WORMS 177 



larval stage as well as adding new segments in the "neck" region — 

 which is also a form of asexual multiplication. 



During their complicated history as parasites all the digenetic 

 flukes, tapeworms and the spiny-headed worms have become involved 

 with various intermediate hosts. In some cases it is difficult to say 

 where the process first began. Possibly the bird, which is now the final 

 host, was a later addition to the original life-cycle and tagged on at the 

 end. It is obvious that by persistently eating an animal infected with 

 flukes a bird must again and again expose itself to infection. Despite this 

 fact, it is sometimes difficult to imagine how the change from inverte- 

 brate to vertebrate host can have occurred, but an important clue has 

 been provided by experiments carried out by Baer. He has shown that if 

 the tapeworm Ligula intestinalis (see p. 195) is "cultured" in an artificial 

 medium, and the temperature raised, the larval form will lay eggs 

 precociously (progenesis) . In view of these experiments it is relatively 

 easy to visualise how, when the worm was introduced into a vertebrate, 

 the sudden change of environment could stimulate egg-production and 

 enhance the species' chances of survival. Under such circumstances the 

 vertebrate host could enter the life-cycle permanently and supplant the 

 original "final" host. There is another advantage which vertebrates 

 enjoy over many invertebrates : on the whole their life is longer, and 

 each individual thus provides the internal parasite with protection and 

 food over a more extensive period. Vertebrates often wander far 

 afield; consequently the parasites which keep up continuous egg- 

 production are enabled to scatter their eggs over a much larger area and 

 during a longer period, if they are lodged, say, in the intestines of a bird 

 instead of the body-cavity of a fly. If man fed regularly upon insects 

 he would probably have acquired many worm parasites which are 

 at present found chiefly in insectivorous birds, but also in bats and 

 other animals, with similar tastes. Hands have relieved him of the 

 grim necessity of eating his own ecto-parasites — otherwise he might 

 easily have become infested with the rat and dog tapeworms which 

 use fleas as intermediate hosts. 



In discussions on parasitism it is customary to compare an ill- 

 adapted parasite, which kills the host, with the farmer who killed the 

 goose which laid the golden eggs. In the case of flukes and tapeworms 

 it is equally important from the point of view of their race that the 

 host should survive in order that they can continue to lay their "golden" 

 eggs, for the bird — by scattering them far and wide in urine, faeces and 



