166 ROUNDWORMS— THE NEMATHELMINTHES 



will thrive best. If it is an internal parasite, it will die when the host dies. 

 A well-adjusted parasite, therefore, is one that does not do too much harm 

 to its host. As an example, most dogs carry several different kinds of 

 worm parasites in their intestines ; yet they show no great loss of vitality 

 because of the presence of these worms in most cases. 



When there is an association which is so well adjusted that the host 

 is not harmed at all, we do not call the organism a parasite, but refer 

 to it as a commensal. Thus, Endamoeba coli would be considered as a 

 commensal since it lives in the human intestine and derives its nourishment 

 from food which we supply, but we are not harmed by its presence. 

 Finally, when an association between two living things is not only peaceful, 

 but positively beneficial to both, it is referred to as symbiosis. You may 

 recall the case of the Portugese man-o-war and Nomeus as an example of 

 this type of association. 



All parasites have one characteristic in common — they are highly 

 specialized for the type of existence they lead, and there are very few 

 parasites that are able to live without their specific hosts. They must 

 make so many adaptations to accommodate themselves to a parasitic 

 existence that they lose the characteristics which enable them to exist as 

 free-living animals or plants. Let us consider ascaris as an example. It 

 lives in the intestine and cannot obtain free atmospheric oxygen, so it has 

 anaerobic respiration. It has lost any digestive glands that its ancestors 

 may have had, for it needs no enzymes while it is living in the presence 

 of digesting food. It may seem strange that ascaris is not digested by 

 the enzymes which are digesting the food around it. Studies of this worm 

 show that if one dies and is lodged in the intestine so it is not expelled, 

 it will be digested. This shows that there is something about the physi- 

 ology of the living worm that resists digestion. It produces enzyme- 

 neutralizing substances which render the enzymes ineffective on it. These 

 substances are somewhat like the antitoxins produced by our own bodies 

 to neutralize the toxins of disease germs and other foreign bodies that 

 may get into our bodies. This is one reason why internal parasites are 

 usually specific as to their hosts. The intestinal secretions of different 

 animals may vary ; and, when a parasite produces the neutralizing agents 

 for one animal, it might find an entirely different set necessary for another 

 animal. We, no doubt, eat many eggs and larvae of parasites which 

 inhabit other animals, but we are not affected because the young parasites 

 are digested by our enzymes. The gametes of the human malarial para- 

 sites can withstand the digestive juices of the Anopheles mosquito, but 

 not those of the Culex mosquito. The ascaris found in man shows no 

 distinguishable morphological difference from that found in pigs, but the 

 human ascaris will not infect the pig and vice versa. 



