Inter-Relations of Living Creatures 60S 



Adaptations of Parasites 



Evolution is not always progressive, and it is illustrated in 

 the adaptations of parasites as well as in the adaptations of birds. 

 Parasites tend to go back, but their very retrogressions may be 

 effectively adaptive to the conditions of their inglorious life of 

 ease. Let us take the case of a tapeworm, floating in the human 

 intestine amid the half -digested food, and moored by its head to 

 the wall. 



It is safe from all enemies (unless, perhaps, the medical prac- 

 titioner with his vermifuge) ; it floats in a plethora of food, 

 which it can absorb by the whole surface of its long tape-like 

 body; it can live and thrive with a minimum of oxygen, and 

 it has a mysterious "anti-body" which preserves it from being 

 digested by its host; it has on its head muscular adhesive 

 suckers, and, in some species, attaching hooks as well, so that 

 it is firmly anchored to the wall of the intestine; it sojourns 

 in warmth and comfort without any expensive sense-organs 

 to keep up ; with its low type of nervous system it lives a life 

 of dull sentience. It has attained to what economists have 

 called complete material well-being. 1 



But there is the seamy side of the tapeworm's "life of ease" 

 namely, degeneracy. There are no sense-organs; the nervous 

 system is of a very low order, without any brain; the muscular 

 system consists of smooth muscle-cells which contract sluggishly; 

 there is no mouth or food-canal. The reproductive system is com- 

 plex, but there is a suggestion of degeneracy in the frequent 

 occurrence of self-fertilisation a very rare thing among animals. 

 Some tapeworms produce eight million eggs, and prolific multi- 

 plication is certainly common among internal parasites. There 

 are two ways of looking at this. Abundant and stimulating food, 

 such as parasites often command, tends to make the in- 

 dividual prolific. This is the individual and physiological 

 aspect. But the continuance of the race is often very diffi- 



1 Thomson, The Wonder of Life, 1914, p. 301. 



