MO LIFE : OUTLINES OF GENERAL BIOLOGY 



an animal is found to Ik* infested externally or internally by other 

 animals which are habitually present and are not found living 

 independently, and when there is some degree of dependence 

 between the infesting animals and the other, the term parasitism 

 is used without distinctions. But as ecology progresses to clearness 

 beyond the old "natural history" from which it arises, we must 

 more clearly distinguish the various scenes or stages of parasitism. 

 Consider some of these: a flea, promenading over the skin and 

 puncturing here and there for blood; a tick, firmly fixed with its 

 mouth-parts deeply inserted in the dermis; a follicle-mite, such 

 as the common "black-head" often seen on the human face [Demo- 

 dex follicttlontm), with its whole worm-like body embedded and 

 absorptive, and only its head showing; a larval red harvest- 

 mite (Trombidium) sometimes felt burrowing in the skin after a 

 gooseberry feast or a grass picnic ; the microscopic larva (cercaria- 

 phase) of a Bilharzia in the act of burrowing from the surface of 

 man's skin towards the intestinal or renal blood-vessels; the large 

 maggots of the ox-warble-fly, which have come to lie passively 

 under the skin, resting after a prolonged internal journey, and 

 awaiting subsequent pupation in the soil; the full-grown female 

 Guinea-worm, lying in a long coil beneath the skin, and until a 

 sore is formed. 



These are but instances of the manifold gradations of habit among 

 parasites which attack the surface of another animal. 



Similarly for endoparasites, there is great diversity. Many Infu- 

 sorians and many Nematodes live in the terminal part of the intestine 

 on the putrefying undigested food-material; they can hardly be 

 called symbions except in special ca.ses, but they do little harm. 

 Tai)eworms and the like have their head attached by suckers or 

 hooks or sometimes proboscis to the wall of the intestine; but 

 this has no nutritive significance, for it is the whole long surface of 

 the body that absorbs the digested food of the host. Here is a 

 passive mode of life, almost eluding the struggle for existence, and 

 here also is unmistakable degeneration. Another grade is illustrated 

 by many parasites which depend not on the food of their host, but 

 on the living tissues; and here again a distinction may be drawn 

 between, say, bladder- worms (the cystic phase of tapeworms and 

 other cestodes) growing in a muscle and forms, like Sacculina, which 

 absorb lymph or other fluids from the surrounding tissue, and 

 others, like larval Ichneumon-flies, that directly devour the living 

 tissues of their hosts. The formidable hookworm (Anchylostomum) 

 sucks blood from the intestinal wall; the liver-fluke (Distomum) 

 feeds on the blood of the sheep's liver; but the malaria organisms 

 and many others are in the blood stream itself, destroying the 

 red blood corpuscles. Not a few cases besides Sacculina and its 

 kindred are known where the parasite castrates its host. 



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