 tracheae of hive-bees with "Isle-of-\Vight" disease. 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 a cancerous growth in fishes by the irritation of a 

 Nematode worm (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. 

 Sarcosixjridia of sheep contain toxic substances which are fatal 

 in very small (quantities when injected into rabbits. (6) When the 

 big cysts of Ticnia cchinococcus burst, and the fluid escapes into the 

 body cavities, there is violent poisoning; and the fluids of other 

 bladderworms have been shown to be toxic, (c) Some adult tape- 

 worms (e.g. Dihothrioccphalus laius) are also toxic, producing a 

 ha?molytic lipoid substance, which is liberated when segments 

 disintegrate, and perhaps also as a secretion. Thus the anaemia 

 of human j)aticnts Ixxomes more intelligible; and it is probable that 

 other tapeworms libiTatc toxic substances, {d) Ascaris produces in 

 the routine of its metabolism volatile aldehydes and fatty acids, 



