STRONG YLUS DOUGLASSIL 157 



being, as is often the case with Entozoa ; the progeny 

 ma}^ have been to all appearances a different animal, 

 finding its habitat in a totally different host, and only 

 returning to its original appearance after two or more 

 generations. But all this does not affect the great 

 fact, that every living thing has had progenitors or a 

 progenitor, and that every Ostrich affected by this or 

 any other worm, must have swallowed one or more 

 worms, or their larvae, before it became so affected. 

 Hence we see how highly commmiicative all parasiti- 

 cal diseases are. 



I should explain, for the benefit of those who do not 

 know anything of the natural history of Entozoa, that 

 the term '^ host '^ is always applied to designate the 

 animal or insect within which the Entozoon is living. 



But it must not be supposed that the picking-up of 

 one or more worms would necessarily give the disease ; 

 a bird in robust health, with its powders of digestion un- 

 injured, may be able to resist a considerable number 

 of attacks, or may be feeding on such food as will 

 prevent the worm getting a footing, or, if it succeeds 

 in this, prevent it increasing to such an extent as to in- 

 juriously affect the bird. Or it may be — and this is the 

 important point to which we have been bringing the 

 reader — that the bird is, say twenty-nine days out of 

 thirty, able to swallow a worm or its ova without 



