l80 FLEAS, FLUKES AND CUCKOOS 



flukes and tapeworms and it has been estimated that a large individual 

 nematode from man can lay over 27,000,000 eggs. Sometimes develop- 

 ment is direct, but unlike the free-living nematodes, the parasitic forms 

 often require intermediate hosts in order to complete their life-cycle. 

 There is another extraordinary phenomenon well known among nema- 

 todes, of which the Ascaris from man, affords the best known example. 

 When an Q;gg of this worm is swallowed by the host it hatches in the 

 small intestine, the site where eventually Ascaris spends its adult life. 

 However, it appears incapable of developing to maturity without first 

 undertaking a ten-day peregrination inside the host's body. After 

 penetrating the mucous membrane of the intestine it is caught in the 

 bloodstream and swept into the liver, thence to the heart and lungs. 

 Possibly in this location it finds additional oxygen which is necessary 

 for its development — but this is a matter of pure conjecture. The 

 young Ascaris then burrows out of the lungs into the trachea and event- 

 ually regains the intestine via the throat and oesophagus, where it 

 continues development. Similar apparently meaningless migrations 

 inside the host's body are undertaken by many nematodes. Some 

 authors put forward the view that this is an extension of a "burrowing" 

 habit exhibited by most of these worms at some period of their develop- 

 ment. They may burrow into the mucosa lining the intestine and then 

 return to the lumen or merely bury their heads in it, or burrow directly 

 through into the body cavity, or burrow into the tissues of an inter- 

 mediate host. Other authors believe that the extensive migrations can 

 be explained on the assumption that these nematodes originally became 

 vertebrate parasites by burrowing through the skin, or that at some 

 period of their history the species in question developed in an inter- 

 mediate host. Now the cycle has been curtailed but the larva still takes 

 a trip which has become redundant since the intermediate host has 

 dropped out. One thing appears certain — these migrations through the 

 host are no joy rides. Like the charge of the gallant Light Brigade — 

 hundreds set out on their apparently pointless mission but only a few 

 come back. 



Chickens are excellent hosts for roundworms and over 50 species 

 have been recorded from the fowl. A rapid glance at any manual 

 deahng with the diseases of poultry gives a good idea of what we can 

 expect in wild birds with the same sort of feeding habits. 



Perhaps the best known of all bird nematodes is Syngamus trachea 

 (order Rhabditida, sub-order Strongylina), a brilliant scarlet worm 



