EFFECTS ON PARASITES 155 



of small ponds is that their ecological niche is discontinuous and 

 short-lived. Like pond animals therefore they need a distributive 

 phase. We have seen many instances where this is an egg or an 

 encysted larva, but most interesting are those where a second host 

 acts as an intermediary. This may itself be an ectoparasite, like the 

 gnat which carries Plasmodium or Filaria bancrofti, or it may 

 be the food of a carnivorous host, like the Trichinella-iniected rat 

 eaten by a pig. In strict usage the host which harbours the sexual 

 phase is called primary, the other secondary, but where there is 

 no sexual phase, as in trypanosomes, or often for medical reasons, 

 man is considered as the primary host. Parasites generally 

 produce many eggs, which may, as has been said above, be merely 

 the result of their good food supply. When the large numbers of 

 parasitic nematodes, almost all of which are bisexual, are con- 

 sidered, it is very doubtful if hermaphroditism is any commoner 

 amongst parasites than in the animal kingdom as a whole. It is 

 obviously not necessary for success, and it is difficult to see how, 

 without self-fertilisation, it could be of any advantage. Even 

 hermaphrodite parasites such as the liver fluke and tapeworm 

 have well-developed copulatory organs, suggesting that cross- 

 fertilisation is either still practised or has only recently been given 

 up. It should be noticed that hermaphroditism is much commoner 

 in free-living nematodes than in the parasitic species. 



There remains to be considered the chemical relationship 

 between parasites and their hosts. This has two aspects. In the 

 first place gut parasites, and to a lesser extent those living else- 

 where, are exposed to enzymes which might at first thought be 

 expected to attack the parasites and dissolve their cells ; this 

 would seem to be especially likely with worms in the small 

 intestine exposed to trypsin. It is clear that the worms survive, 

 and it can be shown that neither trypsin nor pepsin has any effect 

 on them so long as they are alive. The same is, however, true of 

 living earthworms, arthropods, fish, and Protozoa, so that it seems 

 that the living cell surface of all animals is not attacked by, and 

 is impermeable to, proteolytic enzymes. Over and above this, 

 however, some parasites produce anti-enzymes, which diffuse 

 into the medium and neutralise the enzymes there present. The 

 evidence for tapeworms is conflicting, but it seems certain that 

 Ascaris produces, or causes the production of, a substance 

 which combines with and neutralises trypsin (and pepsin). It is 

 a polypeptide, and it so closely resembles an anti-enzyme extracted 



