ECOLOGICAL 151 



like valerianic and butyric. If the worms die and disintegrate in the 

 intestine, the liberation of these substances may readily produce 

 toxic effects; and students have often suffered cutaneously after 

 dissecting the large Ascaris of the horse. The poisoning observed 

 in the formidable disease of trichinosis, due to man's eating infected 

 ham, either raw or insufficiently cooked, is probably due to products 

 of the parasite's metabolism as well as to disintegration of the 

 surrounding muscle in the case of the pig host. 



Of great interest medically is the fact that the irritation produced 

 by the pressure of the parasite may lead to cancer-like growths in 

 the host. Thus cancer-like results are produced in man's bladder 

 by the Egyptian Bilharzia worm, and sarcoma of the rat's liver is 

 believed to be instigated by the bladderworm stage of TcBnia 

 crassicollis, which occurs as a tapeworm in the food-canal of the cat. 

 But the most striking case is that worked out by Professor Johannes 

 Fibiger of Copenhagen, to whom was awarded one of the 1926 

 Nobel prizes. He investigated in detail a threadworm or Nema- 

 tode, Gongylonema neoplasticum, which lives in cockroaches. If the 

 infected insects are eaten by rats, the Nematodes pass into their 

 stomachs and by their irritation instigate the growth of malignant 

 tumours. 



It must be noted, however, that, in an enormous number of cases, 

 the presence of parasites is neither here nor there. A modus vivendi 

 is often established between parasite and host, and no appreciable 

 harm results. But, if the host be otherwise debilitated, the parasites 

 may multiply fatally. The introduction of a parasite into anew 

 host is, of course, in many cases fatal, as when Trypanosomes are 

 introduced by tsetse-flies into man or horses, or into any other 

 appropriate host that is without natural defences against the 

 intruders. It is not known that the Tr5^anosomes found in the 

 blood of tropical African antelopes do their hosts any appreciable 

 harm. 



ADAPTIVE CHARACTERS OF ANIMAL PARASITES.— The 



assumption of a parasitic mode of life is a habitudinal reaction to the 

 intensity of the struggle for existence ; and while there is no discharge 

 from that war, intimate dependence on another organism for food 

 and shelter implies to some extent a life of ease. What adaptations 

 are there to the parasitic mode of life ? 



(i) Many parasites have structures that lessen their risk of 

 dislodgment, e.g. the adhesive suckers on the head of tapeworms, 

 the gripping hooks of the hookworm, the attaching hold-fasts of 

 parasitic Copepods. 



(2) Many parasites are specially adapted for the absorption of 

 food from the host. A very simple adaptation is the great increase 

 of absorptive surface in tapeworms, which may be many feet long. 



